- Email: [email protected]
Introduction to solar heating and cooling systems
Introduction to solar heating and cooling systems
1
R.Z. Wang, Z.Y. Xu, T.S. Ge Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China
1.1
Background
Energy and environment are two vital issues for modern society. Fossil fuels including coal, oil, and natural gas are nonrenewable and cannot provide sufficient energy sources for eternal time. In addition, utilization of these traditional energy resources has caused severe environmental problems, including global warming, air pollution, and so on. Global warming is mainly caused by carbon dioxide (CO2) emissions, which raises the global average temperature and sea level. To solve these problems, several negotiations and conferences have been held, such as the United Nations Framework Convention on Climate Change negotiated in 1992, in which many countries participated. Conferences of the Parties have been held many times in Kyoto, Bali, Copenhagen, and Paris, in which greenhouse gas emission reduction was proposed as an important task in the world. It can be seen that to build a sustainable and green future, both energy resources and the energy-consuming systems should be reconsidered under the modern energy background. For the energy resources, renewable energy resources including solar energy, wind power, and hydropower are among the best choices. Compared with traditional energy resources, renewable energy resources are abundant and environmentally friendly. Among the different renewable energy resources, solar energy is one of the most attractive options. It is a clean and endless power with wide distribution. In this case, there are numerous researches and businesses about solar energy and solar driven systems. For energy-consuming systems, the heating and cooling systems take a big proportion of the entire society energy consumption. It could be as high as 30% of the total energy consumption for those developed countries. If China is taken for an example, then the energy consumption for buildings (heating, cooling, hot water supply, lighting, etc.) is greater than 10% of the total energy used. Green and energy-saving heating and cooling systems should be developed. Considering the merits of renewable energy and high energy consumption of heating/cooling systems, the adoption of a solar energy-driven system to fulfill the heating and cooling demand is a promising solution for the aforementioned problems. Researchers all over the world have conducted innovative studies in this area. To provide a general guideline and roadmap of the solar heating and cooling systems, related technologies, including solar power, solar heating, solar cooling, solar thermal storage, and some advanced systems, will be introduced in this book. Advances in Solar Heating and Cooling. http://dx.doi.org/10.1016/B978-0-08-100301-5.00001-1 Copyright © 2016 Elsevier Ltd. All rights reserved.
4
1.2
Advances in Solar Heating and Cooling
Overview of solar heating and cooling systems
Solar energy is the primary light and heat resource of the Earth. It can provide eternal energy to maintain the atmosphere temperature and germinate plants. With technological developments, solar energy can be utilized more and more efficiently and economically. In a solar heating and cooling system, solar energy has the potential to meet a large proportion of the heating and cooling needs of buildings and industry. There are also numerous technologies for different heat source temperatures and specific demands. To ensure steady and long-term solar utilization, heat storage is also essential. In this chapter, an overview of the solar heating and cooling technologies, including solar energy, solar heating, solar cooling, and heat storage, will be given.
1.2.1
Solar energy
Solar energy is the energy source of solar heating and cooling systems. There are mainly two modern ways to collect solar energy. One is to directly adopt the thermal energy produced by solar radiation with use of a solar collector. The solar heat gained could be then transferred to solar heating or cooling applications; this kind of system is also called a solar thermal system. The other one is to transfer solar radiation into electrical power through photovoltaic (PV) material; this kind of system is also called the solar PV system. When solar energy is integrated with the heating and cooling systems, there are many more options for thermal-driven systems than for electrical-driven systems. In this case, the solar thermal collectors are emphasized and thermal-driven systems have been extensively researched and developed. Because of the significant price reduction of solar photovoltaics in the last 5 years, solar PV-powered systems are also becoming attractive. There are different classifications of the solar collector. It can be classified into nonconcentrating types and concentrating types. It can also be classified into lowtemperature collectors, medium-temperature collectors, and high-temperature collectors according to the working temperature. Low-, medium-, and high-temperature collectors work under 100 C, 100e200 C, and higher than 200 C, respectively. In this chapter, solar collectors are classified into nontracking solar collectors and tracking solar collectors. A brief introduction of solar PV technology is also given.
1.2.1.1
Nontracking solar collectors
This type of solar collector mainly includes the flat-plate collector (FPC), the evacuated-tube collector (ETC), and the compound parabolic concentrator (CPC). They usually work as low- and medium-temperature collectors that are suitable for space-heating and space-cooling. Water, air, or oil can be used as a thermal transport medium. FPCs: The FPCs usually contain the glazing, absorber plate, heat transfer component, and insulation layer. FPCs are typically used for space-heating or hot water
Introduction to solar heating and cooling systems
5
supply. It has low working temperature, but it is simple, cost-effective, and has a long lifetime. It is also easily integrated in buildings. ETCs: When the climate is not so warm or the working temperature is high, the FPC cannot work efficiently because of heat losses, and the ETCs can be used. In the ETC, the absorber surface with selective coating (absorptivity 95%, emissivity <5%) is placed in a double-layer tube with vacuum between two layers. The vacuum surrounding the absorber can greatly reduce the convection and conduction heat losses. In this case, the efficiency can be increased. CPCs: To increase the solar collector efficiency, concentrating collectors such as CPCs can be used. The CPC is a nonimaging concentrator with a low concentration ratio. The CPC uses a compound parabolic reflective surface to reflect and concentrate the solar radiation to the focal line. A tubular absorber is used as a receiver. In some newly developed CPC collectors, a compound parabolic surface and receiver are integrated in the evacuated tube to avoid heat losses and increase the efficiency.
1.2.1.2
Tracking solar collectors
This type of solar collector mainly includes the single-axis tracking collectors and twoaxes tracking collectors. Single-axis tracking collectors include linear parabolic trough collectors (PTCs), linear Fresnel reflectors (LFRs), and cylindrical trough collector (CTCs). They have a two-dimensional concentrating effect. Two-axes tracking collectors include the parabolic dish collector and solar tower (heliostat field) collector. They have a three-dimensional concentrating effect. The tracking collectors usually work as medium- and high-temperature collectors. Water, oil, or molten salt can be used as working fluid. PTCs: The PTC uses a parabolic trough reflector to concentrate the solar radiation. The tubular receiver integrated in the evacuated tube is placed along the focal line of the reflector. The collector needs to track the Sun along a single axis to maximize its efficiency. A higher concentration ratio than that of the CPC can be obtained. PTCs can effectively produce heat at temperatures between 50 C and 400 C. It can be used for solar thermal power generation, solar thermal energy for industry uses, and as the heat source for efficient solar cooling. LFRs: The LFR uses several arrays of flat mirrors to reflect and concentrate the solar radiation together. Compared with PTCs, the LFR is cheaper and takes up less space. The mirror arrays are usually placed on the ground. This makes the installation easier than PTCs, especially in a large system. However, shading and blocking problems can possibly reduce its efficiency. Compact LFR technology can improve this now that it is well accepted for industry heating and solar cooling. Parabolic dish: The parabolic dish utilizes the reflective dish to concentrate the solar radiation to one point. In this case the concentration ratio of a parabolic dish is higher than the PTC and LFR. Higher efficiency or higher working temperature can be obtained. The absorber of a parabolic dish collector is placed at the focal point. As three-dimensional concentrating is adopted, two-axes tracking is needed. Parabolic dishes have been used with power stirling engines to generate electricity.
6
Advances in Solar Heating and Cooling
Table 1.1
Solar thermal collectors [1]
Collector
Motion
Absorber type
Concentration ratio
Indicative temperature (8C)
Flat plate
Stationary
Flat
1
30e80
Evacuated tube
Stationary
Flat
1
50e200
CPC
Stationary
Tubular
1e5
60e240
PTC
Single-axis tracking
Tubular
15e45
60e300
LFR
Single-axis tracking
Tubular
10e40
60e250
Parabolic dish
Two-axes tracking
Point
100e1000
100e500
Solar tower
Two-axes tracking
Point
100e1500
150e2000
CPC, Compound parabolic concentrator; PTC, parabolic trough collector; LFR, linear Fresnel reflector.
Solar tower: The solar tower utilizes the heliostats to concentrate the solar radiation to the receiver on a tower. The heliostats are tracking mirrors spread around the tower. In this case the solar tower is also called the heliostat field or central receiver collector. Because the heliostats are individual components installed on the ground, the total reflective area and the concentration ratio can be large, which increases the system power and working efficiency. Solar tower systems have been considered as an efficient system to generate electricity from solar thermal power. The concentrating types, tracking modes, working temperatures, and efficiencies of the mentioned collectors are given in Table 1.1. The efficiencies of solar thermal collectors are closely related to the working temperature and ambient temperature. In this case the efficiencies are not included.
1.2.1.3
Solar photovoltaics
When solar photovoltaics are used for a heating and cooling system, a conventional vapor compression system can be adopted. In a solar PV system the solar radiation can be converted into direct current electricity through the PV effect of the semiconducting materials. Solar cells could be classified as silicon cells, thin film cells, emerging solar cells, and multijunction solar cells, among which silicon and film solar cells are available on the market. Silicon cells: Silicon-based material is the most maturely developed and commercialized PV material. It is also called “first-generation” technology. Silicon-based materials account for the biggest market share for PV products. Multicrystalline silicon and monocrystalline silicon are the most commonly used materials on the market.
Introduction to solar heating and cooling systems
7
Thin film cells: A thin film cell is made by depositing one or more thin layers of thin film PV material on a substrate. Its thickness varies from nanometers to tens of micrometers, which is easy for building integration. It is also called “second-generation” technology. Commercialized thin film solar cells typically use cadmium telluride, copper indium gallium selenide, and amorphous thin film silicon (a-Si). In 2014 thin film cells accounted for approximately 9% of worldwide deployment whereas the remainder comprised crystalline silicon cells [2]. Emerging solar cells: The emerging solar cells can also be called the “thirdgeneration” solar cells. These solar cells have the potential to overcome the Shockleye Queisser limit for single bandgap solar cells [3]. They include the dye-sensitized cells and organic cells. Other available technologies include the copper zinc tin sulfide cell, perovskite cell, polymer cell, and quantum dot cell. Multijunction cells: Traditional cells have only one pen junction, and there is a theoretical efficiency limit. Multijunction solar cells have multiple pen junctions made of different semiconductor materials. A theoretical efficiency up to 86.8% can be reached by infinite pen junctions [4]. The multijunction cells vary from the junction number and material. These include the InGaP/GaAs/InGaAs cell, amorphous silicon/ hydrogen alloy (a-Si)/nanocrystalline or microcrystalline silicon (nc-Si)/nc-Si thin film cell, a-Si/nc-Si thin film cell, and so on.
1.2.2
Solar heating technologies
The term solar heating means utilizing solar energy to fulfill space-heating and waterheating demands. The solar heating technologies are usually classified into passive and active technologies considering the use of active mechanical and electrical devices. In addition, there are also differences between space and passive water-heating systems.
1.2.2.1
Passive solar space-heating
In the passive solar space-heating system, the façade or roof are used to absorb and store the solar radiation. The stored solar energy will be transferred to heat and fulfill the space-heating demand when it is necessary. No other active mechanical and electrical devices are needed. The key point of passive solar space-heating is the building design. Available technologies include double window, Trombe wall, solar chimney, unglazed transpired solar façade, and solar roof technologies [5]. Passive solar heating can be a complementation of active solar heating.
1.2.2.2
Passive solar water-heating
In the passive solar water-heating system, solar collectors are used to heat the water. Technologies including FPCs, ETCs, integrated collector storage allied to a CPC, and the photovoltaic/thermal (PVT) system can be used. The basic elements of the system include the collector, piping, and hot water tank. The heat transfer from collector to storage tank occurs through the natural convection principle. An electrical pump is not needed.
8
1.2.2.3
Advances in Solar Heating and Cooling
Active solar space- and water-heating
In the active solar space- and water-heating systems, the solar collectors transfer the heat to the heating system through pumps or fans. Nontracking solar collectors are enough for these demands. Sometimes the space- and water-heating functions are integrated in one system. The heating systems can use the solar heat directly or through heat exchange processes. Water or air is used as a transport medium.
1.2.2.4
Other feasible systems
When a medium-temperature solar collector is used, a thermal-driven heat pump can be used for heating. The thermal-driven heat pump cycle usually refers to the sorption heat pump cycle. The sorption heat pump cycle contains sorption, desorption, condensation, throttling, and evaporation processes. The desorption process needs heat input whereas the sorption and condensation processes can output heat. When solar photovoltaics are used, the traditional electrical space- and water-heating technologies are all available. These include electrical heating and vapor compression heat pump systems. The condensation process releases heat output. However, these two systems are seldom seen because the low-temperature solar collector is simple, cheap, and enough for space- or water-heating.
1.2.3
Solar cooling technologies
Cooling demands mainly include refrigeration and dehumidification demands. According to the driving power and demand, solar cooling technologies can be classified into the following kinds.
1.2.3.1
Solar photovoltaic-driven refrigeration and dehumidification
Vapor compression cooling systems can be used for refrigeration and dehumidification. The refrigeration cycle includes the compression, condensation, throttling, and evaporation processes. Electrical power is transferred into mechanical power for vapor compression and then drives the cycle. The evaporation process could then output cooling. The vapor compression air conditioner is now the most widely used refrigeration device in industrial and residential applications. The working fluids include R-134a, R-410a, R-22, R-32, R-407C, and many other organic and inorganic fluids. The cooling coefficient of performance (COP) for air conditioning under normal conditions is approximately 3.0e5.0. For refrigeration and dehumidification application, the difference lies in the evaporation temperature. The dehumidification application requires a lower evaporation temperature to cool the air down to its dew point. Except for the solar PV system, the solar thermal power generation system can also work with a vapor compression cooling system. Such a system could be a combination of Kalina cycle and Rankine cycle. However, the solar thermal power generation is not the topic of this book and it will not be introduced here.
Introduction to solar heating and cooling systems
1.2.3.2
9
Solar thermal-driven refrigeration
Thermal-driven cooling technologies are different for refrigeration and dehumidification applications because of the use of a sorption working pair. For refrigeration application, the closed sorption cooling cycle can be used. The term closed means that the sorption working pair is isolated from the ambient. The cycle also contains the same processes with a sorption heat pump cycle, but the evaporation is the output process. The sorption cycle is built based on the sorption process of refrigerant by the binary working pair. The absorption of vapor by solution and the adsorption of vapor by solid all belong to sorption. Except for the sorption-desorption-condensation-throttlingevaporation loop for refrigerant, there is another loop of sorption-pressurizingdesorption-depressurizing for the binary working pair. For the absorption cooling system, the common working pairs are waterelithium bromide (LiBr) and ammoniaewater. For the adsorption cooling system, the common working pairs are waterezeolite, wateresilica gel, ammonia-calcium chloride (CaCl2), and so on. The efficiencies of the simplest single-stage sorption systems are approximately 0.5e0.8 depending on the working pair and working conditions. The most popular candidate for solar cooling is the single-effect watereLiBr absorption chiller with a COP of approximately 0.7 under a driving temperature, ambient temperature, and evaporation temperature of 90 C, 30 C, and 5 C, respectively. A higher COP can be reached with a double-effect cycle, which also requires higher driving temperature such as 140 C.
1.2.3.3
Solar thermal-driven dehumidification
To fulfill the dehumidification demand, the sorption of water vapor by the binary working pair can also be utilized. In the sorption dehumidification system, the working pair has to contact the ambient and the open sorption cycles can be used. The open sorption system is also called the desiccant cooling system. The working pair has to be related with water here. The open sorption cycle contains the sorption and desorption processes. The sorption process is used for dehumidification whereas the desorption process is used for regeneration of a sorption working pair. The desorption process needs heat input. Compared with the dehumidification completed by vapor compression cooling, the sorption desiccant dehumidification system does not need to cool the air down to dew point temperature, which is thus more energy-saving, but regeneration heat would be needed for desiccant dehumidification. There are mainly two desiccant cooling systems, including liquid desiccant cooling and solid desiccant cooling. In a liquid desiccant cooling system, the working fluid flows between the absorber and the regenerator. In a solid desiccant cooling system, the construction is different because of the nonfluid working medium. A rotary wheel system can be adopted to ensure a continuous operation. Low-temperature solar heat can drive a desiccant cooling system.
1.2.4
Heat storage technologies
The solar power is not steady and available all day long. It varies with time, weather, and season. The instability and intermittency of solar power make high efficiency and
10
Advances in Solar Heating and Cooling
long-term solar utilization difficult. Solar thermal storage is one of the solutions for this. There are now mainly four kinds of solar thermal storage technologies: sensible heat storage, latent heat storage, sorption heat storage, and thermochemical heat storage.
1.2.4.1
Sensible heat storage
Sensible heat storage is the simplest heat storage system. It stores the energy in sensible heat, which can be reflected by the temperature. The fluid storage media include water and oil. The solid storage media include the building fabric, metal, and rock. Take water as an example: it has heat capacity of approximately 4.2 kJ/(kgK) and a density of approximately 1000 kg/m3, which result in an energy density of approximately 11.7 kWh/m3 for a 10 C temperature change.
1.2.4.2
Latent heat storage
Latent heat storage stores the heat in the phase change material (PCM). Compared with sensible heat storage, its energy storage density is much higher. The research about PCM is popular because of this. The phase changing temperature is steady when the system is built. Different PCMs are needed for different energy storage temperatures. The available PCMs include organic PCMs, inorganic PCMs, and eutectic PCMs. One of the most important groups of organic PCMs is paraffin wax. Take paraffin (n-docosane) with a melting temperature of 42e44 C as an example: it has a latent heat of 194.6 kJ/kg and a density of 785 kg/m3 [6]. The energy density is 42.4 kWh/m3. Nonparaffin organic PCMs include the fatty acids and glycols. Inorganic PCMs include salt hydrates, salts, metals, and alloys. Eutectic PCMs are a minimum-melting mixture of several different PCMs [7].
1.2.4.3
Sorption heat storage
The sorption heat storage utilizes the sorption process of the binary working pair to store the heat. The sorption heat contains both the latent heat and another part of heat released by the combination process. The stronger affinity of the working pair will result in higher specific sorption heat. Compared with PCMs, the sorption heat storage material usually has a higher energy density. The sorption heat storage materials can be classified into absorption material, physical adsorption material, and chemical adsorption material. The absorption materials include watereLiBr, ammoniaewater, watereLiCl, and wateresodium hydroxide (NaOH). The storage density of watereLiCl is approximately 253 kWh/m3 and the storage density of watereLiBr is approximately 180e310 kWh/m3 [8]. The physical adsorption materials include waterezeolite and wateresilica gel. The heat storage density of waterezeolite can reach 124 kWh/m3 [8]. The chemical adsorption materials, which also belong to the thermochemical heat storage, include ammoniaeCaCl2 and ammoniaeBaCl2. In addition, the novel three-phase heat storage system integrates the absorption and adsorption processes for energy storage. WatereLiCl is one of the potential materials for three-phase heat storage [9].
Introduction to solar heating and cooling systems
11
Ethanol Drywood
10,000
Energy density (MJ/m3)
Heatsorp
MgH2 Chemical reactions CaCO2 MgCO3 Ca(OH)2
Na2S
LaNiH Ettringite
Sorption Silica gel N
1000
Zeolite PCM Na2SO4H
Zn
Si-earth NiCa battery
Na2HPO4H2O
Ice Water CaCl2H2O (sensible)
NH3/H2O
Pb Flywheels
Paraffin
100 10
20
40
60 80100
200
400
800 1000
Temperature (ºC)
Figure 1.1 Energy density of different heat storage material [8].
1.2.4.4
Thermochemical heat storage
The thermochemical reaction usually has more heat release than the phase change and sorption processes. The reversible thermochemical reaction can be utilized for heat storage with high energy density. Except for the coordination reaction of ammonia mentioned in sorption heat storage, another potential reaction is the hydration reaction of salt hydrate. The materials include magnesium chloride (MgCl2)/water, magnesium sulfate (MgSO4)/water, and sodium sulfide (Na2S)/water [10]. Energy densities of MgSO4/water and Na2S/water can both reach 780 kWh/m3 [8]. Other thermochemical heat storage materials include silicon oxide, iron carbonate, iron hydroxide, and calcium sulfate [7]. To better illustrate the energy densities and working temperatures of these heat storage materials, a cited diagram about the heat storage materials is shown in Fig. 1.1.
1.3
Technology roadmap
The former sections have introduced the available technologies for solar power collection and solar-driven heating, cooling, and heat storage. To make the couplings between different technologies clearer, the contents in this chapter are summarized and a technology roadmap is given in Fig. 1.2. The energy conversion and technology
12
Advances in Solar Heating and Cooling
Solar PV Battery
Silicon
Emerging
Thin film
Multi junction
Cooling Electricity
Dehumidification Compression chiller Liquid desiccant Solid desiccant
Solar radiation
Radiation
Refrigeration Compression chiller
Heat storage
Solar collector
PCM
Sensible
High temperature
Sorption
High energy density
Thermochemical
Heat
Point concentrating: 1. Solar tower 2. Parabolic dish
Heat temperature >200ºC
Line concentrating: 1. PTC 2. LFR 3. CPC
Medium temperature 100~200ºC
Non-concentrating: 1. PTC 2. LFR
Low temperature <100ºC
Absorption chiller Absorption chiller Ejector chiller
Heating Passive space heating 1. Double window 2. Trombe wall Passive water heating Active space heating
Figure 1.2 Solar heating and cooling roadmap.
combinations are shown in this figure. In the following chapters of this book, the mentioned solar heating and cooling technologies will be introduced in detail. The working principle, application, and some advanced researches will be included.
References [1] Kalogirou SA. Solar thermal collectors and applications. Prog Energy Combust Sci 2004; 30(3):231e95. [2] Fraunhofer ISE. Photovoltaics report. 2015. [3] Shockley W, Queisser HJ. Detailed balance limit of efficiency of p-n junction solar cells. J Appl Phys 1961;32(3):510e9. [4] Dimroth F, Kurtz S. High-efficiency multijunction solar cells. MRS Bull 2007;32(03): 230e5. [5] Chan HY, Riffat SB, Zhu J. Review of passive solar heating and cooling technologies. Renewable Sustainable Energy Rev 2010;14(2):781e9. [6] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007;27(8):1271e7. [7] Kousksou T, Bruel P, Jamil A, et al. Energy storage: applications and challenges. Sol Energy Mater Sol Cells 2014;120:59e80. [8] N’Tsoukpoe KE, Liu H, Le Pierres N, et al. A review on long-term sorption solar energy storage. Renewable Sustainable Energy Rev 2009;13(9):2385e96. [9] Yu N, Wang RZ, Lu ZS, et al. Evaluation of a three-phase sorption cycle for thermal energy storage. Energy 2014;67:468e78. [10] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energy Combust Sci 2013;39(5):489e514.
1
R.Z. Wang, Z.Y. Xu, T.S. Ge Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai, China
1.1
Background
Energy and environment are two vital issues for modern society. Fossil fuels including coal, oil, and natural gas are nonrenewable and cannot provide sufficient energy sources for eternal time. In addition, utilization of these traditional energy resources has caused severe environmental problems, including global warming, air pollution, and so on. Global warming is mainly caused by carbon dioxide (CO2) emissions, which raises the global average temperature and sea level. To solve these problems, several negotiations and conferences have been held, such as the United Nations Framework Convention on Climate Change negotiated in 1992, in which many countries participated. Conferences of the Parties have been held many times in Kyoto, Bali, Copenhagen, and Paris, in which greenhouse gas emission reduction was proposed as an important task in the world. It can be seen that to build a sustainable and green future, both energy resources and the energy-consuming systems should be reconsidered under the modern energy background. For the energy resources, renewable energy resources including solar energy, wind power, and hydropower are among the best choices. Compared with traditional energy resources, renewable energy resources are abundant and environmentally friendly. Among the different renewable energy resources, solar energy is one of the most attractive options. It is a clean and endless power with wide distribution. In this case, there are numerous researches and businesses about solar energy and solar driven systems. For energy-consuming systems, the heating and cooling systems take a big proportion of the entire society energy consumption. It could be as high as 30% of the total energy consumption for those developed countries. If China is taken for an example, then the energy consumption for buildings (heating, cooling, hot water supply, lighting, etc.) is greater than 10% of the total energy used. Green and energy-saving heating and cooling systems should be developed. Considering the merits of renewable energy and high energy consumption of heating/cooling systems, the adoption of a solar energy-driven system to fulfill the heating and cooling demand is a promising solution for the aforementioned problems. Researchers all over the world have conducted innovative studies in this area. To provide a general guideline and roadmap of the solar heating and cooling systems, related technologies, including solar power, solar heating, solar cooling, solar thermal storage, and some advanced systems, will be introduced in this book. Advances in Solar Heating and Cooling. http://dx.doi.org/10.1016/B978-0-08-100301-5.00001-1 Copyright © 2016 Elsevier Ltd. All rights reserved.
4
1.2
Advances in Solar Heating and Cooling
Overview of solar heating and cooling systems
Solar energy is the primary light and heat resource of the Earth. It can provide eternal energy to maintain the atmosphere temperature and germinate plants. With technological developments, solar energy can be utilized more and more efficiently and economically. In a solar heating and cooling system, solar energy has the potential to meet a large proportion of the heating and cooling needs of buildings and industry. There are also numerous technologies for different heat source temperatures and specific demands. To ensure steady and long-term solar utilization, heat storage is also essential. In this chapter, an overview of the solar heating and cooling technologies, including solar energy, solar heating, solar cooling, and heat storage, will be given.
1.2.1
Solar energy
Solar energy is the energy source of solar heating and cooling systems. There are mainly two modern ways to collect solar energy. One is to directly adopt the thermal energy produced by solar radiation with use of a solar collector. The solar heat gained could be then transferred to solar heating or cooling applications; this kind of system is also called a solar thermal system. The other one is to transfer solar radiation into electrical power through photovoltaic (PV) material; this kind of system is also called the solar PV system. When solar energy is integrated with the heating and cooling systems, there are many more options for thermal-driven systems than for electrical-driven systems. In this case, the solar thermal collectors are emphasized and thermal-driven systems have been extensively researched and developed. Because of the significant price reduction of solar photovoltaics in the last 5 years, solar PV-powered systems are also becoming attractive. There are different classifications of the solar collector. It can be classified into nonconcentrating types and concentrating types. It can also be classified into lowtemperature collectors, medium-temperature collectors, and high-temperature collectors according to the working temperature. Low-, medium-, and high-temperature collectors work under 100 C, 100e200 C, and higher than 200 C, respectively. In this chapter, solar collectors are classified into nontracking solar collectors and tracking solar collectors. A brief introduction of solar PV technology is also given.
1.2.1.1
Nontracking solar collectors
This type of solar collector mainly includes the flat-plate collector (FPC), the evacuated-tube collector (ETC), and the compound parabolic concentrator (CPC). They usually work as low- and medium-temperature collectors that are suitable for space-heating and space-cooling. Water, air, or oil can be used as a thermal transport medium. FPCs: The FPCs usually contain the glazing, absorber plate, heat transfer component, and insulation layer. FPCs are typically used for space-heating or hot water
Introduction to solar heating and cooling systems
5
supply. It has low working temperature, but it is simple, cost-effective, and has a long lifetime. It is also easily integrated in buildings. ETCs: When the climate is not so warm or the working temperature is high, the FPC cannot work efficiently because of heat losses, and the ETCs can be used. In the ETC, the absorber surface with selective coating (absorptivity 95%, emissivity <5%) is placed in a double-layer tube with vacuum between two layers. The vacuum surrounding the absorber can greatly reduce the convection and conduction heat losses. In this case, the efficiency can be increased. CPCs: To increase the solar collector efficiency, concentrating collectors such as CPCs can be used. The CPC is a nonimaging concentrator with a low concentration ratio. The CPC uses a compound parabolic reflective surface to reflect and concentrate the solar radiation to the focal line. A tubular absorber is used as a receiver. In some newly developed CPC collectors, a compound parabolic surface and receiver are integrated in the evacuated tube to avoid heat losses and increase the efficiency.
1.2.1.2
Tracking solar collectors
This type of solar collector mainly includes the single-axis tracking collectors and twoaxes tracking collectors. Single-axis tracking collectors include linear parabolic trough collectors (PTCs), linear Fresnel reflectors (LFRs), and cylindrical trough collector (CTCs). They have a two-dimensional concentrating effect. Two-axes tracking collectors include the parabolic dish collector and solar tower (heliostat field) collector. They have a three-dimensional concentrating effect. The tracking collectors usually work as medium- and high-temperature collectors. Water, oil, or molten salt can be used as working fluid. PTCs: The PTC uses a parabolic trough reflector to concentrate the solar radiation. The tubular receiver integrated in the evacuated tube is placed along the focal line of the reflector. The collector needs to track the Sun along a single axis to maximize its efficiency. A higher concentration ratio than that of the CPC can be obtained. PTCs can effectively produce heat at temperatures between 50 C and 400 C. It can be used for solar thermal power generation, solar thermal energy for industry uses, and as the heat source for efficient solar cooling. LFRs: The LFR uses several arrays of flat mirrors to reflect and concentrate the solar radiation together. Compared with PTCs, the LFR is cheaper and takes up less space. The mirror arrays are usually placed on the ground. This makes the installation easier than PTCs, especially in a large system. However, shading and blocking problems can possibly reduce its efficiency. Compact LFR technology can improve this now that it is well accepted for industry heating and solar cooling. Parabolic dish: The parabolic dish utilizes the reflective dish to concentrate the solar radiation to one point. In this case the concentration ratio of a parabolic dish is higher than the PTC and LFR. Higher efficiency or higher working temperature can be obtained. The absorber of a parabolic dish collector is placed at the focal point. As three-dimensional concentrating is adopted, two-axes tracking is needed. Parabolic dishes have been used with power stirling engines to generate electricity.
6
Advances in Solar Heating and Cooling
Table 1.1
Solar thermal collectors [1]
Collector
Motion
Absorber type
Concentration ratio
Indicative temperature (8C)
Flat plate
Stationary
Flat
1
30e80
Evacuated tube
Stationary
Flat
1
50e200
CPC
Stationary
Tubular
1e5
60e240
PTC
Single-axis tracking
Tubular
15e45
60e300
LFR
Single-axis tracking
Tubular
10e40
60e250
Parabolic dish
Two-axes tracking
Point
100e1000
100e500
Solar tower
Two-axes tracking
Point
100e1500
150e2000
CPC, Compound parabolic concentrator; PTC, parabolic trough collector; LFR, linear Fresnel reflector.
Solar tower: The solar tower utilizes the heliostats to concentrate the solar radiation to the receiver on a tower. The heliostats are tracking mirrors spread around the tower. In this case the solar tower is also called the heliostat field or central receiver collector. Because the heliostats are individual components installed on the ground, the total reflective area and the concentration ratio can be large, which increases the system power and working efficiency. Solar tower systems have been considered as an efficient system to generate electricity from solar thermal power. The concentrating types, tracking modes, working temperatures, and efficiencies of the mentioned collectors are given in Table 1.1. The efficiencies of solar thermal collectors are closely related to the working temperature and ambient temperature. In this case the efficiencies are not included.
1.2.1.3
Solar photovoltaics
When solar photovoltaics are used for a heating and cooling system, a conventional vapor compression system can be adopted. In a solar PV system the solar radiation can be converted into direct current electricity through the PV effect of the semiconducting materials. Solar cells could be classified as silicon cells, thin film cells, emerging solar cells, and multijunction solar cells, among which silicon and film solar cells are available on the market. Silicon cells: Silicon-based material is the most maturely developed and commercialized PV material. It is also called “first-generation” technology. Silicon-based materials account for the biggest market share for PV products. Multicrystalline silicon and monocrystalline silicon are the most commonly used materials on the market.
Introduction to solar heating and cooling systems
7
Thin film cells: A thin film cell is made by depositing one or more thin layers of thin film PV material on a substrate. Its thickness varies from nanometers to tens of micrometers, which is easy for building integration. It is also called “second-generation” technology. Commercialized thin film solar cells typically use cadmium telluride, copper indium gallium selenide, and amorphous thin film silicon (a-Si). In 2014 thin film cells accounted for approximately 9% of worldwide deployment whereas the remainder comprised crystalline silicon cells [2]. Emerging solar cells: The emerging solar cells can also be called the “thirdgeneration” solar cells. These solar cells have the potential to overcome the Shockleye Queisser limit for single bandgap solar cells [3]. They include the dye-sensitized cells and organic cells. Other available technologies include the copper zinc tin sulfide cell, perovskite cell, polymer cell, and quantum dot cell. Multijunction cells: Traditional cells have only one pen junction, and there is a theoretical efficiency limit. Multijunction solar cells have multiple pen junctions made of different semiconductor materials. A theoretical efficiency up to 86.8% can be reached by infinite pen junctions [4]. The multijunction cells vary from the junction number and material. These include the InGaP/GaAs/InGaAs cell, amorphous silicon/ hydrogen alloy (a-Si)/nanocrystalline or microcrystalline silicon (nc-Si)/nc-Si thin film cell, a-Si/nc-Si thin film cell, and so on.
1.2.2
Solar heating technologies
The term solar heating means utilizing solar energy to fulfill space-heating and waterheating demands. The solar heating technologies are usually classified into passive and active technologies considering the use of active mechanical and electrical devices. In addition, there are also differences between space and passive water-heating systems.
1.2.2.1
Passive solar space-heating
In the passive solar space-heating system, the façade or roof are used to absorb and store the solar radiation. The stored solar energy will be transferred to heat and fulfill the space-heating demand when it is necessary. No other active mechanical and electrical devices are needed. The key point of passive solar space-heating is the building design. Available technologies include double window, Trombe wall, solar chimney, unglazed transpired solar façade, and solar roof technologies [5]. Passive solar heating can be a complementation of active solar heating.
1.2.2.2
Passive solar water-heating
In the passive solar water-heating system, solar collectors are used to heat the water. Technologies including FPCs, ETCs, integrated collector storage allied to a CPC, and the photovoltaic/thermal (PVT) system can be used. The basic elements of the system include the collector, piping, and hot water tank. The heat transfer from collector to storage tank occurs through the natural convection principle. An electrical pump is not needed.
8
1.2.2.3
Advances in Solar Heating and Cooling
Active solar space- and water-heating
In the active solar space- and water-heating systems, the solar collectors transfer the heat to the heating system through pumps or fans. Nontracking solar collectors are enough for these demands. Sometimes the space- and water-heating functions are integrated in one system. The heating systems can use the solar heat directly or through heat exchange processes. Water or air is used as a transport medium.
1.2.2.4
Other feasible systems
When a medium-temperature solar collector is used, a thermal-driven heat pump can be used for heating. The thermal-driven heat pump cycle usually refers to the sorption heat pump cycle. The sorption heat pump cycle contains sorption, desorption, condensation, throttling, and evaporation processes. The desorption process needs heat input whereas the sorption and condensation processes can output heat. When solar photovoltaics are used, the traditional electrical space- and water-heating technologies are all available. These include electrical heating and vapor compression heat pump systems. The condensation process releases heat output. However, these two systems are seldom seen because the low-temperature solar collector is simple, cheap, and enough for space- or water-heating.
1.2.3
Solar cooling technologies
Cooling demands mainly include refrigeration and dehumidification demands. According to the driving power and demand, solar cooling technologies can be classified into the following kinds.
1.2.3.1
Solar photovoltaic-driven refrigeration and dehumidification
Vapor compression cooling systems can be used for refrigeration and dehumidification. The refrigeration cycle includes the compression, condensation, throttling, and evaporation processes. Electrical power is transferred into mechanical power for vapor compression and then drives the cycle. The evaporation process could then output cooling. The vapor compression air conditioner is now the most widely used refrigeration device in industrial and residential applications. The working fluids include R-134a, R-410a, R-22, R-32, R-407C, and many other organic and inorganic fluids. The cooling coefficient of performance (COP) for air conditioning under normal conditions is approximately 3.0e5.0. For refrigeration and dehumidification application, the difference lies in the evaporation temperature. The dehumidification application requires a lower evaporation temperature to cool the air down to its dew point. Except for the solar PV system, the solar thermal power generation system can also work with a vapor compression cooling system. Such a system could be a combination of Kalina cycle and Rankine cycle. However, the solar thermal power generation is not the topic of this book and it will not be introduced here.
Introduction to solar heating and cooling systems
1.2.3.2
9
Solar thermal-driven refrigeration
Thermal-driven cooling technologies are different for refrigeration and dehumidification applications because of the use of a sorption working pair. For refrigeration application, the closed sorption cooling cycle can be used. The term closed means that the sorption working pair is isolated from the ambient. The cycle also contains the same processes with a sorption heat pump cycle, but the evaporation is the output process. The sorption cycle is built based on the sorption process of refrigerant by the binary working pair. The absorption of vapor by solution and the adsorption of vapor by solid all belong to sorption. Except for the sorption-desorption-condensation-throttlingevaporation loop for refrigerant, there is another loop of sorption-pressurizingdesorption-depressurizing for the binary working pair. For the absorption cooling system, the common working pairs are waterelithium bromide (LiBr) and ammoniaewater. For the adsorption cooling system, the common working pairs are waterezeolite, wateresilica gel, ammonia-calcium chloride (CaCl2), and so on. The efficiencies of the simplest single-stage sorption systems are approximately 0.5e0.8 depending on the working pair and working conditions. The most popular candidate for solar cooling is the single-effect watereLiBr absorption chiller with a COP of approximately 0.7 under a driving temperature, ambient temperature, and evaporation temperature of 90 C, 30 C, and 5 C, respectively. A higher COP can be reached with a double-effect cycle, which also requires higher driving temperature such as 140 C.
1.2.3.3
Solar thermal-driven dehumidification
To fulfill the dehumidification demand, the sorption of water vapor by the binary working pair can also be utilized. In the sorption dehumidification system, the working pair has to contact the ambient and the open sorption cycles can be used. The open sorption system is also called the desiccant cooling system. The working pair has to be related with water here. The open sorption cycle contains the sorption and desorption processes. The sorption process is used for dehumidification whereas the desorption process is used for regeneration of a sorption working pair. The desorption process needs heat input. Compared with the dehumidification completed by vapor compression cooling, the sorption desiccant dehumidification system does not need to cool the air down to dew point temperature, which is thus more energy-saving, but regeneration heat would be needed for desiccant dehumidification. There are mainly two desiccant cooling systems, including liquid desiccant cooling and solid desiccant cooling. In a liquid desiccant cooling system, the working fluid flows between the absorber and the regenerator. In a solid desiccant cooling system, the construction is different because of the nonfluid working medium. A rotary wheel system can be adopted to ensure a continuous operation. Low-temperature solar heat can drive a desiccant cooling system.
1.2.4
Heat storage technologies
The solar power is not steady and available all day long. It varies with time, weather, and season. The instability and intermittency of solar power make high efficiency and
10
Advances in Solar Heating and Cooling
long-term solar utilization difficult. Solar thermal storage is one of the solutions for this. There are now mainly four kinds of solar thermal storage technologies: sensible heat storage, latent heat storage, sorption heat storage, and thermochemical heat storage.
1.2.4.1
Sensible heat storage
Sensible heat storage is the simplest heat storage system. It stores the energy in sensible heat, which can be reflected by the temperature. The fluid storage media include water and oil. The solid storage media include the building fabric, metal, and rock. Take water as an example: it has heat capacity of approximately 4.2 kJ/(kgK) and a density of approximately 1000 kg/m3, which result in an energy density of approximately 11.7 kWh/m3 for a 10 C temperature change.
1.2.4.2
Latent heat storage
Latent heat storage stores the heat in the phase change material (PCM). Compared with sensible heat storage, its energy storage density is much higher. The research about PCM is popular because of this. The phase changing temperature is steady when the system is built. Different PCMs are needed for different energy storage temperatures. The available PCMs include organic PCMs, inorganic PCMs, and eutectic PCMs. One of the most important groups of organic PCMs is paraffin wax. Take paraffin (n-docosane) with a melting temperature of 42e44 C as an example: it has a latent heat of 194.6 kJ/kg and a density of 785 kg/m3 [6]. The energy density is 42.4 kWh/m3. Nonparaffin organic PCMs include the fatty acids and glycols. Inorganic PCMs include salt hydrates, salts, metals, and alloys. Eutectic PCMs are a minimum-melting mixture of several different PCMs [7].
1.2.4.3
Sorption heat storage
The sorption heat storage utilizes the sorption process of the binary working pair to store the heat. The sorption heat contains both the latent heat and another part of heat released by the combination process. The stronger affinity of the working pair will result in higher specific sorption heat. Compared with PCMs, the sorption heat storage material usually has a higher energy density. The sorption heat storage materials can be classified into absorption material, physical adsorption material, and chemical adsorption material. The absorption materials include watereLiBr, ammoniaewater, watereLiCl, and wateresodium hydroxide (NaOH). The storage density of watereLiCl is approximately 253 kWh/m3 and the storage density of watereLiBr is approximately 180e310 kWh/m3 [8]. The physical adsorption materials include waterezeolite and wateresilica gel. The heat storage density of waterezeolite can reach 124 kWh/m3 [8]. The chemical adsorption materials, which also belong to the thermochemical heat storage, include ammoniaeCaCl2 and ammoniaeBaCl2. In addition, the novel three-phase heat storage system integrates the absorption and adsorption processes for energy storage. WatereLiCl is one of the potential materials for three-phase heat storage [9].
Introduction to solar heating and cooling systems
11
Ethanol Drywood
10,000
Energy density (MJ/m3)
Heatsorp
MgH2 Chemical reactions CaCO2 MgCO3 Ca(OH)2
Na2S
LaNiH Ettringite
Sorption Silica gel N
1000
Zeolite PCM Na2SO4H
Zn
Si-earth NiCa battery
Na2HPO4H2O
Ice Water CaCl2H2O (sensible)
NH3/H2O
Pb Flywheels
Paraffin
100 10
20
40
60 80100
200
400
800 1000
Temperature (ºC)
Figure 1.1 Energy density of different heat storage material [8].
1.2.4.4
Thermochemical heat storage
The thermochemical reaction usually has more heat release than the phase change and sorption processes. The reversible thermochemical reaction can be utilized for heat storage with high energy density. Except for the coordination reaction of ammonia mentioned in sorption heat storage, another potential reaction is the hydration reaction of salt hydrate. The materials include magnesium chloride (MgCl2)/water, magnesium sulfate (MgSO4)/water, and sodium sulfide (Na2S)/water [10]. Energy densities of MgSO4/water and Na2S/water can both reach 780 kWh/m3 [8]. Other thermochemical heat storage materials include silicon oxide, iron carbonate, iron hydroxide, and calcium sulfate [7]. To better illustrate the energy densities and working temperatures of these heat storage materials, a cited diagram about the heat storage materials is shown in Fig. 1.1.
1.3
Technology roadmap
The former sections have introduced the available technologies for solar power collection and solar-driven heating, cooling, and heat storage. To make the couplings between different technologies clearer, the contents in this chapter are summarized and a technology roadmap is given in Fig. 1.2. The energy conversion and technology
12
Advances in Solar Heating and Cooling
Solar PV Battery
Silicon
Emerging
Thin film
Multi junction
Cooling Electricity
Dehumidification Compression chiller Liquid desiccant Solid desiccant
Solar radiation
Radiation
Refrigeration Compression chiller
Heat storage
Solar collector
PCM
Sensible
High temperature
Sorption
High energy density
Thermochemical
Heat
Point concentrating: 1. Solar tower 2. Parabolic dish
Heat temperature >200ºC
Line concentrating: 1. PTC 2. LFR 3. CPC
Medium temperature 100~200ºC
Non-concentrating: 1. PTC 2. LFR
Low temperature <100ºC
Absorption chiller Absorption chiller Ejector chiller
Heating Passive space heating 1. Double window 2. Trombe wall Passive water heating Active space heating
Figure 1.2 Solar heating and cooling roadmap.
combinations are shown in this figure. In the following chapters of this book, the mentioned solar heating and cooling technologies will be introduced in detail. The working principle, application, and some advanced researches will be included.
References [1] Kalogirou SA. Solar thermal collectors and applications. Prog Energy Combust Sci 2004; 30(3):231e95. [2] Fraunhofer ISE. Photovoltaics report. 2015. [3] Shockley W, Queisser HJ. Detailed balance limit of efficiency of p-n junction solar cells. J Appl Phys 1961;32(3):510e9. [4] Dimroth F, Kurtz S. High-efficiency multijunction solar cells. MRS Bull 2007;32(03): 230e5. [5] Chan HY, Riffat SB, Zhu J. Review of passive solar heating and cooling technologies. Renewable Sustainable Energy Rev 2010;14(2):781e9. [6] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007;27(8):1271e7. [7] Kousksou T, Bruel P, Jamil A, et al. Energy storage: applications and challenges. Sol Energy Mater Sol Cells 2014;120:59e80. [8] N’Tsoukpoe KE, Liu H, Le Pierres N, et al. A review on long-term sorption solar energy storage. Renewable Sustainable Energy Rev 2009;13(9):2385e96. [9] Yu N, Wang RZ, Lu ZS, et al. Evaluation of a three-phase sorption cycle for thermal energy storage. Energy 2014;67:468e78. [10] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energy Combust Sci 2013;39(5):489e514.