- Email: [email protected]
Hybrid fuel cells technologies for electrical microgrids
Electric Power Systems Research 80 (2010) 993–1005
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Review
Hybrid fuel cells technologies for electrical microgrids Jose Ignacio San Martín, Inmaculada Zamora ∗ , Jose Javier San Martín, Victor Aperribay, Pablo Eguia Department of Electrical Engineering, University of the Basque Country, Alda. de Urquijo, s/n, 48013 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 5 March 2008 Received in revised form 28 December 2009 Accepted 6 January 2010 Available online 4 February 2010 Keywords: Hybrid system Fuel cell Microgrid Polygeneration
a b s t r a c t Hybrid systems are characterized by containing two or more electrical generation technologies, in order to optimize the global efficiency of the processes involved. These systems can present different operating modes. Besides, they take into account aspects that not only concern the electrical and thermal efficiencies, but also the reduction of pollutant emissions. There is a wide range of possible configurations to form hybrid systems, including hydrogen, renewable energies, gas cycles, vapour cycles or both. Nowadays, these technologies are mainly used for energy production in electrical microgrids. Some examples of these technologies are: hybridization processes of fuel cells with wind turbines and photovoltaic plants, cogeneration and trigeneration processes that can be configured with fuel cell technologies, etc. This paper reviews and analyses the main characteristics of electrical microgrids and the systems based on fuel cells for polygeneration and hybridization processes. © 2010 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
4.
5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 Electrical microgrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 Fuel cell technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 3.1. Fuel cell classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 3.2. Thermodynamic aspects of fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997 3.3. Fuel cell main parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998 Polygeneration with fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 4.1. Cogeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 4.2. Trigeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 Hybrid systems technologies with fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.1. Solid oxide fuel cell–gas microturbine hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.2. SOFC–PEMFC hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.3. Fuel cell–wind turbine–PV hybrid systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.4. Comparative analysis of hybrid technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
1. Introduction One of the main objectives of energy policy is to ensure a secure and continuous power supply coupled with the reduction of emissions associated with climate change. The achievement of this objective requires the development of renewable energy
∗ Corresponding author. Tel.: +34 946014063; fax: +34 946014200. E-mail address: [email protected] (I. Zamora). 0378-7796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2010.01.006
sources, alternative fuels for transportation and an increase in the energy efficiency of production, transmission and consumption processes. Besides, electricity market liberalization and environmental restrictions configure a future directed to energy diversification, with a significant increase in the use of clean energies. Within the context of Distributed Generation some emergent technologies for electrical generation, cogeneration and trigeneration have to be considered, such as: fuel cells, gas microturbines, Stirling motors, low cost photovoltaic systems, wind generators, etc. These technologies participate as active devices in electrical
994
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 1. Technologies for electrical microgrids.
microgrids, which can operate connected to a low voltage distribution network or as an islanded network [1]. One of the main concerns with renewable energies is their discontinuous character that imposes reserve needs for balancing generation and load. But this issue can be overcome with the development of hydrogen production technology, because it will allow to store the renewable energies when it is convenient, especially during power system valley hours. From an economic standpoint, wind power is one of the most competitive renewable sources, so it could be the cheapest way to produce hydrogen from renewable energy sources. Another renewable energy source to be considered is small hydro. Once the energy from these sources is stored as hydrogen, it can be used by fuel cells without pollutant emissions. Fuel cells are devices capable of producing electricity by an electrochemical transformation of the potential energy of a specific fuel, without classical combustion. The type of fuel used ranges from hydrogen to simple and derived hydrocarbons such as alcohols. The use of pure fuels eliminates problems associated with stack contamination by S, NO, V, etc. On the other hand, hydrogen does not generate by-products such as CO and CO2 . Hydrocarbons do it, but due to the high efficiency of the fuel cells, for equivalent quantities of produced energy, CO2 emissions can be reduced in half or less, with the corresponding environmental benefit.
Therefore, hydrogen production from a renewable source and its use in fuel cells promises a clean energy source than can be used in interconnected or islanded microgrids. Considering the previous aspects, hybrid systems are contemplated as a very promising future solution for the development of flexible and adaptable energy systems with enough capacity to meet demand, safely and with a reduced environmental impact. Having this goal in mind, this paper presents the main characteristics of electrical microgrids, fuel cells, as well as the integration of hybrid generation systems in these microgrids.
2. Electrical microgrids Although, at present, cogeneration plants and wind farms are the main sources of distributed generation, modern microgeneration systems are acquiring greater importance. This is due to their smaller rated power and the technology improvements that are extending the application of distributed generation to small industrial, commercial and residential sectors. Micro-generation can be defined as any kind of generating source of electricity and thermal energy. These sources are based on small power units, lower than 100 kW, placed in a location close
Fig. 2. Main characteristics of energy storage devices.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 3. Solid oxide fuel cell.
to the consumption point [2,3]. These sizes are sufficiently small to be connected to the local distribution system, without requiring the connection to the transmission network. The efficiency of these generators varies, usually, between 18% and 60%. Most of these generators have heat interchangers that allow using the residual heat for hot water production, space heating or for producing heat for an industrial process, increasing the total efficiency of the system up to 90%. The residual heat can also be used to operate refrigeration systems. In order to decrease the emission of pollutants to the atmosphere, mainly nitrogen oxides, “thin flame” devices and catalytic converters have been adapted to the stationary uses of electrical generation. The noise, inherent to several technologies, can be reduced in residential areas soundproofing the equipments.
995
Because of the previous characteristics, these devices can be competitive in front of traditional stations, although imply a higher cost. The most applied technologies in micro-generation are shown in Fig. 1 [4–7]. In addition, the use of storage technologies can facilitate the connection of microgenerators to the network, allowing to smooth the production from intermittent energy sources. Flywheels, batteries and supercapacitors are the devices developed at the moment to store energy [1–8]. The main characteristics of these storage devices are shown in Fig. 2. Within this context, microgrids present two operating modes: connected to the public distribution network or islanded from the grid, operating autonomously. With regard to the first mode of operation, it is necessary to highlight that microgrids are relevant when aspects like quality of supply, global efficiency, flexibility of use and costs reduction are considered. In relation to the second mode, it is focused mainly to guarantee the energy supply to remote communities. Finally, an important characteristic of microgrids is derived of its role as a consumer, when interconnected to the electrical distribution network. This characteristic depends on the flexibility of the advanced electronic devices that control the interconnection between the microgrid and the distribution network [9–11]. Therefore, the main requirement of the generation sources used in microgrids is centred in its interconnection by means of electronic devices and not in a specific nominal power. So, in order to be able to implant these micro-generation technologies, it will be necessary to develop other products and services, such as new electronic topologies for interconnection of micro-systems to electrical network.
Fig. 4. Fuel cells relevant characteristics.
996
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 5. Thermodynamic properties with standard values.
3. Fuel cell technologies A fuel cell is an electrochemical device capable of converting chemical energy of two reagents (a fuel and an oxidant) directly into low voltage d.c. electricity. The transformation is made by means of an electrochemical reaction, in which, the oxidant is usually air or oxygen. On the other hand, since the product that is oxidized is not part of the fuel cell structure and both products can be supplied uninterruptedly, the production of electricity will persist while there are reagents [12]. The basic operation of a SOFC is shown in Fig. 3. Comparing fuel cells with rechargeable batteries, in the later the chemical energy of the electrodes becomes electricity and, when it is over, it needs a recharge process that regenerates the chemical energy from electricity. However, in fuel cells the chemical energy comes from a fuel that is supplied from the outside, the operation method is the opposite of electrolysis. Fuel cells have a high electrical efficiency. In practice, around 50–60% due to overvoltages. But, it must be kept in mind that the electrochemical reactions are exothermic, so the simultaneous use of heat and electricity (cogeneration) produces a higher global efficiency, approximately up to 85% [13]. Eq. (1) shows the global efficiency considering both types of energy. Total =
Q +E mH2 · LHVH2
(1)
where Q is the thermal energy obtained, E is the electrical energy produced in the fuel cell, mH2 is the hydrogen mass flow rate and LHVH2 is the hydrogen low heating power.
Fig. 6. Ideal operation of a fuel cell.
Fig. 7. Thermodynamic efficiency with several fuels, in p.u.
3.1. Fuel cell classification Fuel cells can be classified based on two fundamental aspects: operating temperature (low or high) and the electrolyte material (substance used as a bridge to exchange ions between the anode and cathode). Regarding the temperature, four models of low temperature (AFC, PEMFC, DMFC, PAFC) and two of high temperature (MCFC and SOFC) are considered [14]. Some relevant aspects of these fuel cells are: - AFC – Alkaline Fuel Cell. The electrolyte used is a solution of diluted potassium hydroxide. It uses pure hydrogen as fuel, with null concentration of CO or CO2 , to avoid notably reducing the efficiency. They operate at atmospheric pressure and the electrodes are usually made of nickel and nickel oxide, or carbon doped with platinum. The cell voltage is in the order of 0.8 V and the current density is around 150 mA/cm2 . Their useful life is usually a year of operation. - PEMFC – Proton Exchange Membrane Fuel Cell. The electrolyte consists of a solid polymer layer, usually Nafion (based on a polyethylene polymer). The anode, or fuel electrode, is Pt/C deposited on coal paper and the cathode, or air electrode, is also Pt/C. They can be fed with reformed fuel and air. Operation temperature is low and when having a solid electrolyte their useful life is 20,000 h for stationary applications like primary power supply and it is expected to reach 40,000 h in 2011 [15]. They can supply maximum energy after 3 min of operation. Every cell supplies around 0.7 V and its current density is close to 900 mA/cm2 . “Air bleed” technique allows to operate the cells with hydrogen obtained from alcohols or carbonated fuels. - DMFC – Direct Methanol Fuel Cell. It is a PEMFC variant that uses direct methanol as fuel instead of hydrogen; this fuel is obtained generally from natural gas or biomass. Their current density is low and they have not achieved a competitive development.
Fig. 8. Parameters of a fuel cell fed with H2 , with different temperatures.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
997
Fig. 9. Fuel cells for CHP. Typical performance parameters.
- PAFC – Phosphoric Acid Fuel Cell. It uses phosphoric acid as electrolyte, contained inside a silicon carbon matrix placed together with Teflon. Catalysts are made of platinum and electrodes are made of porous carbon. The quantity of thermal energy that can be obtained from this kind of cells is very similar to the electrical energy. Power density is around 150–300 mW/cm2 . Their response time is higher than PEMFC ones and they are used in fix installations with power ranges of 0.2–10 MW. Their application field is considered to be in cogeneration systems. - MCFC – Molten Carbonate Fuel Cell. The electrolyte is a liquid solution of lithium carbonate or potassium carbonate, contained inside a porous and inert ceramic matrix, usually LiAlO2 . Anode consists of sintered nickel powder (porous) with some chromium to avoid material agglomeration. Cathode consists of nickel oxide with some lithium. It can operate with hydrogen, carbon monoxide, natural gas, propane, etc. Its efficiency is around 55–57% and, when used in cogeneration, up to 85%. Compared with other technologies, this cell is capable of operating with a higher voltage than PAFC for the same current. When temperature falls around 30 ◦ C, the voltage output is reduced 15%, approximately.
- SOFC – Solid Oxide Fuel Cell. The electrolyte is a solid ceramic material (zirconium) that operates with temperatures in the range of 750–1050 ◦ C, where the ceramic material presents an acceptable ionic conductivity. Besides, they allow internal reforming and cogeneration using the residual heat. On the other hand, the high operating temperature involves a higher starting time. This technology is very sensible to temperature variations; a reduction of 10% in temperature causes a 12% efficiency drop. This is because resistance in the oxygen ions conductivity increases. Anode is made of porous zirconium/nickel and cathode is a lanthanum manganate doped with magnesium. They reach voltages of 0.6 V per cell and current densities close to 250 mA/cm2 . Their useful life is around 30,000 h. Additionally, Fig. 4 shows the main characteristics of these six types of fuel cells.
3.2. Thermodynamic aspects of fuel cells For constant temperature and pressure, and under reversible conditions, the maximum electrical power that can be obtained from the fuel cell is given by the variation in the Gibbs free energy,
Fig. 10. Efficiency and power ranges of several technologies.
998
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 11. Performance characteristics for fuel cell based cogeneration systems.
G, as shown in the following equation: Welec = G = −nFE
(2)
G = H − TS
(3)
where Welec is electrical power, n is number of electrons that participate in the electrochemical reaction (2 for hydrogen), F is Faraday constant: 96,485 C/mol, E is e.m.f. of the basic fuel cell, T is absolute temperature, G is Gibbs free energy, determined by Eq. (3). The ideal efficiency (thermodynamic efficiency) of the fuel cell, under reversible conditions, is defined as (4): t =
nFE G =− H H
(4)
and keeping in mind (2), we can write (5): t =
S H − T · S =1−T H H
and, using the Gibbs function, the electrical power, Welec , can be obtained, as shown in (10): −G = Welec = 237.13 kJ/mol
Based on the previous calculations the electrochemical reaction in the fuel cell produces 237.13 kJ/mol of electrical power and 48.7 kJ/mol of heat. These values are shown graphically in Fig. 6. 3.3. Fuel cell main parameters From Eq. (3), the theoretical output voltage (E0 ) corresponding to the oxidation of hydrogen in a reversible and isothermal process can be calculated using (11), for a fuel cell under standard
(5)
In Fig. 5, the thermodynamic properties of H2 , O2 and H2 O are indicated, for standard conditions (p = 1 atm = 1.01325 bar and T = 25 ◦ C = 298 K) [13]. Using the numerical values shown in Fig. 5 and the Hess law, we can obtain the following values: H = Hreaction =
Hproducts −
Hreagents ,
H = −285.83 kJ/mol
S = Sreaction =
Sproducts −
(6)
Sreagents ,
S = −163.34 J/K · mol
(7)
The heat absorbed by the system is shown in (8): Q = T · S = −48.7 kJ/mol
(8)
Besides, according to Eq. (2), the Gibbs free energy can be obtained, as indicated in (9): G = H − T · S = −237.13 kJ/mol
(9)
(10)
Fig. 12. Electrical and thermal efficiencies in cogeneration processes.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
999
to connect the individual cells in series (stacks), in order to have the standard industrial voltages of 12 or 24 Vdc. Another important parameter is the efficiency. The ideal efficiency of a fuel cell (thermodynamic efficiency) takes the value shown in (14): t =
conditions [16]. −G = 1.23 V nF
(11)
This theoretical value happens if output water is in liquid form. In a real fuel cell the output voltage is lower. For example, for a PEM fuel cell with an operating temperature around 80 ◦ C, admitting ideal behaviour of gases, and that the enthalpy and entropy variation of the chemical reaction do not vary significantly with temperature, the value of G can be obtained in accordance with Eq. (2). G = H − T · S ∼ = H 0 − T · S 0 = −228.17 kJ/mol H0
(12)
S0
and are the thermodynamic properties with stanwhere dard values. Substituting (12) in (3) the output voltage for the example fuel cell is obtained. E=
−G = 1.18 V nF
(14)
This value is for output water in liquid form and pure hydrogen and oxygen. For other fuels, Fig. 7 shows the thermodynamic efficiency under standard conditions. Fig. 8 shows the estimated values of G, E and the ideal efficiency, for fuel cells of H2 , with a wide range of temperatures. In this figure, water appears in the liquid and gas states, and CO2 and NO2 are in gas state.
Fig. 13. Residential PEMFC cogeneration.
E0 =
−nFE 0 = 83% H
(13)
This value corresponds to the fuel cell operating with pure oxygen. When using air, the resulting output voltage will be around 2% lower. As each fuel cell generates around 1.18 V, it will be necessary
4. Polygeneration with fuel cells Simultaneous generation of electrical and useful thermal energy (heat, cold, or both) is an obvious way to optimize the energy efficiency of demand. In the last decade, polygeneration has had a great development worldwide, thanks to the use of gas and by taking advantage of biomass and other waste products useful for conversion into energy. Fig. 9 highlights the most relevant properties of fuel cells, from the point of view of polygeneration [17]. The results shown correspond to the following six devices: PAFC (200 kW), PEMFC (10 kW), PEMFC (200 kW), MCFC (300 kW), MCFC (1200 kW) and SOFC (125 kW). The fuel cells operate in cogeneration mode. Within this context, it can be highlighted that most fuel cells use hydrogen as fuel while the bulk of primary energy sources are hydrocarbons. It is therefore necessary to include a fuel reformer to obtain hydrogen. The six fuel cells in Fig. 9 use hydrogen obtained by natural gas reforming. For low temperature fuel cells, as PMFC and PAFC, the reformer is relatively complex and usually includes a desulfurizer, a steam reformer or partial oxidation reactor, shift converters and a cleanup system to remove carbon monoxide from the anode gas stream.
Fig. 14. Operation patterns of residential PEMFC cogeneration.
1000
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 15. Trigeneration system with a fuel cell.
In high temperature fuel cells, as MCFC and SOFC, fuel processing for simple fuels such as natural gas may simply consist of desulfurizing and preheating the fuel stream before introducing it into the internally reforming anode compartment of the fuel cell stack. The typical efficiency of natural gas reformers for hydrogen production is in the order of 80–85% [18]. In relation to the installation costs of the fuel cell system, the stack subsystem represents 25–40% of the total cost, the reforming subsystem 25–30%, the electronic power conditioning subsystem 10–20%, the thermal management subsystem 10–20% and the ancillary subsystem 5–15% [19]. The electrical efficiency for the six fuel cells of Fig. 9 can be obtained using Eq. (15) [17]. Electrical efficiency = (FPS Eff) · (H2 utilization) · (Stack Eff) · (PC Eff) ·
HHV LHV
ratio of the fuel
(15)
where FPS is Fuel Processing Subsystem Efficiency, LHV = LHV of H2 generated/LHV of the fuel consumed, H2 utilization is percent
of H2 actually consumed in the stack, Stack Eff is operating voltage/oxidation potential and PC Eff is AC power delivered/DC power generated. On the other hand, a fourth type of production can be established when the process that saves energy (associated with the three mentioned before) also allows to sequester CO2 from the exhaust gases. The CO2 can later be used to produce drinks or in hothouses. This last version allows improvements in a double way: increasing the efficiency and reducing pollutant emissions of micro-generation technologies. In this sense, CO2 dissolved in beer increases the acidity and eliminates oxygen, allowing a better preserving of the natural or added components. In hothouses, increasing the level of CO2 in the air makes the chlorophyllian function of plants easier, and the improvement in production can reach 20% [20]. Finally, Fig. 10 shows the global generation efficiency of different technologies, considering the low heat value of the fuel and the most appropriate power ranges. This efficiency takes into account the electrical and the thermal energy supplied by the different technologies.
Fig. 16. Schematic diagram of the basic configuration of a combined system.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
1001
Fig. 17. Scheme of SOFC–Gas Microturbine hybrid system.
4.1. Cogeneration In cogeneration systems, the fuel needed to generate electrical and thermal energy is much lower than when electricity and heat are produced separately as in a conventional system. So, from the point of view of primary energy efficiency, cogeneration systems are very promising, since the sum of the efficiencies of the partial processes implies an efficient use of the primary energy source. However, the applicability of these systems is limited by the heat demand and its later use in an industrial, commercial or domestic process. Thus, while in a conventional thermal plant, only 33% of the primary energy becomes electricity, or 55% in a modern combined cycle gas plant, with cogeneration this value could reach 90%. It is only necessary to install it next to where a simultaneous demand of electricity and heat exists, like in the industrial or residential sectors. For supplying loads with variable heat demand, like residential loads, fuel cells have a quite constant efficiency for a wide load range (30–100%), while traditional systems are less flexible. Fig. 11 shows the main performance characteristics of different types of fuel cells used in cogeneration systems [21], while Fig. 12 compares the electrical and thermal efficiencies for different fuel cells and other traditional technologies for cogeneration [22]. The electrical efficiency of the fuel cells in Fig. 11 has been obtained using expression (15). Finally, Figs. 13 and 14 show an application example of cogeneration in a residential building using a PEM fuel cell [23]. Thus, for a given demand of electricity and heat, these figures show how this goal can be achieved. The operation method is basically DSS (Daily Star-up and Shutdown) and load following operation, to avoid hot water surplus, as described in the central part of Fig. 14.
4.2. Trigeneration Trigeneration systems include those processes of production and simultaneous use of electricity, heat and cold, from a single fuel source. Within this context, one of the technologies that has the best performance for being integrated into a trigeneration system is the fuel cell. The simultaneous use of energy allows obtaining
high levels of overall energy efficiency, lower emissions, security of supply, as well as lower losses and investments in networks. Besides, trigeneration presents several advantages, being the most relevant: • An increase in the equivalent electrical efficiency of the cogeneration plant, due to a better use of waste heat. • A smooth operation of the plant over the year, as an increase in the demand for cooling often coincides with a reduction in the demand for heating. • An improvement of the environmental conditions, avoiding the use of CFCs and HCFCs by using natural refrigerants. Fig. 15 shows the process diagram of a trigeneration system with a fuel cell [24]. The cold is obtained using an absorption system. Absorption cooling systems are based on the evaporation and condensation of a concentrated solution for producing cold. They can use any type of waste heat, steam, hot liquid or hot gas, providing cold for air conditioning or for low temperature processes. If the waste heat is a gas, a gas to water heat exchanger is needed within an intermediate circuit. These systems are very well adapted for the recovery of waste heat, as it is possible to achieve high rates of efficiency with residual flows of relatively low temperature. The coolant and the absorbent constitute what is called a working pair. Essentially, the pairs used are: ammonia together with water as absorbent and water together with a water solution of lithium bromide as absorbent. Water/ammonia systems are used for the production of cold at low temperatures (up to −60 ◦ C), and require a heat source from 100 ◦ C to 120 ◦ C for simple effect absorption systems. Systems based on water/lithium bromide are used for air conditioning or for cooling processes, in which the temperature is between 5 ◦ C and 10 ◦ C. Fig. 16 presents the SOFC technology used to obtain air conditioning or hot water in buildings [25]. The system uses water/lithium bromide as working pair, in an absorption cooling cycle. The results show that this combination of technologies presents major technical and environmental advantages. The SOFC proposed in this example is a pre-commercial tubular model of 110 kW, developed by Siemens–Westinghouse. With this fuel cell, the electric efficiency of the system is 43.3%, the thermal efficiency
1002
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 19. Scheme of SOFC–PEMFC hybrid power system.
5.2. SOFC–PEMFC hybrid system
Fig. 18. Characteristics of SOFC–Gas Microturbine hybrid system.
in heating is 43.7%, the thermal efficiency in cooling is 52.6% and the thermal efficiency in hot water production is 46.7%. This performance provides results of global efficiency in the three operating modes, up to 87.95%, 95.9% and 90%, respectively. 5. Hybrid systems technologies with fuel cells The hybridization of technologies consists on integrating different complementary technologies, with the objective of obtaining benefits of its respective advantages and to alleviate the possible disadvantages of each one of them. In this sense, modelling and characterization of hybrid systems allows evaluating the technical and economic aspects of configurations under particular conditions. Also, it is possible to evaluate different scenarios and determining the critical values of the parameters that make the solution leans toward one configuration or another. These systems suggest a more productive use of energy. Next sections show some application examples of hybrid systems. 5.1. Solid oxide fuel cell–gas microturbine hybrid system Fig. 17 shows the block diagram of a 30-kW hybrid system, constituted by a SOFC fuel cell and a gas microturbine (MGT) [26]. In this system the combustion chamber of the microturbine is equipped to completely oxidize the residuals included in the exhaust gases of the anode. The whole system is fed with methane at atmospheric temperature and pressure. On the other hand, the compressed air warms up before entering in the SOFC cathode. The oxygen of the exhaust air of the cathode is used in the combustion process to burn the residual hydrogen, carbon monoxide and the remains of methane. The effluents coming from the combustor expand in the turbine and, by means of a recuperator, the intake air is heated. In the reformer, methane is pre-reformed with the vapour of the anode effluents and the surplus heat coming from the SOFC. The numerical values that appear in Fig. 17 show a typical example, with results based on secure design conditions. The electrical efficiency is, approximately, 66.5% (LHV). For this technology, efficiencies around 70% are foreseen. On the other hand, if in this hybrid system the SOFC works up to 800 ◦ C, the temperature of the exhaust gasses will be around 190 ◦ C. These gasses can then be used to activate trigeneration systems [27]. Fig. 18 shows a summary of the performance characteristics of this hybrid system [28].
The advantages of a system that combines the technology of fuel cells of high and low temperature are being evaluated by means of predictive algorithms [29]. In Fig. 19, the block diagram corresponding to the combination of SOFC–PEMFC technologies can be seen. This model has been analysed by simulation and presents a total electrical efficiency of 61%. The solid oxide fuel cell (SOFC) generates electricity and exhaust gases that contain CO and unused H2 . These gases are cooled down and introduced to the shift reactors, in which CO reacts with H2 O, producing CO2 and H2 . After the shift reactors, the remaining traces of CO are eliminated by means of a selective catalytic reaction. This process is necessary to prevent the poisoning of the catalysts used in the PEMFC. The result is a synthesis gas, rich in hydrogen, which is cooled down later to around 70 ◦ C, before entering in the PEMFC. As the anode fluid of the PEMFC contains non-used H2 , this is reheated to be burnt with the air flow coming from the SOFC cathode. In order to optimize the processes, many factors are being investigated, for example, the fuel rate or the increment of the cells surfaces. In this sense, it should be remembered that SOFCs have a higher cost that PEMFCs, but the former are more efficient. Another process that is being studied is the design of the stages of heat recovery. To optimize the thermal efficiency of the global system the heat should be transferred efficiently to the coldest fluids; for example, to the incoming air and the fuel, as well as to the exhaust gases coming from the PEMFC anode. 5.3. Fuel cell–wind turbine–PV hybrid systems Renewable energies, especially wind and solar, generate electricity in a discontinuous way. As this power cannot always be stored or sent to the network, it finds in hydrogen an energy vector to store it. In this sense, hydrogen supplements the limitations of both energies and vice versa, wind and photovoltaic energies can help with the high cost of producing hydrogen from the electrolysis of water.
Fig. 20. PV–FC hybrid system.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
1003
Fig. 21. Wind Turbine–FC–UC hybrid system.
In this scenario, renewable energies and hydrogen constitute an interesting binomial in numerous applications [30], such as generation of hydrogen in big plants for their later use in stationary applications, transport or to feed systems connected to the electrical network. In valley hours or with energy surpluses, wind turbine–PV systems connected to the network can supply electricity to run the electrolyser. The hydrogen is stored and later on, it can be used in a fuel cell. The electrical power supplied for this device can be used during the periods of low activity of the wind farm or with a limited threshold of solar radiation [31].
Another interesting application is the use of hydrogen generated with renewable energy in isolated autonomous systems. In these systems, the demand curve does not usually adapt to a great extent to the generation curve. The intermittent character of renewable energy makes necessary to store the energy for its later use in appropriate hours. In the design of autonomous generation systems, daily energy balances are carried out based on the analysis of the energy demand and an estimation of the energy production. These balances use statistical data of the natural resources: solar radiation, wind speed
Fig. 22. Matrix of hybrid technologies.
1004
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
and frequency, etc. Under this design, the hydrogen tank has enough capacity, not only to assume the daily balance of generation and demand, but also to provide the system with an appropriate autonomy. Thus, this system is operative in periods of reduced wind activity. With photovoltaic systems, the most used method to carry out this design is named “the worst month”. The system is designed under the most restrictive conditions, that is to say, when the relationship between production and demand is the lowest. This situation usually happens in winter periods, when the solar radiation is smaller. This way, electrical supply will be guaranteed during the whole year. The differences of resources among the seasons cause these systems to be over-rated in summer and more adjusted in winter. From this point of view, the combination of wind and solar energy is usually interesting, since they are complementary resources. The way in which hydrogen can be used for the storage of photovoltaic solar energy is shown in Fig. 20. This hybrid system is also known as regenerative system. During the day, the photovoltaic panels generate hydrogen and oxygen. During the night, electric power is generated by means of fuel cells. The water coming from the electrochemical reaction of the fuel cell is directed to the electrolyser. Regarding to technologies for storing pressurized hydrogen, the following procedures can be pointed out: as compressed gas in pressurized tanks (70,000 kPa), in low-pressure containers, in liquid form at very low temperatures (−253 ◦ C) at 1500 kPa, in nanostructures, etc. Finally, Fig. 21 shows the block diagram corresponding to a hybrid system composed by “Wind Turbine–Fuel Cells–Ultra Capacitors” [32]. The system consists on a wind generator, an induction generator with a capacitor bank for power factor correction, an ac/dc converter, a FC/UC system, a dc/dc converter, two dc/ac inverters and a three-winding transformer that connects the system to the electrical network. 5.4. Comparative analysis of hybrid technologies Fig. 22 shows the technologies susceptible of being used in hybrid systems [33]. Different proposals are included, considering those that are commercial, those that are under research and development, as well as those that are not applicable. 6. Conclusions This paper presents a review of the major characteristics of hybrid systems that integrate fuel cells and other technologies in electrical microgrids. These microgrids combine energy systems to produce a superior overall efficiency, compared with their separated operation. This is so, because this configuration allows compensating the limitations of some technologies in terms of: fuel flexibility, utilization of waste heat, pollution, etc. First, this paper presents the most outstanding characteristics of the following fuel cells technologies: AFC, PEMFC, DMFC, PAFC, MCFC and SOFC. Also, the thermodynamic, electrical and thermal aspects of these electrochemical devices are considered. In the field of cogeneration, cost data, pollutant emission and operating characteristics of various fuel cell systems that operate in cogeneration mode are shown. In addition, data reflect overall efficiency (electrical and thermal), comparing the fuel cells technology with other different technologies, such as: internal combustion engines, combined cycle cogeneration, and gas microturbines. Regarding trigeneration systems with fuel cell, this paper shows the configuration used to supply electricity, hot water and cold water for air conditioning in buildings. In this practical application, the SOFC technology has been used.
In the area of hybrid systems with fuel cells, the hybridization of a fuel cell SOFC and a gas microturbine is presented. In this hybrid system, the exhaust gases of the microturbine allow to raise the temperature of the air that enters the cathode of the fuel cell. Besides, the exhaust gases emitted for the SOFC fuel cell are burned in the combustion chamber, allowing obtaining a high electrical efficiency. Also, the combination of SOFC and PEMFC technologies, which is particularly attractive from the point of view of cost and efficiency, are analysed. Additionally, considering the fuel cell hybridization with renewable energy technologies, the use of renewable energy is analysed to activate an electrolyser, which will feed the fuel cells with hydrogen. In this sense, the combination of fuel cells with wind turbines and photovoltaic systems are shown. Finally, a matrix of hybrid technologies is included. This matrix presents possible combinations of different technologies, in order to maximize the benefits of working together rather than separately. Acknowledgements The work presented in this paper has been carried out by the research team of Project ENE2006-15700-CO2-02/CON, supported by the Ministry of Education and Science of Spain and by the Regional Council of Guipuzcoa. References [1] I. Zamora, J.I. San Martín, A.J. Mazón, J.J. San Martín, V. Aperribay, Emergent technologies in electrical microgeneration, International Journal of Emerging Electric Power Systems 3 (October (2)) (2005) 1–28, Article 1092. [2] R. Lasseter, Microgrids, IEEE Power Engineering Society Winter Meeting, 27–31 January 2002, vol. 1, 2002, pp. 305–308. [3] A. Dimeas, N. Hatziargyriou, A multiagent system for microgrids, in: Power Engineering Society General Meeting, 6–10 June 2004, vol. 1, 2004, pp. 55–58. [4] S.R. Bull, Renewable energy today and tomorrow, Proceedings of the IEEE 89 (8) (2001) 1216–1226. [5] D.W. Wu, R.Z. Wang, Combined cooling, heating and power: a review, Progress in Energy and Combustion Science. (2006) 459–495. [6] A. Bining, California’s advanced reciprocating internal combustion engine program—technology development focus, current status and progress, in: 4th Annual Advanced Stationary Reciprocating Engines Conference, Energy Resource Center, Downey, 2007. [7] E. Lysen, S.V. Egmond, S. Hagedoorn, Opslag van Elektriciteit: Status en toekomst perspectief voor Nederland, Utrecht Centrum voor Energieonderzoek, July 2006, http://www.uce-uu.nl/downloads/Eindrapport% 20Opslag%20Elektriciteit%2027Jul06.pdf. [8] CIGRE TF 38.01.10, Modelling new forms of generation and storage, Brochure N◦ 185, April 2001. [9] IEEE Standard for Interconnecting Distributed Resources with Electric Power System, Std. 1547, 2003. [10] European Project Large Scale Integration of Micro-Generation to Low Voltage Grids, DA1 Digital Models for Micro Sources, December 2003, http://www.microgrids.eu/micro2000/delivarables/Deliverable DA1.pdf. [11] R. Lasseter, A. Akhil, C. Marnay, J. Stevens, J. Dagle, R. Guttromson, A. Meliopoulous, R. Yinger, J. Eto, Integration of distributed energy resources. The CERTS microgrid concept, California Energy Commission, P500-03-089F, October 2003, http://certs.lbl.gov/pdf/50829.pdf. [12] U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, 2003, http://www.nrel.gov/programs/hydrogen.html. [13] Fuel Cell Handbook, fifth ed., U.S. Department of Energy, National Energy Technology Laboratory, B/T books, 2000. [14] U.S. Department of Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, 2005, http://www.nrel.gov/programs/hydrogen.html. [15] R. Borup, et al., Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chemical Reviews (2007) 3904–3951. [16] J. Padulles, G.W. Ault, J.R. McDonald, An approach to the dynamic modelling of fuel cell characteristics for distributed generation operation, in: IEEE Power Engineering Society Winter Meeting, vol. 1, 2000, pp. 134–138. [17] Energy and Environmental Analysis, Technology Characterization: Fuel Cells, Arlington, 2008, http://www.epa.gov/chp/documents/catalog chptech fuel cells.pdf. [18] B. Gnörich, Hydrogen and fuel cell state of the art assessment in the Roads2HyCom Project, in: World Hydrogen Technology Convention, 4–7 November, Montecatini Terme (Italy), 2007. [19] Energy Nexus Group for Energy Protection Agency, Technology Characterization: Fuel Cells, Arlington, April 2002, http://www.dleg.state.mi.us/ mpsc/electric/capacity/energyplan/alttech/fuelcells april 2002.pdf.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005 [20] I. Zamora, J.I. San Martín, A.J. Mazón, J.J. San Martín, V. Aperribay, J.M. Arrieta, Cogeneration in electrical microgrids, in: International Conference on Renewable Energies and Power Quality 2006, 5–7 April, Palma de Mallorca, Spain, 2006. [21] I. Knight, I. Ugursal, Residential Cogeneration Systems: A Review of the Current Technologies, Annex 42 of the International Energy Agency, Ottawa, 2005. [22] S. Meo, G. Paparo, G. Velotto, Fuel cell based distributed generation feeding electrical and thermal loads, in: International Conference on Renewable Energies and Power Quality 2004, 31 March–2 April, Barcelona (Spain), 2004. [23] K. Kobayashi, M. Kawamura, T. Takahashi, Y. Nishizaka, K. Nishizaki, Development of PEFC Co-generation System for Japanese Residential Market, PEFC Project Technology Development Department Tokyo Gas Co., Ltd, Tokyo, 2005. [24] A. Jones, A Climate Change Agency for London, 2006, http://www.revolve.ws/ downloads/presentations/LondonClimateChangeAgency.pps. [25] F. Zink, Y. Lu, L. Schaefer, A solid oxide fuel cell system for buildings, Energy Conversion & Management 48 (2007) 809–818. [26] S. Kimijima, N. Kasagi, Performance evaluation of gas turbine-fuel cell hybrid microgeneration system, in: Proceedings of ASME TURBO EXPO, Amsterdam, 2002.
1005
[27] TIAX, An Assessment of a Hydrogen Cities Concept Applied to a Representative Community, Prepared for DOE/NETL (D0204), Cambridge MA, July 2004, http://www.tiaxllc.com/aboutus/pdfs/hydrocities 072304.pdf. [28] A.D. Little Inc., Opportunities for Micropower and Fuel Cell/Gas Turbine Hybrid Systems in Industrial Applications, Final Report to Lockheed Martin Energy Research Corporation and the DOE Office of Industrial Technologies, January 2000. http://www.eere.energy.gov/de/pdfs/micropower opp vol2 appx.pdf. [29] A.L. Dicks, R.G. Fellows, C. Martin Mescal, C. Seymour, A study of SOFC–PEM hybrid systems, Journal of Power Sources 86 (March (1–2)) (2000) 501–506. [30] O. Alonso, A. Vegas, F. Isorna, J. Benz, Fuel Cells Integration in autonomous systems fed from renewable sources, INTA (2005) (in Spanish). [31] K. Rajashekara, Hybrid fuel-cell strategies for clean power generation, IEEE Transactions on Industry Applications 41 (3) (2005) 682–689. [32] O.C. Onar, M. Uzunoglu, M.S. Alam, Dynamic modelling, design and simulation of a wind/fuel cell/ultra-capacitor-based hybrid power generation system, Journal of Power Sources 161 (October (1)) (2006) 707–722. [33] G.D. Burch, Hybrid Renewable Energy Systems, Renewable Energy Workshops, August 21, Golden, Colorado, 2001, http://www.netl.doe.gov/publications/ proceedings/01/hybrids/Gary%20Burch%208.21.01.pdf.
Contents lists available at ScienceDirect
Electric Power Systems Research journal homepage: www.elsevier.com/locate/epsr
Review
Hybrid fuel cells technologies for electrical microgrids Jose Ignacio San Martín, Inmaculada Zamora ∗ , Jose Javier San Martín, Victor Aperribay, Pablo Eguia Department of Electrical Engineering, University of the Basque Country, Alda. de Urquijo, s/n, 48013 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 5 March 2008 Received in revised form 28 December 2009 Accepted 6 January 2010 Available online 4 February 2010 Keywords: Hybrid system Fuel cell Microgrid Polygeneration
a b s t r a c t Hybrid systems are characterized by containing two or more electrical generation technologies, in order to optimize the global efficiency of the processes involved. These systems can present different operating modes. Besides, they take into account aspects that not only concern the electrical and thermal efficiencies, but also the reduction of pollutant emissions. There is a wide range of possible configurations to form hybrid systems, including hydrogen, renewable energies, gas cycles, vapour cycles or both. Nowadays, these technologies are mainly used for energy production in electrical microgrids. Some examples of these technologies are: hybridization processes of fuel cells with wind turbines and photovoltaic plants, cogeneration and trigeneration processes that can be configured with fuel cell technologies, etc. This paper reviews and analyses the main characteristics of electrical microgrids and the systems based on fuel cells for polygeneration and hybridization processes. © 2010 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
4.
5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993 Electrical microgrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 994 Fuel cell technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 3.1. Fuel cell classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 996 3.2. Thermodynamic aspects of fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 997 3.3. Fuel cell main parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 998 Polygeneration with fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999 4.1. Cogeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 4.2. Trigeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001 Hybrid systems technologies with fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.1. Solid oxide fuel cell–gas microturbine hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.2. SOFC–PEMFC hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.3. Fuel cell–wind turbine–PV hybrid systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1002 5.4. Comparative analysis of hybrid technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004
1. Introduction One of the main objectives of energy policy is to ensure a secure and continuous power supply coupled with the reduction of emissions associated with climate change. The achievement of this objective requires the development of renewable energy
∗ Corresponding author. Tel.: +34 946014063; fax: +34 946014200. E-mail address: [email protected] (I. Zamora). 0378-7796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2010.01.006
sources, alternative fuels for transportation and an increase in the energy efficiency of production, transmission and consumption processes. Besides, electricity market liberalization and environmental restrictions configure a future directed to energy diversification, with a significant increase in the use of clean energies. Within the context of Distributed Generation some emergent technologies for electrical generation, cogeneration and trigeneration have to be considered, such as: fuel cells, gas microturbines, Stirling motors, low cost photovoltaic systems, wind generators, etc. These technologies participate as active devices in electrical
994
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 1. Technologies for electrical microgrids.
microgrids, which can operate connected to a low voltage distribution network or as an islanded network [1]. One of the main concerns with renewable energies is their discontinuous character that imposes reserve needs for balancing generation and load. But this issue can be overcome with the development of hydrogen production technology, because it will allow to store the renewable energies when it is convenient, especially during power system valley hours. From an economic standpoint, wind power is one of the most competitive renewable sources, so it could be the cheapest way to produce hydrogen from renewable energy sources. Another renewable energy source to be considered is small hydro. Once the energy from these sources is stored as hydrogen, it can be used by fuel cells without pollutant emissions. Fuel cells are devices capable of producing electricity by an electrochemical transformation of the potential energy of a specific fuel, without classical combustion. The type of fuel used ranges from hydrogen to simple and derived hydrocarbons such as alcohols. The use of pure fuels eliminates problems associated with stack contamination by S, NO, V, etc. On the other hand, hydrogen does not generate by-products such as CO and CO2 . Hydrocarbons do it, but due to the high efficiency of the fuel cells, for equivalent quantities of produced energy, CO2 emissions can be reduced in half or less, with the corresponding environmental benefit.
Therefore, hydrogen production from a renewable source and its use in fuel cells promises a clean energy source than can be used in interconnected or islanded microgrids. Considering the previous aspects, hybrid systems are contemplated as a very promising future solution for the development of flexible and adaptable energy systems with enough capacity to meet demand, safely and with a reduced environmental impact. Having this goal in mind, this paper presents the main characteristics of electrical microgrids, fuel cells, as well as the integration of hybrid generation systems in these microgrids.
2. Electrical microgrids Although, at present, cogeneration plants and wind farms are the main sources of distributed generation, modern microgeneration systems are acquiring greater importance. This is due to their smaller rated power and the technology improvements that are extending the application of distributed generation to small industrial, commercial and residential sectors. Micro-generation can be defined as any kind of generating source of electricity and thermal energy. These sources are based on small power units, lower than 100 kW, placed in a location close
Fig. 2. Main characteristics of energy storage devices.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 3. Solid oxide fuel cell.
to the consumption point [2,3]. These sizes are sufficiently small to be connected to the local distribution system, without requiring the connection to the transmission network. The efficiency of these generators varies, usually, between 18% and 60%. Most of these generators have heat interchangers that allow using the residual heat for hot water production, space heating or for producing heat for an industrial process, increasing the total efficiency of the system up to 90%. The residual heat can also be used to operate refrigeration systems. In order to decrease the emission of pollutants to the atmosphere, mainly nitrogen oxides, “thin flame” devices and catalytic converters have been adapted to the stationary uses of electrical generation. The noise, inherent to several technologies, can be reduced in residential areas soundproofing the equipments.
995
Because of the previous characteristics, these devices can be competitive in front of traditional stations, although imply a higher cost. The most applied technologies in micro-generation are shown in Fig. 1 [4–7]. In addition, the use of storage technologies can facilitate the connection of microgenerators to the network, allowing to smooth the production from intermittent energy sources. Flywheels, batteries and supercapacitors are the devices developed at the moment to store energy [1–8]. The main characteristics of these storage devices are shown in Fig. 2. Within this context, microgrids present two operating modes: connected to the public distribution network or islanded from the grid, operating autonomously. With regard to the first mode of operation, it is necessary to highlight that microgrids are relevant when aspects like quality of supply, global efficiency, flexibility of use and costs reduction are considered. In relation to the second mode, it is focused mainly to guarantee the energy supply to remote communities. Finally, an important characteristic of microgrids is derived of its role as a consumer, when interconnected to the electrical distribution network. This characteristic depends on the flexibility of the advanced electronic devices that control the interconnection between the microgrid and the distribution network [9–11]. Therefore, the main requirement of the generation sources used in microgrids is centred in its interconnection by means of electronic devices and not in a specific nominal power. So, in order to be able to implant these micro-generation technologies, it will be necessary to develop other products and services, such as new electronic topologies for interconnection of micro-systems to electrical network.
Fig. 4. Fuel cells relevant characteristics.
996
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 5. Thermodynamic properties with standard values.
3. Fuel cell technologies A fuel cell is an electrochemical device capable of converting chemical energy of two reagents (a fuel and an oxidant) directly into low voltage d.c. electricity. The transformation is made by means of an electrochemical reaction, in which, the oxidant is usually air or oxygen. On the other hand, since the product that is oxidized is not part of the fuel cell structure and both products can be supplied uninterruptedly, the production of electricity will persist while there are reagents [12]. The basic operation of a SOFC is shown in Fig. 3. Comparing fuel cells with rechargeable batteries, in the later the chemical energy of the electrodes becomes electricity and, when it is over, it needs a recharge process that regenerates the chemical energy from electricity. However, in fuel cells the chemical energy comes from a fuel that is supplied from the outside, the operation method is the opposite of electrolysis. Fuel cells have a high electrical efficiency. In practice, around 50–60% due to overvoltages. But, it must be kept in mind that the electrochemical reactions are exothermic, so the simultaneous use of heat and electricity (cogeneration) produces a higher global efficiency, approximately up to 85% [13]. Eq. (1) shows the global efficiency considering both types of energy. Total =
Q +E mH2 · LHVH2
(1)
where Q is the thermal energy obtained, E is the electrical energy produced in the fuel cell, mH2 is the hydrogen mass flow rate and LHVH2 is the hydrogen low heating power.
Fig. 6. Ideal operation of a fuel cell.
Fig. 7. Thermodynamic efficiency with several fuels, in p.u.
3.1. Fuel cell classification Fuel cells can be classified based on two fundamental aspects: operating temperature (low or high) and the electrolyte material (substance used as a bridge to exchange ions between the anode and cathode). Regarding the temperature, four models of low temperature (AFC, PEMFC, DMFC, PAFC) and two of high temperature (MCFC and SOFC) are considered [14]. Some relevant aspects of these fuel cells are: - AFC – Alkaline Fuel Cell. The electrolyte used is a solution of diluted potassium hydroxide. It uses pure hydrogen as fuel, with null concentration of CO or CO2 , to avoid notably reducing the efficiency. They operate at atmospheric pressure and the electrodes are usually made of nickel and nickel oxide, or carbon doped with platinum. The cell voltage is in the order of 0.8 V and the current density is around 150 mA/cm2 . Their useful life is usually a year of operation. - PEMFC – Proton Exchange Membrane Fuel Cell. The electrolyte consists of a solid polymer layer, usually Nafion (based on a polyethylene polymer). The anode, or fuel electrode, is Pt/C deposited on coal paper and the cathode, or air electrode, is also Pt/C. They can be fed with reformed fuel and air. Operation temperature is low and when having a solid electrolyte their useful life is 20,000 h for stationary applications like primary power supply and it is expected to reach 40,000 h in 2011 [15]. They can supply maximum energy after 3 min of operation. Every cell supplies around 0.7 V and its current density is close to 900 mA/cm2 . “Air bleed” technique allows to operate the cells with hydrogen obtained from alcohols or carbonated fuels. - DMFC – Direct Methanol Fuel Cell. It is a PEMFC variant that uses direct methanol as fuel instead of hydrogen; this fuel is obtained generally from natural gas or biomass. Their current density is low and they have not achieved a competitive development.
Fig. 8. Parameters of a fuel cell fed with H2 , with different temperatures.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
997
Fig. 9. Fuel cells for CHP. Typical performance parameters.
- PAFC – Phosphoric Acid Fuel Cell. It uses phosphoric acid as electrolyte, contained inside a silicon carbon matrix placed together with Teflon. Catalysts are made of platinum and electrodes are made of porous carbon. The quantity of thermal energy that can be obtained from this kind of cells is very similar to the electrical energy. Power density is around 150–300 mW/cm2 . Their response time is higher than PEMFC ones and they are used in fix installations with power ranges of 0.2–10 MW. Their application field is considered to be in cogeneration systems. - MCFC – Molten Carbonate Fuel Cell. The electrolyte is a liquid solution of lithium carbonate or potassium carbonate, contained inside a porous and inert ceramic matrix, usually LiAlO2 . Anode consists of sintered nickel powder (porous) with some chromium to avoid material agglomeration. Cathode consists of nickel oxide with some lithium. It can operate with hydrogen, carbon monoxide, natural gas, propane, etc. Its efficiency is around 55–57% and, when used in cogeneration, up to 85%. Compared with other technologies, this cell is capable of operating with a higher voltage than PAFC for the same current. When temperature falls around 30 ◦ C, the voltage output is reduced 15%, approximately.
- SOFC – Solid Oxide Fuel Cell. The electrolyte is a solid ceramic material (zirconium) that operates with temperatures in the range of 750–1050 ◦ C, where the ceramic material presents an acceptable ionic conductivity. Besides, they allow internal reforming and cogeneration using the residual heat. On the other hand, the high operating temperature involves a higher starting time. This technology is very sensible to temperature variations; a reduction of 10% in temperature causes a 12% efficiency drop. This is because resistance in the oxygen ions conductivity increases. Anode is made of porous zirconium/nickel and cathode is a lanthanum manganate doped with magnesium. They reach voltages of 0.6 V per cell and current densities close to 250 mA/cm2 . Their useful life is around 30,000 h. Additionally, Fig. 4 shows the main characteristics of these six types of fuel cells.
3.2. Thermodynamic aspects of fuel cells For constant temperature and pressure, and under reversible conditions, the maximum electrical power that can be obtained from the fuel cell is given by the variation in the Gibbs free energy,
Fig. 10. Efficiency and power ranges of several technologies.
998
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 11. Performance characteristics for fuel cell based cogeneration systems.
G, as shown in the following equation: Welec = G = −nFE
(2)
G = H − TS
(3)
where Welec is electrical power, n is number of electrons that participate in the electrochemical reaction (2 for hydrogen), F is Faraday constant: 96,485 C/mol, E is e.m.f. of the basic fuel cell, T is absolute temperature, G is Gibbs free energy, determined by Eq. (3). The ideal efficiency (thermodynamic efficiency) of the fuel cell, under reversible conditions, is defined as (4): t =
nFE G =− H H
(4)
and keeping in mind (2), we can write (5): t =
S H − T · S =1−T H H
and, using the Gibbs function, the electrical power, Welec , can be obtained, as shown in (10): −G = Welec = 237.13 kJ/mol
Based on the previous calculations the electrochemical reaction in the fuel cell produces 237.13 kJ/mol of electrical power and 48.7 kJ/mol of heat. These values are shown graphically in Fig. 6. 3.3. Fuel cell main parameters From Eq. (3), the theoretical output voltage (E0 ) corresponding to the oxidation of hydrogen in a reversible and isothermal process can be calculated using (11), for a fuel cell under standard
(5)
In Fig. 5, the thermodynamic properties of H2 , O2 and H2 O are indicated, for standard conditions (p = 1 atm = 1.01325 bar and T = 25 ◦ C = 298 K) [13]. Using the numerical values shown in Fig. 5 and the Hess law, we can obtain the following values: H = Hreaction =
Hproducts −
Hreagents ,
H = −285.83 kJ/mol
S = Sreaction =
Sproducts −
(6)
Sreagents ,
S = −163.34 J/K · mol
(7)
The heat absorbed by the system is shown in (8): Q = T · S = −48.7 kJ/mol
(8)
Besides, according to Eq. (2), the Gibbs free energy can be obtained, as indicated in (9): G = H − T · S = −237.13 kJ/mol
(9)
(10)
Fig. 12. Electrical and thermal efficiencies in cogeneration processes.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
999
to connect the individual cells in series (stacks), in order to have the standard industrial voltages of 12 or 24 Vdc. Another important parameter is the efficiency. The ideal efficiency of a fuel cell (thermodynamic efficiency) takes the value shown in (14): t =
conditions [16]. −G = 1.23 V nF
(11)
This theoretical value happens if output water is in liquid form. In a real fuel cell the output voltage is lower. For example, for a PEM fuel cell with an operating temperature around 80 ◦ C, admitting ideal behaviour of gases, and that the enthalpy and entropy variation of the chemical reaction do not vary significantly with temperature, the value of G can be obtained in accordance with Eq. (2). G = H − T · S ∼ = H 0 − T · S 0 = −228.17 kJ/mol H0
(12)
S0
and are the thermodynamic properties with stanwhere dard values. Substituting (12) in (3) the output voltage for the example fuel cell is obtained. E=
−G = 1.18 V nF
(14)
This value is for output water in liquid form and pure hydrogen and oxygen. For other fuels, Fig. 7 shows the thermodynamic efficiency under standard conditions. Fig. 8 shows the estimated values of G, E and the ideal efficiency, for fuel cells of H2 , with a wide range of temperatures. In this figure, water appears in the liquid and gas states, and CO2 and NO2 are in gas state.
Fig. 13. Residential PEMFC cogeneration.
E0 =
−nFE 0 = 83% H
(13)
This value corresponds to the fuel cell operating with pure oxygen. When using air, the resulting output voltage will be around 2% lower. As each fuel cell generates around 1.18 V, it will be necessary
4. Polygeneration with fuel cells Simultaneous generation of electrical and useful thermal energy (heat, cold, or both) is an obvious way to optimize the energy efficiency of demand. In the last decade, polygeneration has had a great development worldwide, thanks to the use of gas and by taking advantage of biomass and other waste products useful for conversion into energy. Fig. 9 highlights the most relevant properties of fuel cells, from the point of view of polygeneration [17]. The results shown correspond to the following six devices: PAFC (200 kW), PEMFC (10 kW), PEMFC (200 kW), MCFC (300 kW), MCFC (1200 kW) and SOFC (125 kW). The fuel cells operate in cogeneration mode. Within this context, it can be highlighted that most fuel cells use hydrogen as fuel while the bulk of primary energy sources are hydrocarbons. It is therefore necessary to include a fuel reformer to obtain hydrogen. The six fuel cells in Fig. 9 use hydrogen obtained by natural gas reforming. For low temperature fuel cells, as PMFC and PAFC, the reformer is relatively complex and usually includes a desulfurizer, a steam reformer or partial oxidation reactor, shift converters and a cleanup system to remove carbon monoxide from the anode gas stream.
Fig. 14. Operation patterns of residential PEMFC cogeneration.
1000
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 15. Trigeneration system with a fuel cell.
In high temperature fuel cells, as MCFC and SOFC, fuel processing for simple fuels such as natural gas may simply consist of desulfurizing and preheating the fuel stream before introducing it into the internally reforming anode compartment of the fuel cell stack. The typical efficiency of natural gas reformers for hydrogen production is in the order of 80–85% [18]. In relation to the installation costs of the fuel cell system, the stack subsystem represents 25–40% of the total cost, the reforming subsystem 25–30%, the electronic power conditioning subsystem 10–20%, the thermal management subsystem 10–20% and the ancillary subsystem 5–15% [19]. The electrical efficiency for the six fuel cells of Fig. 9 can be obtained using Eq. (15) [17]. Electrical efficiency = (FPS Eff) · (H2 utilization) · (Stack Eff) · (PC Eff) ·
HHV LHV
ratio of the fuel
(15)
where FPS is Fuel Processing Subsystem Efficiency, LHV = LHV of H2 generated/LHV of the fuel consumed, H2 utilization is percent
of H2 actually consumed in the stack, Stack Eff is operating voltage/oxidation potential and PC Eff is AC power delivered/DC power generated. On the other hand, a fourth type of production can be established when the process that saves energy (associated with the three mentioned before) also allows to sequester CO2 from the exhaust gases. The CO2 can later be used to produce drinks or in hothouses. This last version allows improvements in a double way: increasing the efficiency and reducing pollutant emissions of micro-generation technologies. In this sense, CO2 dissolved in beer increases the acidity and eliminates oxygen, allowing a better preserving of the natural or added components. In hothouses, increasing the level of CO2 in the air makes the chlorophyllian function of plants easier, and the improvement in production can reach 20% [20]. Finally, Fig. 10 shows the global generation efficiency of different technologies, considering the low heat value of the fuel and the most appropriate power ranges. This efficiency takes into account the electrical and the thermal energy supplied by the different technologies.
Fig. 16. Schematic diagram of the basic configuration of a combined system.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
1001
Fig. 17. Scheme of SOFC–Gas Microturbine hybrid system.
4.1. Cogeneration In cogeneration systems, the fuel needed to generate electrical and thermal energy is much lower than when electricity and heat are produced separately as in a conventional system. So, from the point of view of primary energy efficiency, cogeneration systems are very promising, since the sum of the efficiencies of the partial processes implies an efficient use of the primary energy source. However, the applicability of these systems is limited by the heat demand and its later use in an industrial, commercial or domestic process. Thus, while in a conventional thermal plant, only 33% of the primary energy becomes electricity, or 55% in a modern combined cycle gas plant, with cogeneration this value could reach 90%. It is only necessary to install it next to where a simultaneous demand of electricity and heat exists, like in the industrial or residential sectors. For supplying loads with variable heat demand, like residential loads, fuel cells have a quite constant efficiency for a wide load range (30–100%), while traditional systems are less flexible. Fig. 11 shows the main performance characteristics of different types of fuel cells used in cogeneration systems [21], while Fig. 12 compares the electrical and thermal efficiencies for different fuel cells and other traditional technologies for cogeneration [22]. The electrical efficiency of the fuel cells in Fig. 11 has been obtained using expression (15). Finally, Figs. 13 and 14 show an application example of cogeneration in a residential building using a PEM fuel cell [23]. Thus, for a given demand of electricity and heat, these figures show how this goal can be achieved. The operation method is basically DSS (Daily Star-up and Shutdown) and load following operation, to avoid hot water surplus, as described in the central part of Fig. 14.
4.2. Trigeneration Trigeneration systems include those processes of production and simultaneous use of electricity, heat and cold, from a single fuel source. Within this context, one of the technologies that has the best performance for being integrated into a trigeneration system is the fuel cell. The simultaneous use of energy allows obtaining
high levels of overall energy efficiency, lower emissions, security of supply, as well as lower losses and investments in networks. Besides, trigeneration presents several advantages, being the most relevant: • An increase in the equivalent electrical efficiency of the cogeneration plant, due to a better use of waste heat. • A smooth operation of the plant over the year, as an increase in the demand for cooling often coincides with a reduction in the demand for heating. • An improvement of the environmental conditions, avoiding the use of CFCs and HCFCs by using natural refrigerants. Fig. 15 shows the process diagram of a trigeneration system with a fuel cell [24]. The cold is obtained using an absorption system. Absorption cooling systems are based on the evaporation and condensation of a concentrated solution for producing cold. They can use any type of waste heat, steam, hot liquid or hot gas, providing cold for air conditioning or for low temperature processes. If the waste heat is a gas, a gas to water heat exchanger is needed within an intermediate circuit. These systems are very well adapted for the recovery of waste heat, as it is possible to achieve high rates of efficiency with residual flows of relatively low temperature. The coolant and the absorbent constitute what is called a working pair. Essentially, the pairs used are: ammonia together with water as absorbent and water together with a water solution of lithium bromide as absorbent. Water/ammonia systems are used for the production of cold at low temperatures (up to −60 ◦ C), and require a heat source from 100 ◦ C to 120 ◦ C for simple effect absorption systems. Systems based on water/lithium bromide are used for air conditioning or for cooling processes, in which the temperature is between 5 ◦ C and 10 ◦ C. Fig. 16 presents the SOFC technology used to obtain air conditioning or hot water in buildings [25]. The system uses water/lithium bromide as working pair, in an absorption cooling cycle. The results show that this combination of technologies presents major technical and environmental advantages. The SOFC proposed in this example is a pre-commercial tubular model of 110 kW, developed by Siemens–Westinghouse. With this fuel cell, the electric efficiency of the system is 43.3%, the thermal efficiency
1002
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
Fig. 19. Scheme of SOFC–PEMFC hybrid power system.
5.2. SOFC–PEMFC hybrid system
Fig. 18. Characteristics of SOFC–Gas Microturbine hybrid system.
in heating is 43.7%, the thermal efficiency in cooling is 52.6% and the thermal efficiency in hot water production is 46.7%. This performance provides results of global efficiency in the three operating modes, up to 87.95%, 95.9% and 90%, respectively. 5. Hybrid systems technologies with fuel cells The hybridization of technologies consists on integrating different complementary technologies, with the objective of obtaining benefits of its respective advantages and to alleviate the possible disadvantages of each one of them. In this sense, modelling and characterization of hybrid systems allows evaluating the technical and economic aspects of configurations under particular conditions. Also, it is possible to evaluate different scenarios and determining the critical values of the parameters that make the solution leans toward one configuration or another. These systems suggest a more productive use of energy. Next sections show some application examples of hybrid systems. 5.1. Solid oxide fuel cell–gas microturbine hybrid system Fig. 17 shows the block diagram of a 30-kW hybrid system, constituted by a SOFC fuel cell and a gas microturbine (MGT) [26]. In this system the combustion chamber of the microturbine is equipped to completely oxidize the residuals included in the exhaust gases of the anode. The whole system is fed with methane at atmospheric temperature and pressure. On the other hand, the compressed air warms up before entering in the SOFC cathode. The oxygen of the exhaust air of the cathode is used in the combustion process to burn the residual hydrogen, carbon monoxide and the remains of methane. The effluents coming from the combustor expand in the turbine and, by means of a recuperator, the intake air is heated. In the reformer, methane is pre-reformed with the vapour of the anode effluents and the surplus heat coming from the SOFC. The numerical values that appear in Fig. 17 show a typical example, with results based on secure design conditions. The electrical efficiency is, approximately, 66.5% (LHV). For this technology, efficiencies around 70% are foreseen. On the other hand, if in this hybrid system the SOFC works up to 800 ◦ C, the temperature of the exhaust gasses will be around 190 ◦ C. These gasses can then be used to activate trigeneration systems [27]. Fig. 18 shows a summary of the performance characteristics of this hybrid system [28].
The advantages of a system that combines the technology of fuel cells of high and low temperature are being evaluated by means of predictive algorithms [29]. In Fig. 19, the block diagram corresponding to the combination of SOFC–PEMFC technologies can be seen. This model has been analysed by simulation and presents a total electrical efficiency of 61%. The solid oxide fuel cell (SOFC) generates electricity and exhaust gases that contain CO and unused H2 . These gases are cooled down and introduced to the shift reactors, in which CO reacts with H2 O, producing CO2 and H2 . After the shift reactors, the remaining traces of CO are eliminated by means of a selective catalytic reaction. This process is necessary to prevent the poisoning of the catalysts used in the PEMFC. The result is a synthesis gas, rich in hydrogen, which is cooled down later to around 70 ◦ C, before entering in the PEMFC. As the anode fluid of the PEMFC contains non-used H2 , this is reheated to be burnt with the air flow coming from the SOFC cathode. In order to optimize the processes, many factors are being investigated, for example, the fuel rate or the increment of the cells surfaces. In this sense, it should be remembered that SOFCs have a higher cost that PEMFCs, but the former are more efficient. Another process that is being studied is the design of the stages of heat recovery. To optimize the thermal efficiency of the global system the heat should be transferred efficiently to the coldest fluids; for example, to the incoming air and the fuel, as well as to the exhaust gases coming from the PEMFC anode. 5.3. Fuel cell–wind turbine–PV hybrid systems Renewable energies, especially wind and solar, generate electricity in a discontinuous way. As this power cannot always be stored or sent to the network, it finds in hydrogen an energy vector to store it. In this sense, hydrogen supplements the limitations of both energies and vice versa, wind and photovoltaic energies can help with the high cost of producing hydrogen from the electrolysis of water.
Fig. 20. PV–FC hybrid system.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
1003
Fig. 21. Wind Turbine–FC–UC hybrid system.
In this scenario, renewable energies and hydrogen constitute an interesting binomial in numerous applications [30], such as generation of hydrogen in big plants for their later use in stationary applications, transport or to feed systems connected to the electrical network. In valley hours or with energy surpluses, wind turbine–PV systems connected to the network can supply electricity to run the electrolyser. The hydrogen is stored and later on, it can be used in a fuel cell. The electrical power supplied for this device can be used during the periods of low activity of the wind farm or with a limited threshold of solar radiation [31].
Another interesting application is the use of hydrogen generated with renewable energy in isolated autonomous systems. In these systems, the demand curve does not usually adapt to a great extent to the generation curve. The intermittent character of renewable energy makes necessary to store the energy for its later use in appropriate hours. In the design of autonomous generation systems, daily energy balances are carried out based on the analysis of the energy demand and an estimation of the energy production. These balances use statistical data of the natural resources: solar radiation, wind speed
Fig. 22. Matrix of hybrid technologies.
1004
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005
and frequency, etc. Under this design, the hydrogen tank has enough capacity, not only to assume the daily balance of generation and demand, but also to provide the system with an appropriate autonomy. Thus, this system is operative in periods of reduced wind activity. With photovoltaic systems, the most used method to carry out this design is named “the worst month”. The system is designed under the most restrictive conditions, that is to say, when the relationship between production and demand is the lowest. This situation usually happens in winter periods, when the solar radiation is smaller. This way, electrical supply will be guaranteed during the whole year. The differences of resources among the seasons cause these systems to be over-rated in summer and more adjusted in winter. From this point of view, the combination of wind and solar energy is usually interesting, since they are complementary resources. The way in which hydrogen can be used for the storage of photovoltaic solar energy is shown in Fig. 20. This hybrid system is also known as regenerative system. During the day, the photovoltaic panels generate hydrogen and oxygen. During the night, electric power is generated by means of fuel cells. The water coming from the electrochemical reaction of the fuel cell is directed to the electrolyser. Regarding to technologies for storing pressurized hydrogen, the following procedures can be pointed out: as compressed gas in pressurized tanks (70,000 kPa), in low-pressure containers, in liquid form at very low temperatures (−253 ◦ C) at 1500 kPa, in nanostructures, etc. Finally, Fig. 21 shows the block diagram corresponding to a hybrid system composed by “Wind Turbine–Fuel Cells–Ultra Capacitors” [32]. The system consists on a wind generator, an induction generator with a capacitor bank for power factor correction, an ac/dc converter, a FC/UC system, a dc/dc converter, two dc/ac inverters and a three-winding transformer that connects the system to the electrical network. 5.4. Comparative analysis of hybrid technologies Fig. 22 shows the technologies susceptible of being used in hybrid systems [33]. Different proposals are included, considering those that are commercial, those that are under research and development, as well as those that are not applicable. 6. Conclusions This paper presents a review of the major characteristics of hybrid systems that integrate fuel cells and other technologies in electrical microgrids. These microgrids combine energy systems to produce a superior overall efficiency, compared with their separated operation. This is so, because this configuration allows compensating the limitations of some technologies in terms of: fuel flexibility, utilization of waste heat, pollution, etc. First, this paper presents the most outstanding characteristics of the following fuel cells technologies: AFC, PEMFC, DMFC, PAFC, MCFC and SOFC. Also, the thermodynamic, electrical and thermal aspects of these electrochemical devices are considered. In the field of cogeneration, cost data, pollutant emission and operating characteristics of various fuel cell systems that operate in cogeneration mode are shown. In addition, data reflect overall efficiency (electrical and thermal), comparing the fuel cells technology with other different technologies, such as: internal combustion engines, combined cycle cogeneration, and gas microturbines. Regarding trigeneration systems with fuel cell, this paper shows the configuration used to supply electricity, hot water and cold water for air conditioning in buildings. In this practical application, the SOFC technology has been used.
In the area of hybrid systems with fuel cells, the hybridization of a fuel cell SOFC and a gas microturbine is presented. In this hybrid system, the exhaust gases of the microturbine allow to raise the temperature of the air that enters the cathode of the fuel cell. Besides, the exhaust gases emitted for the SOFC fuel cell are burned in the combustion chamber, allowing obtaining a high electrical efficiency. Also, the combination of SOFC and PEMFC technologies, which is particularly attractive from the point of view of cost and efficiency, are analysed. Additionally, considering the fuel cell hybridization with renewable energy technologies, the use of renewable energy is analysed to activate an electrolyser, which will feed the fuel cells with hydrogen. In this sense, the combination of fuel cells with wind turbines and photovoltaic systems are shown. Finally, a matrix of hybrid technologies is included. This matrix presents possible combinations of different technologies, in order to maximize the benefits of working together rather than separately. Acknowledgements The work presented in this paper has been carried out by the research team of Project ENE2006-15700-CO2-02/CON, supported by the Ministry of Education and Science of Spain and by the Regional Council of Guipuzcoa. References [1] I. Zamora, J.I. San Martín, A.J. Mazón, J.J. San Martín, V. Aperribay, Emergent technologies in electrical microgeneration, International Journal of Emerging Electric Power Systems 3 (October (2)) (2005) 1–28, Article 1092. [2] R. Lasseter, Microgrids, IEEE Power Engineering Society Winter Meeting, 27–31 January 2002, vol. 1, 2002, pp. 305–308. [3] A. Dimeas, N. Hatziargyriou, A multiagent system for microgrids, in: Power Engineering Society General Meeting, 6–10 June 2004, vol. 1, 2004, pp. 55–58. [4] S.R. Bull, Renewable energy today and tomorrow, Proceedings of the IEEE 89 (8) (2001) 1216–1226. [5] D.W. Wu, R.Z. Wang, Combined cooling, heating and power: a review, Progress in Energy and Combustion Science. (2006) 459–495. [6] A. Bining, California’s advanced reciprocating internal combustion engine program—technology development focus, current status and progress, in: 4th Annual Advanced Stationary Reciprocating Engines Conference, Energy Resource Center, Downey, 2007. [7] E. Lysen, S.V. Egmond, S. Hagedoorn, Opslag van Elektriciteit: Status en toekomst perspectief voor Nederland, Utrecht Centrum voor Energieonderzoek, July 2006, http://www.uce-uu.nl/downloads/Eindrapport% 20Opslag%20Elektriciteit%2027Jul06.pdf. [8] CIGRE TF 38.01.10, Modelling new forms of generation and storage, Brochure N◦ 185, April 2001. [9] IEEE Standard for Interconnecting Distributed Resources with Electric Power System, Std. 1547, 2003. [10] European Project Large Scale Integration of Micro-Generation to Low Voltage Grids, DA1 Digital Models for Micro Sources, December 2003, http://www.microgrids.eu/micro2000/delivarables/Deliverable DA1.pdf. [11] R. Lasseter, A. Akhil, C. Marnay, J. Stevens, J. Dagle, R. Guttromson, A. Meliopoulous, R. Yinger, J. Eto, Integration of distributed energy resources. The CERTS microgrid concept, California Energy Commission, P500-03-089F, October 2003, http://certs.lbl.gov/pdf/50829.pdf. [12] U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, 2003, http://www.nrel.gov/programs/hydrogen.html. [13] Fuel Cell Handbook, fifth ed., U.S. Department of Energy, National Energy Technology Laboratory, B/T books, 2000. [14] U.S. Department of Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, 2005, http://www.nrel.gov/programs/hydrogen.html. [15] R. Borup, et al., Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chemical Reviews (2007) 3904–3951. [16] J. Padulles, G.W. Ault, J.R. McDonald, An approach to the dynamic modelling of fuel cell characteristics for distributed generation operation, in: IEEE Power Engineering Society Winter Meeting, vol. 1, 2000, pp. 134–138. [17] Energy and Environmental Analysis, Technology Characterization: Fuel Cells, Arlington, 2008, http://www.epa.gov/chp/documents/catalog chptech fuel cells.pdf. [18] B. Gnörich, Hydrogen and fuel cell state of the art assessment in the Roads2HyCom Project, in: World Hydrogen Technology Convention, 4–7 November, Montecatini Terme (Italy), 2007. [19] Energy Nexus Group for Energy Protection Agency, Technology Characterization: Fuel Cells, Arlington, April 2002, http://www.dleg.state.mi.us/ mpsc/electric/capacity/energyplan/alttech/fuelcells april 2002.pdf.
J.I. San Martín et al. / Electric Power Systems Research 80 (2010) 993–1005 [20] I. Zamora, J.I. San Martín, A.J. Mazón, J.J. San Martín, V. Aperribay, J.M. Arrieta, Cogeneration in electrical microgrids, in: International Conference on Renewable Energies and Power Quality 2006, 5–7 April, Palma de Mallorca, Spain, 2006. [21] I. Knight, I. Ugursal, Residential Cogeneration Systems: A Review of the Current Technologies, Annex 42 of the International Energy Agency, Ottawa, 2005. [22] S. Meo, G. Paparo, G. Velotto, Fuel cell based distributed generation feeding electrical and thermal loads, in: International Conference on Renewable Energies and Power Quality 2004, 31 March–2 April, Barcelona (Spain), 2004. [23] K. Kobayashi, M. Kawamura, T. Takahashi, Y. Nishizaka, K. Nishizaki, Development of PEFC Co-generation System for Japanese Residential Market, PEFC Project Technology Development Department Tokyo Gas Co., Ltd, Tokyo, 2005. [24] A. Jones, A Climate Change Agency for London, 2006, http://www.revolve.ws/ downloads/presentations/LondonClimateChangeAgency.pps. [25] F. Zink, Y. Lu, L. Schaefer, A solid oxide fuel cell system for buildings, Energy Conversion & Management 48 (2007) 809–818. [26] S. Kimijima, N. Kasagi, Performance evaluation of gas turbine-fuel cell hybrid microgeneration system, in: Proceedings of ASME TURBO EXPO, Amsterdam, 2002.
1005
[27] TIAX, An Assessment of a Hydrogen Cities Concept Applied to a Representative Community, Prepared for DOE/NETL (D0204), Cambridge MA, July 2004, http://www.tiaxllc.com/aboutus/pdfs/hydrocities 072304.pdf. [28] A.D. Little Inc., Opportunities for Micropower and Fuel Cell/Gas Turbine Hybrid Systems in Industrial Applications, Final Report to Lockheed Martin Energy Research Corporation and the DOE Office of Industrial Technologies, January 2000. http://www.eere.energy.gov/de/pdfs/micropower opp vol2 appx.pdf. [29] A.L. Dicks, R.G. Fellows, C. Martin Mescal, C. Seymour, A study of SOFC–PEM hybrid systems, Journal of Power Sources 86 (March (1–2)) (2000) 501–506. [30] O. Alonso, A. Vegas, F. Isorna, J. Benz, Fuel Cells Integration in autonomous systems fed from renewable sources, INTA (2005) (in Spanish). [31] K. Rajashekara, Hybrid fuel-cell strategies for clean power generation, IEEE Transactions on Industry Applications 41 (3) (2005) 682–689. [32] O.C. Onar, M. Uzunoglu, M.S. Alam, Dynamic modelling, design and simulation of a wind/fuel cell/ultra-capacitor-based hybrid power generation system, Journal of Power Sources 161 (October (1)) (2006) 707–722. [33] G.D. Burch, Hybrid Renewable Energy Systems, Renewable Energy Workshops, August 21, Golden, Colorado, 2001, http://www.netl.doe.gov/publications/ proceedings/01/hybrids/Gary%20Burch%208.21.01.pdf.