- Email: [email protected]
Proposal of a nearly zero energy building electrical power generator with an optimal temporary generation–consumption correlation
Accepted Manuscript Title: A proposal of nearly Zero Energy Building (nZEB) electrical power generator for optimal temporary generation-consumption correlation Author: J. Renau L. Domenech V. Garc´ıa A. Real N. Mont´es F. S´anchez PII: DOI: Reference:
S0378-7788(14)00362-4 http://dx.doi.org/doi:10.1016/j.enbuild.2014.03.083 ENB 5016
To appear in:
ENB
Received date: Revised date: Accepted date:
11-11-2013 24-3-2014 26-3-2014
Please cite this article as: J. Renau, L. Domenech, V. Garc´ia, A. Real, N. Mont´es, F. S´anchez, A proposal of nearly Zero Energy Building (nZEB) electrical power generator for optimal temporary generation-consumption correlation, Energy & Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.03.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript_local_compilation_TEX2013
A proposal of nearly Zero Energy Building (nZEB) electrical power generator for optimal temporary generation-consumption correlation. J.Renau, L.Domenech, V.Garc´ıa, A.Real, N.Mont´es, F.S´anchez
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Department of Building Engineering and Industrial Production, University CEU Cardenal Herrera (CEU-UCH), Valencia, Spain
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Abstract
This work presents an Electrical PV Power Generator system that was designed, build and tested during Solar De-
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cathlon Europe 2012 competition at the SMLsystem, the CEU Cardenal Herrera University prototype. One of the main purposes of the contest is to evaluate the prototypes capacity for electrical energy balance in terms of self-sufficiency,
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low energy consumptions and the temporary generation-consumption correlation demanded. Design and evaluation are driven through the competition rules, that build a framework for a kind of nearly-Zero Energy House (nZEH) with the particular criteria of obtaining all the necessary energy from the sun.
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Different considerations that were carried out to design the PV system are shown and discussed: energy demand and disposability, possibility to grid connection, power installed, appropriate battery capacity, Electrical-PV system configuration (AC-coupling or/ and DC-coupling), and the temporary generation-consumption correlation demanded.
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In order to achieve an optimal energy balance, the Electrical PV Power Generator system design strategy presented is focussed on the energy correlation analysis behavior and the efficient management of the energy system keeping the main load of a nZEH low enough and time controlled.
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Keywords: Solar Decathlon Europe 2012, nZEB, NZEB, ZEB, Photovoltaic Systems, Self-sufficient PV systems,
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Energy correlation, SMLsystem
1. Introduction
The recast of the European Directive 2010/31/EU of 19 of May 2010 [1] define the nearly Zero Energy Building as a building that has a very high energy performance, which nearly zero or very low amount of energy should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby.
The use of on-site even nearby energy is know as energy self-consumption. The term on-site energy is easy to understand because the energy is produced in the same building where is produced. Nevertheless, nearby energy is an extended term for the same concept which implies a higher control in energy flows. So, an other electrical power grid concept called Smart Grid [2] has to be used for the energy management. Email address: [email protected] (J.Renau) Preprint submitted to Energy and Building Journal
March 14, 2014
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In medium term, the use of an small scale Smart Grid as a neighbourhood energy exchange system maximize the local photovoltaic energy consumption [3] and minimize the energy losses for the energy storage conversions. This situation can be powered by the use of a Demand-Side Management (DSM), defined by the European Commission as a tool to influence the global energy market and hence the security of energy supply in the medium and long term.
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DSM brings many benefits such as, reduction of the generation margin capacity and improving transmission grid investment and operation efficiency, but more challenges will have to be faced: advanced metering and control technologies –lack of ICT infrastructure–, the use of real-time information, more complexity of the system operation
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and lack of understanding of benefits and the difficult to evaluate them; as Professor Goran Strbac shows [4].
The authors present in this paper an Electrical PV Power Generator System designed and implemented through the
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participation of the Department of Building Engineering and Industrial Production of the University CEU Cardenal Herrera in the Solar Decathlon Europe 2012 (SDE12) with the project SMLsystem. This generator was designed in accordance with the needs of the contest rules, but also with the European laws in order to be applied in future real
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buildings. 1.1. Competition and rules framework
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The Solar Decathlon Europe is an international competition in which universities from all over the world meet to design, build and operate an energetically self-sufficient house, grid-connected, using solar energy as the only energy source and equipped with all of the technologies that permit maximum energy efficiency.
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During the final phase of the competition, teams assembled their houses in Madrid, and participate during September 2012 in ten different challenges regarding renewable energy use and savings. The SDE organization specified a framework rules for the competition comprising the prototype evaluation criteria and procedures for juries. The houses
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presented by the teams were constantly monitored and the information related to the houses behaviour was assessed and open.
namely:
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The winner of SDE12 is known after the evaluation of ten contests or challenges with different scoring each one,
1. Architecture
2. Engineering and Construction 3. Energy Efficiency
4. Electrical Energy Balance 5. Comfort Conditions 6. Functioning of the house
7. Communication and Raising Social Awareness 8. Industrialization and Market viability 9. Innovation 10. Sustainability 2
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1.2. Electrical Energy Balance Contest Contest 4 is which has a special significance for this paper. Electrical Energy Balance is one of the main objectives of the Solar Decathlon Europe which “promote research in the development of efficient houses. [...]. Particular emphasis is put on reducing energy consumption and on obtaining all the necessary energy from the sun” [5]
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So the objective is to evaluate the houses’ electrical energy self-sufficiency provided by solar active technology and their electricity use intensity.
evaluate three different concepts that completes the idea of a self-sufficient house:
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1. Autonomy
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For that propose the energy production and energy consumed in each house was measured. The main idea was
2. Temporary Generation-Consumption Correlation
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3. Consumption per measurable area 1.2.1. Autonomy
EG−yearly − E L−yearly ≥ 0
(1)
EG − E L ≥ 10kWh
(2)
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This sub-contest evaluate the self-supply of the house during the competition weeks.
For a positive annual electrical energy balance, the house must follow the equation 1. Where:
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EG−yearly Represent the energy generated throughout a whole year.
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E L−yearly Represent the consumption of the loads throughout a whole year. But during the competition it is impossible to determinate the yearly production and consumption, so as the equation 2 shows a value of 10 kWh, which it was considered as sufficient during September (as the equivalence of a year electrical energy balance).
However, if the house doesn’t exceed the 10 kWh could obtain reduced point between −10 kWh ≤ EG − E L < 10 kWh.
The measure period was determined between the free shadow period (generation frame). Between 10 to 17 h. Out of these generation frame, the PV system must be disconnected. 1.2.2. Temporary generation-consumption correlation One of the main advantages of Electrical Distributed Generation is that electricity is consumed in the same place where it is generated. This reduces the need for transmission lines and minimizes the electricity transport losses. This 3
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effect is maximized if electricity is consumed at the same time as it is generated (this concept is discussed deeper in section 2). To evaluate the score of this sub-contest, equation 5 gives the score in function of ξ, which is the correlation between generated and simultaneously consumed energy by the loads.
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It depends on we have batteries (equation 4) or not (equation 3). The value ξ is continuously integrated during the measuring period of the competition, which is a longest period than the generation one. The value of ξ is between
ξ
=
EG−L R EL
(EG−L + E Bat−L ) R EL ξ · Pointstotal−contest
=
Points
=
an
R
ξ
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R
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[0, 1].
(3) (4) (5)
As for the autonomy sub-contest, there was a measuring frame for the correlation sub-contest. The measures was
1.2.3. Consumption per measurable area
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taken between 8 am to 23 pm.
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In order to reduce the CO2 emissions and external energy dependence of the countries, it is just as important to
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have a large renewable energy production as efficient energy consumption. E L−average A
(6)
Where the E L−average is the average energy consumed by the loads daily during the contest weeks. Lower the value,
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greater the scoring.
1.3. SMLsystem: the SDE 2012 prototype SMLsystem project (figure 1) is the research continuous upgrading initiated with SMLhouse (Small, Medium and Large) proposed in the Solar Decathlon Europe 2010. SMLsystem returns to prefabrication as a proposal defining an architectural concept where structural, composition and functional values are introduced in a sustainable building approach. The Unit or basic Module –considered as a box–, is formed totally by prefabricated materials and dryassembled, being wood the predominant material in SMLsystem prototype. Each of these Units is transported fully equipped. The courtyard acts like a composition element. In all constructive Units there is a courtyard –generated with a louvre–, which divides the space, providing an special access to the house. Also, offering natural lighting and ventilation control in a great space value. The effectiveness of the courtyard is increased with the use of both louvres 4
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Figure 1: SMLsystem in Madrid during competition
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–horizontal and vertical–, reducing the excess radiation at certain times of the day, as well as sift and offer more privacy to the housing.
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Reducing energy demand is the main objective of SMLsystem as a nearly-Zero Energy House [1], but also the efficient management of the renewable energy produced on-site. Passive systems incorporated into its design allow minimizing the need for and dependence on active systems for cooling and heating of the house. In other hand,
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efficiency of active systems optimizes the use of energy and reduces demand. For the HVAC system, a heat pump combined with Phase Change Materials for a Thermal Energy Storage (TES) system has been implemented. For the optimization of consumption compared to generation, was used a Domotic Prediction System (DPS). The Learning
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Ability (LA) of the system will allow it to predict the behaviour of the user based on past behaviours to displace consumption and optimizing the generation. This automation system will monitor air quality, humidity, etc., activating
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the corresponding systems, namely, ventilation for air exchange, and air conditioning. The project will also have a LED lighting interaction with intelligent home automation system which will regulate the levels of lighting intensity depending on the needs at all times. The use of high efficiency appliances as well as proper use of them also reduces the energy demand of SMLsystem.
The Sun is a clean energy source but the energy depends on the weather conditions, for ensure the energy disposability. In SMLsystem all the possible photovoltaic areas were exploited and the architecture design is a key issue in the integration of the PV technologies. This surfaces can defined as building’s energy footprint, so in contrast with traditional energy sources they are visible, the PV systems are a ”form” –i. e. shapes, colors and features–, and architects are responsible to manage this ”form” [6]. The different orientations of the construction elements –roof and fac¸ades– require different photovoltaic systems with appropriate energy management strategies –discussed in section 2–. The solar panels are placed on the top of each Module with an independent structure per Module, which properly 5
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joins with the roof structure of them. That point is important because all the facilities which are complex are set up, so the work is just execute joints between the module and facilities pack, that minimizes personal risks implicit while working on the roof, and construction problems like damages on the waterproof layer.
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2. The SMLsystem generator A self-sufficient house needs an electrical energy generator for works as nearly-Zero Energy as much as possible. Balancing the consumption and generation as well as possible and minimizing the energy consumption from the grid.
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Some authors like Huafen Hu et al. [7] assert that the next level of the Net Zero Energy Houses (NZEH) is to take them off from the grid. Of course is right when you consider an outage energy as a quality indicator.
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In the design proposed in this paper, it is assumed the necessity of a grid to ensure the energy, because of it is not consider an energy shortage as a possible situation, in that case the energy will be buy from the grid.
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To achieve high self consumption rates from the PV system, a storage system has to be used for buffering the daily energy to use it during the night [8]. Nevertheless, the coexistence of PV and batteries with an electrical grid is a new situation.
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In the past, the PV generators were mainly assembled to sell the energy to the electrical grid, because it energy was remunerated with feed-in tariffs.
Currently, this is changing in all the European Countries a grid parity has reached, that means that solar energy can
and more interesting.
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compete with conventional energy from the grid in several markets [9]. So, the self-consumption is more economical
The authors present a PV generator system that can work with the currently situation: there aren’t feed-in tariffs,
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later consumptions.
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neither an ADSM system for maximize the consumption from PV generation, so all the energy has to be stored for
2.1. Description
The requirement of an electrical grid imposes the necessity of a multi-coupling system. In figure 2 five parts can be highlighted: (1) the roof generator, (2) two generators in each facade, (3) the storage system, (4) the energy manager and (5) the existence of an electrical grid –smart grid of villa solar 2012–. The system proposed uses a combined AC and DC coupling system for achieve maximum efficiency and a year reliability. Of course, with a high self consumption rate. This kind of systems were thought for isolated consumer like the system presented by Karim Moutawakkil and Steffen Elster [10]. This proposal include an energy rejecter to ensure the stability of the micro-grid. For SMLsystem the stability of the micro-grid can be achieved thanks to the injection of the energy surplus to the electrical grid –smart grid–. The energy sources of the SMLsystem Generator are the PV Roof Generator and the PV fa¸cades Generators (both east and west fac¸ades). 6
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Figure 2: SMLsystem electrical system sketch
Both generators are joint between the hybrid inverter/charger XW4548, working as a grid transfer relay or as
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an energy seller to the grid, never as a battery charger, so the charger disabled to charging from the grid (figure 3). The XW4548 just can charge the batteries from the energy surplus in the AC loads side, provided by the PV Roof
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Generator.
Figure 3: XW4548 functioning
The PV Roof Generator is a pure AC-coupled system connected in the AC-Loads side. The AC-Loads output is configured as a micro-grid feeding the whole consumptions of SMLsystem. So, the energy of this micro-grid can be provided by the PV Roof Generator or by the XW4548 output, that keeps the stability using a few energy from the DC bus. Of course, the XW4548 output maximize the energy consumption from the renewable sources, namely, the PV fa¸cades Generator and the Battery Bank, that is considered as solar energy too. 7
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Battery Bank
Model
Enersys SBS190F
Monoblock
12 Vdc
Bank
4 units
Energy stored
5.5 kWh
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Table 1: Battery bank characteristics
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Manufacturer
The Battery Bank was designed to cover the worst energy daily demand, during the contest period. Nevertheless,
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Johannes Weniger et al. [8] shows how to sizing a PV battery system for residential applications, taking relevant the economical aspects. For the design exposed in this paper just an energy and a secure functioning were considered.
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The main parameters of the battery bank can be seen in table 1.
The PV Roof Generator, as mentioned above, was designed to be an AC-coupled system. That means a high performance energy generator because the energy from the PV just needs one conversion to feed the loads.
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So, to configure the generator, the better performance panels at the moment were used. Namely, the Sunpower 225 panel (table 2), which is a monocrystalline technology one. Considering the technical conditions of that technology, the PV Roof Generator was designed for improve the catchment of direct radiation from the sun.
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The Photovoltaic Roof Generator was assembled using the Sunpower 225 module (table 2), a monocrystalline silicon technology. This technology is better than other for maximum production with direct radiation. The number of modules depends on the space of the roof. The best configuration for September in Madrid is an
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azimuth of 2o to the east and a slope of 34o yearly. Nevertheless, for September, the maximum production power is at 37o . The difference between 37o and 30o is less than 2% in energy production.
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In an energy analysis this difference can be depict, but the difference of 7o in architecture is quite significant. So the final design was 21 modules sloping 30o and 0o azimuth of the house, but for the real location was ≈ 10o azimuth. The table 3 shows the essential information of the Roof Generator. The design of the PV Fa¸cades Generator depends on the fac¸ade available surface, because of is a Building Integrated PV system (BIPV). Just the east and the west fac¸ades were available in the prototype. So, the main solar energy was the diffuse one. To benefit that the PV CIGS technology has the better performance (table 4). The energy from the less power PV Fac¸ades Generator was used directly to charge the battery bank using two MPPT charger, one per fac¸ade. 2.2. System functioning Figure 4 clarify the system functioning and the sign criteria for next section (3), in power or energy flows. The diagram shows that sometimes the energy can be bi-directional in the same branch, but never at the same time. 8
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225 Wp
Rated current
5.59 A
Rated voltage
41,0 V
Open circuit voltage
48,5 V
Short circuit current
5,87 A
String
Modules
3x7=21
5,47
16,47
287
287
Open circuit voltage (V)
339,5
339,5
Short circuit current (A)
5,87
17,61
Rated current (A)
18%
Rated voltage (V)
Temperature coefficients −0, 38%W/C
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−132, 5mV/C
Table 3: Roof PV Generator characteristics
3, 5mA/C 798x1559x46mm
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Table 2: Sunpower 225 characteristics
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Dimensions
Generator
7
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Efficiency
Photovoltaic Roof Generator
cr
Peak power
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Sunpower 225
CIGS Module
PV Fac¸ades Generator
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Peak power
75,7 Wp
Rated current
1,7 A
Rated voltage
44,5 V
Open circuit voltage
65,3 V
Short circuit current
2,03 A
Efficiency
Dimensions
String
Generator
2
2x8=16
Rated current (A)
1,7
13,6
Rated voltage (V)
89
89
Open circuit voltage (V)
130,6
130,6
Short circuit current (A)
2,03
16,24
Modules
11,59 %
640x1245x32,5mm Table 5: PV Fac¸ades Generator
Table 4: CIGS module characteristics
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Figure 4: Power flows - Sign Criteria
To calculate the energy provided by PV modules, time series of meteorological data are required. For this analysis
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was used a file provided by the Organizer of the SDE12 with the meteorological data of the Barajas Airport. The gross energy per time of the PV modules is represented by PRMOD for the Roof and PFMOD for the Fac¸ade; for the usable power some performance ratios has to be consider because of the energy conversions. inverter was ηGT = 0.95.
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The PV Roof Generator use a grid-tie solar inverter, so just one conversion is need. The performance used for this
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The usable power for the PV Roof Generator (PRAC ) is calculated with equation:
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PRAC = PRMOD · ηGT
(7)
The PV Fac¸ade Generator is a DC-coupled generator. So, a minimum of two conversions are needed. Namely, DC-DC energy conditioning for battery coupling and a DC-AC conversion for usable energy form. Both powers can be calculated using equation (8) for the power conditioned by the MPPT battery charger; and the equation (9) for the power converted by the XW4548 Joint into usable AC power to be consumed in SMLsystem.
PDC F
=
PFMOD · η MPPT
(8)
PAC F
=
DC−AC PDC F · ηXW
(9)
The performances considered were: η MPPT = 0.8 for the DC-DC conditioning and ηDC−AC = 0.88 for the converXW sion from the DC bus to the AC-Loads grid. When the XW4548 works as a battery charger from the AC-Load side, the conversion performance is the same: ηAC−DC = 0.88. XW 10
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The XW4548 behavior depends on the power balance of the AC bus (PAC XW ), calculated as indicated in equation (10) and the sign criteria of the figure 4. AC AC PAC XW = PR − PL
(10)
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AC Where the PAC L is the power consumed by loads. When the value of PXW is negative means that the loads need
more energy than the PV Roof Generator can provide.
This extra needed energy can come from three different sources: (1) from the batteries, (2) from the fac¸ades or
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(3) from the grid. Even a mix of those three ones. XW4548 is the energy manager that has to choice the best energy source in each moment, which depends on the strategy programmed by the customer.
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The energy source in the competition has to prior PV energy during the Correlation measurement frame, so the XW4548 has to prior the energy from the fac¸ades Generator over the Battery energy and over the Grid energy. Nevertheless, when the value of PAC XW is positive, more energy is produced than consumed. In this case, the energy
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surplus goes to the XW4548, where it will be stored in the battery bank if it has enough capacity. The energy surplus that can not be stored in the batteries is sold to the grid, in order to raise the autonomy contest of the prototype.
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When the energy goes in or out the batteries an electrochemical conversion takes place, so exist two efficiencies for that conversions. Sometimes the conversions can need different electrochemical processes and that means different
0.8.
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conversion performances, but the same performance is considered for both conversions in this case: ηQ−DC = ηDC−Q = B B So, the charging equation is (11) for calculate the chemical power (PQB ) that is stored at the moment and, of course,
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the chemical power decrease when the batteries are discharged (12).
PQB
=
ηDC−Q · PDC−E B B
(11)
PQB
=
−ηQ−DC · PDC−S B B
(12)
3. Comparative of different configurations There are several configurations or designs for self-consumption PV generators, mainly they are differentiated by the side where they are exchanging the energy between the PV, the Battery Bank and the Loads. If the system is a DC-coupled one, all the energy is managed in the DC bus but the load still using AC; but also, if the system is an AC-coupled one, the energy flows around the AC grid. In the first case, a DC-DC battery charger is need and in the second case an AC-DC charger is need. In both cases, the battery bank is working as an energy back-up. For the DC-coupled system, the energy control is easiest, but worst the energy performance.
11
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For the analysis in this paper a third configuration is defined. The whole self-sufficiency generator include a battery bank for buffering the energy. Nevertheless, in the short term a Net-Metering could be possible in Spain with no remunerations, because of a pure AC system is used to compare with the others. Where the energy is consumed or exported to the grid.
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Summarizing the different configurations: Pure AC A pure AC coupling system is a grid-tie one. The whole energy generated is injected to the inner grid of
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the consumer where it can be consumed or transferred to the electrical grid.
Pure DC Pure DC coupling system is similar to a stand-alone PV system. The energy from the PV modules has to
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be DC-DC conditioned for store it in the batteries, and then DC-AC converted to a usable energy. Mixed AC-DC Mixed AC-DC coupling system is the proposed system for SMLsystem prototype. Both systems, Pure
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AC and Pure DC are mixed in the same system, joints with the XW4548 which create a micro-grid for feed the loads.
Some researchers like M. Castillo-Cagigal et al. [11] investigate the synchronization between the energy de-
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manded with the energy produced, using the Active Demand-Side Management (ADSM). It will be the future for a high efficiency electrical system, improving the performance behaviour with the use of high level electrical grids –smart grids–.
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Nowadays, using ADSM is very difficult, so the proposal in this paper is to evaluate the potential of different PV generator configurations without a control of the demand.
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In order to compare the different PV generator configurations, a simulation was defined and run. It does the calculations process as figure 5 shows. But the energy way depends on the system configuration. Namely, for pure AC system the positive Roof-Load Balance goes direct to the grid –Power to grid–. And for
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the Pure DC systems the Roof-Load Balance is always negative, where the Fac¸ade Balance is considered with all PV power installed in SMLsystem: Roof and Fac¸ades. The instantaneously power generated by the solar panels was calculated using the time series of meteorological data provided. To modelling the energy demand, as real as possible, the measures during the contest period were used. Figure 6 is the demand chart –consumption– of SMLsystem during the first week of the contest. This chart was extended for simulate a yearly consumption, but increasing winter and summer consumptions for simulate the heating a cooling system usage. The demand chart shows a repeatability in the consumption peaks. This is because of the ADSM used during the contest, where all the loads could be scheduled, which was a part of the contest strategy. So, for this analysis, the extended chart of figure 6 –with the yearly variation added– is considered as the 0 phase condition of the analysis –origin of the consumptions–, showed as the x axis of figure 7. 12
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Roof- Load Balance AC AC PAC XW = PR − PL
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Yes PAC XW > 0
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No
Fac¸ade Balance
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AC AC PAC D = PF + PXW
Yes
PAC D >0
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No
Charging
Yes Q E BQ ≤ E B−MAX
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State of Charge:
Discharging State of Charge:
Power to Grid
ed
No
AC PGAC = PAC XW + PF
Charging power
Yes Q E BQ ≤ E B−MIN
Power from Grid
No
PGAC = |PAC D |
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DC DC PDC D = PXW + PF
Discharging power
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Yes
DC DC PDC D = PXW + PF
DC PDC D > P B−MAX
Power to grid:
No
Yes
DC PGDC = |PDC D − P B−MAX |
DC |PDC D | > P B−MAX
Power to batteries: DC PDC B = P B−MAX
No Power from Batteries DC PDC B = P B−MAX
Power to batteries: DC PDC B = PD
Power from Grid
Power from Batteries
DC PGDC = |PDC D | − P B−MAX
DC PDC B = |PD |
Figure 5: Functioning process
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5
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3
2
cr
Power (kW)
4
0
50
100
150
200
250
300
350 400 Measures
450
500
550
600
650
700
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0
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1
Figure 6: Demand chart in 1st Competition week
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In other words –to understand what the phase means–, the value of the phase is an hour displacement from the origin of the consumptions. A value of the phase of 2 hours means a displacement of all the consumption values 2 hours yearly.
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The two charts of the figure 7 are the self-sufficiency ratio, calculated as equation (4), and the yearly energy balance. A positive balance means injected energy to the grid –achieved in the three cases–. The energy produced in the three different configurations is the same, the produced energy is the value of the
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energy outputted by the solar panels, added as an only generator or as multiple ones. So, the main difference is the performance of the conversions. Better the global performance, higher the yearly energy balance.
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As mentioned earlier, the use of ADSM during the contest makes the extended demand chart as a quasi-controlled demand-side. So, the phase 0 is the better correlated ratio achieved. Never is 100% because the energy produced depends on the meteorology and not always can be produced the energy than is needed. The battery bank is just a back up for a daily consumption.
When the consumption change the phase from the 0 one, correlation ratios decrease because the demand peak change the time position. For the systems including batteries, the correlation ratio losses are softer than for the pure AC ones because of the buffering of the batteries. That can be shown in the yearly energy balance: when the use of the batteries raise the global performance decrease, so, more energy is wasted in the conversions. The self-sufficiency achieved with the two systems with batteries is quite similar, both systems are more independent of the demand chart than the pure AC, and the performance of the mixed AC-DC is greater than the pure DC one, but the cost too. In other words, the use of a mixed AC-DC system was a good decision for the SDE contest, because 14
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Self-Sufficiency ratio 100 80
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ξ (%)
60 40
−10
−8
−4
−6
−2
0 phase (h)
2
6
8
10
12
Pure AC coupling system
Pure DC coupling system
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Mixed AC-DC coupling system
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4000
2000 −12
−10
−8
−6
−4
−2
0 phase (h)
ed
Yearly Energy Balance (kWh)
Yearly Energy Balance 6000
4
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0 −12
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20
2
4
6
8
10
12
pt
Figure 7: Year simulations of the three models
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high self-sufficiency ratios and high performance energy surplus were achieved.
4. Competitions results
A real test of the system took place at Madrid between 17th and 28th of September in the Solar Decathlon Europe 2012, international competition of solar houses, that means is not a laboratory test with controlled conditions. So, the measures were limited to four data from three meters installed as figure 8 shows. According with the nomenclature of the figure 4, the energy measures are: electrical energy to the grid (EGAC = ES ell ) and from the grid (−EGAC = EGrid ); energy consumed in the whole system (−E LAC = E L ); and the energy produced by the PV Roof Generator (−ERAC = E PV ). The entire competition was 14 days, but just only two days are shown in figures 9 and 10 as the most representative ones. The data of the Roof PV, Grid and Consumption are direct measures of the meters. The DC bus data is the result of a balance between the measures in the AC-Loads side of the XW4548 joint. 15
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Figure 8: SMLsystem electrical system diagram
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Dashed line is the instantaneously power of the PV Roof Generator. It shows at the right side the abrupt finish of the generation, that is the close of generation frame, when the power suddenly goes to zero. Unfortunately, the functioning the of the XW4548 was not be the expected, the energy surplus of the PV Roof Generator was transferred to the electrical grid, because of a configuration problem that couldn’t be solved. In figure 9 the Grid line is the energy surplus of the PV Roof Generator after the loads were feed. In the same figure, around 15 hours there was a energy peak demand that the PV Roof Generator could not supply but rapidly the system inject energy from the DC bus to the AC side. The same happens when the generation must shut down (17 hours). The control system extract the energy from the batteries until they can not supply the demand and it had to be feed with energy from the grid. The Battery Bank could not supply that demand peak by two main reasons: (1) there was a extremely high peak, so the internal resistance of the batteries made the energy performance lower than in a normal situation –softly demand–
16
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wasting more energy; (2) and the energy stored in the batteries was not enough to supply, because just the energy produced in the PV Fac¸ade Generator was stored. Power charts-4th day of competition Roof PV Grid Consumption DC bus
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4000
us
W
cr
2000
0
12 Time
6
18
24
M
−2000
an
0
Figure 9: Power charts - 4th day in competition
The solution to those problems during the contest were not easy. More power was directly connected to the
PV Roof Generator (Roof PV line).
ed
batteries, in order to increase the self-sufficiency. It can be seen in figure 10 as reduction of the power produced in the
of the competition.
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In table 6 can be seen the results of the 14 days of competition and in table 7 the simulations done before the start
Energy Measurement
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Energy Measurement
Consumptions
197,7 kWh
Roof Generator
174,0 kWh
Total production
211,4 kWh
Sold to Grid
13,7 kWh
Yearly consumption
4903 kWh
Yearly PV production
8758 kWh
Competition PV production
449 kWh
Competition consumption
178 kWh
Sold to grid yearly
271 kWh
Table 6: Total energy during contest Table 7: Simulations and analysis done before contest
There is a significant difference between the competition results and the simulations in the PV generation, because the weather was not so good during the test and the modifications in the PV generator made worst the whole energy performance, wasting more energy for the conversions.
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Power charts-6th day of competition 3000 Roof PV Grid Consumption DC bus
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1000
cr
W
2000
0
12 Time
6
18
24
an
−1000
us
0
Figure 10: Power charts - 6th day in competition
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5. Conclusions and future improvement
SMLsystem was awarded with the Second Place in the Electrical Energy Balance Contest, which was a very good
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result for the CEU Valencia Team.
During the competition period some problems appeared and were solved successfully, showing some hardware problems in the energy management of the main joint of the system, but also the SMLsystem Generator was working
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very well, showing a hardy behaviour, even in a non controlled demand. All the analysis done show that for achieve a high self-sufficiency ratio (greater than 60%), with an stochastic
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demand, a battery bank is need.
As the current situation impose, the use of the Pure AC part in the Mixed AC-DC system has to be according with a good ADSM, because to store that energy the cost are higher than for a pure DC. This Pure AC sub-system was a requirement to get score in the Autonomy sub-contest (section 1.2.1), this subsystem raise the energy injection to the electrical grid. In the currently situation, without a remuneration with feed-in tariffs, the whole energy from the pure AC has to be consumed, in order to increase the global performance, but not be injected to the grid that could be even forbidden. Nowadays, the prototype is been modified to implement an hydrogen storage system, using the electrolysis to produce hydrogen for use it in a 3 kW High Temperature PEM Fuel Cell. The use of a High Temperature Fuel Cell can work as a cogenerator providing electrical energy and some hot domestic water.
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Acknowledgements All the CEU Valencia Team want to thanks to the Government of Spain the support to the Solar Decathlon Europe 2012. Thanks to all the sponsors and partners of the Team. An special acknowledge to Schneider Electric which technical
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and economical support made the electrical generator proposed possible.
And, of course, thanks to the CEU Cardenal Herrera University and CEU San Pablo Foundation, who done all of
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this possible. Helping and supporting the whole CEU Valencia Team.
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References
[1] E. Recast, Directive 2010/31/eu of the european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast), Official Journal of the European Union 18 (06).
[2] D. Ilic, P. G. Da Silva, S. Karnouskos, M. Griesemer, An energy market for trading electricity in smart grid neighbourhoods, in: Digital
an
Ecosystems Technologies (DEST), 2012 6th IEEE International Conference on, IEEE, 2012, pp. 1–6.
[3] R. Velik, Battery storage versus neighbourhood energy exchange to maximize local photovoltaics energy consumption in grid-connected residential neighbourhoods, IJARER International Journal of Advanced Renewable Energy Research 2 (6).
[5] Solar Decathlon Europe 2012 Rules for the competition.
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[4] G. Strbac, Demand side management: Benefits and challenges, Energy Policy 36 (12) (2008) 4419–4426.
[6] A. Scognamiglio, H. N. Røstvik, Photovoltaics and zero energy buildings: a new opportunity and challenge for design, Progress in Photovoltaics: Research and Applications 21 (6) (2013) 1319–1336.
(2012) 90–103.
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[7] H. Hu, G. Augenbroe, A stochastic model based energy management system for off-grid solar houses, Building and Environment 50 (0)
[8] J. Weniger, T. Tjaden, V. Quaschning, Sizing of residential pv battery systems, Energy Procedia 46 (2014) 78–87. [9] C. Breyer, A. Gerlach, J. Mueller, H. Behacker, A. Milner, Grid-parity analysis for eu and us regions and market segments–dynamics of
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grid-parity and dependence on solar irradiance, local electricity prices and pv progress ratio, in: 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany, 2009, pp. 21–25.
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[10] K. Moutawakkil, S. Elster, Re hybrid systems: Coupling of renewable energy sources on the ac and dc side of the inverter, Refocus 7 (5) (2006) 46 – 48.
[11] M. Castillo-Cagigal, E. Caama˜no-Mart´ın, E. Matallanas, D. Masa-Bote, A. Guti´errez, F. Monasterio-Huelin, J. Jim´enez-Leube, Pv selfconsumption optimization with storage and active dsm for the residential sector, Solar Energy 85 (9) (2011) 2338–2348.
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Ac ce p
te
d
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Highlights Electrical system developed for the Solar Decathlon Europe 2012. A Mixed AC-DC coupling system for an electrical grid-tie installation. Analysis of different self-sufficiency systems for nZEH with low capacity batteries. How appropriate Active Demand-Side Management saves electrical energy.
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S0378-7788(14)00362-4 http://dx.doi.org/doi:10.1016/j.enbuild.2014.03.083 ENB 5016
To appear in:
ENB
Received date: Revised date: Accepted date:
11-11-2013 24-3-2014 26-3-2014
Please cite this article as: J. Renau, L. Domenech, V. Garc´ia, A. Real, N. Mont´es, F. S´anchez, A proposal of nearly Zero Energy Building (nZEB) electrical power generator for optimal temporary generation-consumption correlation, Energy & Buildings (2014), http://dx.doi.org/10.1016/j.enbuild.2014.03.083 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Manuscript_local_compilation_TEX2013
A proposal of nearly Zero Energy Building (nZEB) electrical power generator for optimal temporary generation-consumption correlation. J.Renau, L.Domenech, V.Garc´ıa, A.Real, N.Mont´es, F.S´anchez
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Department of Building Engineering and Industrial Production, University CEU Cardenal Herrera (CEU-UCH), Valencia, Spain
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Abstract
This work presents an Electrical PV Power Generator system that was designed, build and tested during Solar De-
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cathlon Europe 2012 competition at the SMLsystem, the CEU Cardenal Herrera University prototype. One of the main purposes of the contest is to evaluate the prototypes capacity for electrical energy balance in terms of self-sufficiency,
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low energy consumptions and the temporary generation-consumption correlation demanded. Design and evaluation are driven through the competition rules, that build a framework for a kind of nearly-Zero Energy House (nZEH) with the particular criteria of obtaining all the necessary energy from the sun.
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Different considerations that were carried out to design the PV system are shown and discussed: energy demand and disposability, possibility to grid connection, power installed, appropriate battery capacity, Electrical-PV system configuration (AC-coupling or/ and DC-coupling), and the temporary generation-consumption correlation demanded.
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In order to achieve an optimal energy balance, the Electrical PV Power Generator system design strategy presented is focussed on the energy correlation analysis behavior and the efficient management of the energy system keeping the main load of a nZEH low enough and time controlled.
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Keywords: Solar Decathlon Europe 2012, nZEB, NZEB, ZEB, Photovoltaic Systems, Self-sufficient PV systems,
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Energy correlation, SMLsystem
1. Introduction
The recast of the European Directive 2010/31/EU of 19 of May 2010 [1] define the nearly Zero Energy Building as a building that has a very high energy performance, which nearly zero or very low amount of energy should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby.
The use of on-site even nearby energy is know as energy self-consumption. The term on-site energy is easy to understand because the energy is produced in the same building where is produced. Nevertheless, nearby energy is an extended term for the same concept which implies a higher control in energy flows. So, an other electrical power grid concept called Smart Grid [2] has to be used for the energy management. Email address: [email protected] (J.Renau) Preprint submitted to Energy and Building Journal
March 14, 2014
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In medium term, the use of an small scale Smart Grid as a neighbourhood energy exchange system maximize the local photovoltaic energy consumption [3] and minimize the energy losses for the energy storage conversions. This situation can be powered by the use of a Demand-Side Management (DSM), defined by the European Commission as a tool to influence the global energy market and hence the security of energy supply in the medium and long term.
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DSM brings many benefits such as, reduction of the generation margin capacity and improving transmission grid investment and operation efficiency, but more challenges will have to be faced: advanced metering and control technologies –lack of ICT infrastructure–, the use of real-time information, more complexity of the system operation
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and lack of understanding of benefits and the difficult to evaluate them; as Professor Goran Strbac shows [4].
The authors present in this paper an Electrical PV Power Generator System designed and implemented through the
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participation of the Department of Building Engineering and Industrial Production of the University CEU Cardenal Herrera in the Solar Decathlon Europe 2012 (SDE12) with the project SMLsystem. This generator was designed in accordance with the needs of the contest rules, but also with the European laws in order to be applied in future real
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buildings. 1.1. Competition and rules framework
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The Solar Decathlon Europe is an international competition in which universities from all over the world meet to design, build and operate an energetically self-sufficient house, grid-connected, using solar energy as the only energy source and equipped with all of the technologies that permit maximum energy efficiency.
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During the final phase of the competition, teams assembled their houses in Madrid, and participate during September 2012 in ten different challenges regarding renewable energy use and savings. The SDE organization specified a framework rules for the competition comprising the prototype evaluation criteria and procedures for juries. The houses
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presented by the teams were constantly monitored and the information related to the houses behaviour was assessed and open.
namely:
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The winner of SDE12 is known after the evaluation of ten contests or challenges with different scoring each one,
1. Architecture
2. Engineering and Construction 3. Energy Efficiency
4. Electrical Energy Balance 5. Comfort Conditions 6. Functioning of the house
7. Communication and Raising Social Awareness 8. Industrialization and Market viability 9. Innovation 10. Sustainability 2
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1.2. Electrical Energy Balance Contest Contest 4 is which has a special significance for this paper. Electrical Energy Balance is one of the main objectives of the Solar Decathlon Europe which “promote research in the development of efficient houses. [...]. Particular emphasis is put on reducing energy consumption and on obtaining all the necessary energy from the sun” [5]
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So the objective is to evaluate the houses’ electrical energy self-sufficiency provided by solar active technology and their electricity use intensity.
evaluate three different concepts that completes the idea of a self-sufficient house:
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1. Autonomy
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For that propose the energy production and energy consumed in each house was measured. The main idea was
2. Temporary Generation-Consumption Correlation
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3. Consumption per measurable area 1.2.1. Autonomy
EG−yearly − E L−yearly ≥ 0
(1)
EG − E L ≥ 10kWh
(2)
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This sub-contest evaluate the self-supply of the house during the competition weeks.
For a positive annual electrical energy balance, the house must follow the equation 1. Where:
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EG−yearly Represent the energy generated throughout a whole year.
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E L−yearly Represent the consumption of the loads throughout a whole year. But during the competition it is impossible to determinate the yearly production and consumption, so as the equation 2 shows a value of 10 kWh, which it was considered as sufficient during September (as the equivalence of a year electrical energy balance).
However, if the house doesn’t exceed the 10 kWh could obtain reduced point between −10 kWh ≤ EG − E L < 10 kWh.
The measure period was determined between the free shadow period (generation frame). Between 10 to 17 h. Out of these generation frame, the PV system must be disconnected. 1.2.2. Temporary generation-consumption correlation One of the main advantages of Electrical Distributed Generation is that electricity is consumed in the same place where it is generated. This reduces the need for transmission lines and minimizes the electricity transport losses. This 3
Page 3 of 20
effect is maximized if electricity is consumed at the same time as it is generated (this concept is discussed deeper in section 2). To evaluate the score of this sub-contest, equation 5 gives the score in function of ξ, which is the correlation between generated and simultaneously consumed energy by the loads.
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It depends on we have batteries (equation 4) or not (equation 3). The value ξ is continuously integrated during the measuring period of the competition, which is a longest period than the generation one. The value of ξ is between
ξ
=
EG−L R EL
(EG−L + E Bat−L ) R EL ξ · Pointstotal−contest
=
Points
=
an
R
ξ
us
R
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[0, 1].
(3) (4) (5)
As for the autonomy sub-contest, there was a measuring frame for the correlation sub-contest. The measures was
1.2.3. Consumption per measurable area
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taken between 8 am to 23 pm.
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In order to reduce the CO2 emissions and external energy dependence of the countries, it is just as important to
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have a large renewable energy production as efficient energy consumption. E L−average A
(6)
Where the E L−average is the average energy consumed by the loads daily during the contest weeks. Lower the value,
Ac ce
greater the scoring.
1.3. SMLsystem: the SDE 2012 prototype SMLsystem project (figure 1) is the research continuous upgrading initiated with SMLhouse (Small, Medium and Large) proposed in the Solar Decathlon Europe 2010. SMLsystem returns to prefabrication as a proposal defining an architectural concept where structural, composition and functional values are introduced in a sustainable building approach. The Unit or basic Module –considered as a box–, is formed totally by prefabricated materials and dryassembled, being wood the predominant material in SMLsystem prototype. Each of these Units is transported fully equipped. The courtyard acts like a composition element. In all constructive Units there is a courtyard –generated with a louvre–, which divides the space, providing an special access to the house. Also, offering natural lighting and ventilation control in a great space value. The effectiveness of the courtyard is increased with the use of both louvres 4
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Figure 1: SMLsystem in Madrid during competition
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–horizontal and vertical–, reducing the excess radiation at certain times of the day, as well as sift and offer more privacy to the housing.
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Reducing energy demand is the main objective of SMLsystem as a nearly-Zero Energy House [1], but also the efficient management of the renewable energy produced on-site. Passive systems incorporated into its design allow minimizing the need for and dependence on active systems for cooling and heating of the house. In other hand,
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efficiency of active systems optimizes the use of energy and reduces demand. For the HVAC system, a heat pump combined with Phase Change Materials for a Thermal Energy Storage (TES) system has been implemented. For the optimization of consumption compared to generation, was used a Domotic Prediction System (DPS). The Learning
pt
Ability (LA) of the system will allow it to predict the behaviour of the user based on past behaviours to displace consumption and optimizing the generation. This automation system will monitor air quality, humidity, etc., activating
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the corresponding systems, namely, ventilation for air exchange, and air conditioning. The project will also have a LED lighting interaction with intelligent home automation system which will regulate the levels of lighting intensity depending on the needs at all times. The use of high efficiency appliances as well as proper use of them also reduces the energy demand of SMLsystem.
The Sun is a clean energy source but the energy depends on the weather conditions, for ensure the energy disposability. In SMLsystem all the possible photovoltaic areas were exploited and the architecture design is a key issue in the integration of the PV technologies. This surfaces can defined as building’s energy footprint, so in contrast with traditional energy sources they are visible, the PV systems are a ”form” –i. e. shapes, colors and features–, and architects are responsible to manage this ”form” [6]. The different orientations of the construction elements –roof and fac¸ades– require different photovoltaic systems with appropriate energy management strategies –discussed in section 2–. The solar panels are placed on the top of each Module with an independent structure per Module, which properly 5
Page 5 of 20
joins with the roof structure of them. That point is important because all the facilities which are complex are set up, so the work is just execute joints between the module and facilities pack, that minimizes personal risks implicit while working on the roof, and construction problems like damages on the waterproof layer.
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2. The SMLsystem generator A self-sufficient house needs an electrical energy generator for works as nearly-Zero Energy as much as possible. Balancing the consumption and generation as well as possible and minimizing the energy consumption from the grid.
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Some authors like Huafen Hu et al. [7] assert that the next level of the Net Zero Energy Houses (NZEH) is to take them off from the grid. Of course is right when you consider an outage energy as a quality indicator.
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In the design proposed in this paper, it is assumed the necessity of a grid to ensure the energy, because of it is not consider an energy shortage as a possible situation, in that case the energy will be buy from the grid.
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To achieve high self consumption rates from the PV system, a storage system has to be used for buffering the daily energy to use it during the night [8]. Nevertheless, the coexistence of PV and batteries with an electrical grid is a new situation.
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In the past, the PV generators were mainly assembled to sell the energy to the electrical grid, because it energy was remunerated with feed-in tariffs.
Currently, this is changing in all the European Countries a grid parity has reached, that means that solar energy can
and more interesting.
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compete with conventional energy from the grid in several markets [9]. So, the self-consumption is more economical
The authors present a PV generator system that can work with the currently situation: there aren’t feed-in tariffs,
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later consumptions.
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neither an ADSM system for maximize the consumption from PV generation, so all the energy has to be stored for
2.1. Description
The requirement of an electrical grid imposes the necessity of a multi-coupling system. In figure 2 five parts can be highlighted: (1) the roof generator, (2) two generators in each facade, (3) the storage system, (4) the energy manager and (5) the existence of an electrical grid –smart grid of villa solar 2012–. The system proposed uses a combined AC and DC coupling system for achieve maximum efficiency and a year reliability. Of course, with a high self consumption rate. This kind of systems were thought for isolated consumer like the system presented by Karim Moutawakkil and Steffen Elster [10]. This proposal include an energy rejecter to ensure the stability of the micro-grid. For SMLsystem the stability of the micro-grid can be achieved thanks to the injection of the energy surplus to the electrical grid –smart grid–. The energy sources of the SMLsystem Generator are the PV Roof Generator and the PV fa¸cades Generators (both east and west fac¸ades). 6
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Figure 2: SMLsystem electrical system sketch
Both generators are joint between the hybrid inverter/charger XW4548, working as a grid transfer relay or as
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an energy seller to the grid, never as a battery charger, so the charger disabled to charging from the grid (figure 3). The XW4548 just can charge the batteries from the energy surplus in the AC loads side, provided by the PV Roof
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Generator.
Figure 3: XW4548 functioning
The PV Roof Generator is a pure AC-coupled system connected in the AC-Loads side. The AC-Loads output is configured as a micro-grid feeding the whole consumptions of SMLsystem. So, the energy of this micro-grid can be provided by the PV Roof Generator or by the XW4548 output, that keeps the stability using a few energy from the DC bus. Of course, the XW4548 output maximize the energy consumption from the renewable sources, namely, the PV fa¸cades Generator and the Battery Bank, that is considered as solar energy too. 7
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Battery Bank
Model
Enersys SBS190F
Monoblock
12 Vdc
Bank
4 units
Energy stored
5.5 kWh
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Table 1: Battery bank characteristics
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Manufacturer
The Battery Bank was designed to cover the worst energy daily demand, during the contest period. Nevertheless,
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Johannes Weniger et al. [8] shows how to sizing a PV battery system for residential applications, taking relevant the economical aspects. For the design exposed in this paper just an energy and a secure functioning were considered.
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The main parameters of the battery bank can be seen in table 1.
The PV Roof Generator, as mentioned above, was designed to be an AC-coupled system. That means a high performance energy generator because the energy from the PV just needs one conversion to feed the loads.
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So, to configure the generator, the better performance panels at the moment were used. Namely, the Sunpower 225 panel (table 2), which is a monocrystalline technology one. Considering the technical conditions of that technology, the PV Roof Generator was designed for improve the catchment of direct radiation from the sun.
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The Photovoltaic Roof Generator was assembled using the Sunpower 225 module (table 2), a monocrystalline silicon technology. This technology is better than other for maximum production with direct radiation. The number of modules depends on the space of the roof. The best configuration for September in Madrid is an
pt
azimuth of 2o to the east and a slope of 34o yearly. Nevertheless, for September, the maximum production power is at 37o . The difference between 37o and 30o is less than 2% in energy production.
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In an energy analysis this difference can be depict, but the difference of 7o in architecture is quite significant. So the final design was 21 modules sloping 30o and 0o azimuth of the house, but for the real location was ≈ 10o azimuth. The table 3 shows the essential information of the Roof Generator. The design of the PV Fa¸cades Generator depends on the fac¸ade available surface, because of is a Building Integrated PV system (BIPV). Just the east and the west fac¸ades were available in the prototype. So, the main solar energy was the diffuse one. To benefit that the PV CIGS technology has the better performance (table 4). The energy from the less power PV Fac¸ades Generator was used directly to charge the battery bank using two MPPT charger, one per fac¸ade. 2.2. System functioning Figure 4 clarify the system functioning and the sign criteria for next section (3), in power or energy flows. The diagram shows that sometimes the energy can be bi-directional in the same branch, but never at the same time. 8
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225 Wp
Rated current
5.59 A
Rated voltage
41,0 V
Open circuit voltage
48,5 V
Short circuit current
5,87 A
String
Modules
3x7=21
5,47
16,47
287
287
Open circuit voltage (V)
339,5
339,5
Short circuit current (A)
5,87
17,61
Rated current (A)
18%
Rated voltage (V)
Temperature coefficients −0, 38%W/C
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−132, 5mV/C
Table 3: Roof PV Generator characteristics
3, 5mA/C 798x1559x46mm
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Table 2: Sunpower 225 characteristics
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Dimensions
Generator
7
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Efficiency
Photovoltaic Roof Generator
cr
Peak power
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Sunpower 225
CIGS Module
PV Fac¸ades Generator
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Peak power
75,7 Wp
Rated current
1,7 A
Rated voltage
44,5 V
Open circuit voltage
65,3 V
Short circuit current
2,03 A
Efficiency
Dimensions
String
Generator
2
2x8=16
Rated current (A)
1,7
13,6
Rated voltage (V)
89
89
Open circuit voltage (V)
130,6
130,6
Short circuit current (A)
2,03
16,24
Modules
11,59 %
640x1245x32,5mm Table 5: PV Fac¸ades Generator
Table 4: CIGS module characteristics
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Figure 4: Power flows - Sign Criteria
To calculate the energy provided by PV modules, time series of meteorological data are required. For this analysis
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was used a file provided by the Organizer of the SDE12 with the meteorological data of the Barajas Airport. The gross energy per time of the PV modules is represented by PRMOD for the Roof and PFMOD for the Fac¸ade; for the usable power some performance ratios has to be consider because of the energy conversions. inverter was ηGT = 0.95.
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The PV Roof Generator use a grid-tie solar inverter, so just one conversion is need. The performance used for this
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The usable power for the PV Roof Generator (PRAC ) is calculated with equation:
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PRAC = PRMOD · ηGT
(7)
The PV Fac¸ade Generator is a DC-coupled generator. So, a minimum of two conversions are needed. Namely, DC-DC energy conditioning for battery coupling and a DC-AC conversion for usable energy form. Both powers can be calculated using equation (8) for the power conditioned by the MPPT battery charger; and the equation (9) for the power converted by the XW4548 Joint into usable AC power to be consumed in SMLsystem.
PDC F
=
PFMOD · η MPPT
(8)
PAC F
=
DC−AC PDC F · ηXW
(9)
The performances considered were: η MPPT = 0.8 for the DC-DC conditioning and ηDC−AC = 0.88 for the converXW sion from the DC bus to the AC-Loads grid. When the XW4548 works as a battery charger from the AC-Load side, the conversion performance is the same: ηAC−DC = 0.88. XW 10
Page 10 of 20
The XW4548 behavior depends on the power balance of the AC bus (PAC XW ), calculated as indicated in equation (10) and the sign criteria of the figure 4. AC AC PAC XW = PR − PL
(10)
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AC Where the PAC L is the power consumed by loads. When the value of PXW is negative means that the loads need
more energy than the PV Roof Generator can provide.
This extra needed energy can come from three different sources: (1) from the batteries, (2) from the fac¸ades or
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(3) from the grid. Even a mix of those three ones. XW4548 is the energy manager that has to choice the best energy source in each moment, which depends on the strategy programmed by the customer.
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The energy source in the competition has to prior PV energy during the Correlation measurement frame, so the XW4548 has to prior the energy from the fac¸ades Generator over the Battery energy and over the Grid energy. Nevertheless, when the value of PAC XW is positive, more energy is produced than consumed. In this case, the energy
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surplus goes to the XW4548, where it will be stored in the battery bank if it has enough capacity. The energy surplus that can not be stored in the batteries is sold to the grid, in order to raise the autonomy contest of the prototype.
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When the energy goes in or out the batteries an electrochemical conversion takes place, so exist two efficiencies for that conversions. Sometimes the conversions can need different electrochemical processes and that means different
0.8.
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conversion performances, but the same performance is considered for both conversions in this case: ηQ−DC = ηDC−Q = B B So, the charging equation is (11) for calculate the chemical power (PQB ) that is stored at the moment and, of course,
Ac ce
pt
the chemical power decrease when the batteries are discharged (12).
PQB
=
ηDC−Q · PDC−E B B
(11)
PQB
=
−ηQ−DC · PDC−S B B
(12)
3. Comparative of different configurations There are several configurations or designs for self-consumption PV generators, mainly they are differentiated by the side where they are exchanging the energy between the PV, the Battery Bank and the Loads. If the system is a DC-coupled one, all the energy is managed in the DC bus but the load still using AC; but also, if the system is an AC-coupled one, the energy flows around the AC grid. In the first case, a DC-DC battery charger is need and in the second case an AC-DC charger is need. In both cases, the battery bank is working as an energy back-up. For the DC-coupled system, the energy control is easiest, but worst the energy performance.
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For the analysis in this paper a third configuration is defined. The whole self-sufficiency generator include a battery bank for buffering the energy. Nevertheless, in the short term a Net-Metering could be possible in Spain with no remunerations, because of a pure AC system is used to compare with the others. Where the energy is consumed or exported to the grid.
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Summarizing the different configurations: Pure AC A pure AC coupling system is a grid-tie one. The whole energy generated is injected to the inner grid of
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the consumer where it can be consumed or transferred to the electrical grid.
Pure DC Pure DC coupling system is similar to a stand-alone PV system. The energy from the PV modules has to
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be DC-DC conditioned for store it in the batteries, and then DC-AC converted to a usable energy. Mixed AC-DC Mixed AC-DC coupling system is the proposed system for SMLsystem prototype. Both systems, Pure
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AC and Pure DC are mixed in the same system, joints with the XW4548 which create a micro-grid for feed the loads.
Some researchers like M. Castillo-Cagigal et al. [11] investigate the synchronization between the energy de-
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manded with the energy produced, using the Active Demand-Side Management (ADSM). It will be the future for a high efficiency electrical system, improving the performance behaviour with the use of high level electrical grids –smart grids–.
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Nowadays, using ADSM is very difficult, so the proposal in this paper is to evaluate the potential of different PV generator configurations without a control of the demand.
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In order to compare the different PV generator configurations, a simulation was defined and run. It does the calculations process as figure 5 shows. But the energy way depends on the system configuration. Namely, for pure AC system the positive Roof-Load Balance goes direct to the grid –Power to grid–. And for
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the Pure DC systems the Roof-Load Balance is always negative, where the Fac¸ade Balance is considered with all PV power installed in SMLsystem: Roof and Fac¸ades. The instantaneously power generated by the solar panels was calculated using the time series of meteorological data provided. To modelling the energy demand, as real as possible, the measures during the contest period were used. Figure 6 is the demand chart –consumption– of SMLsystem during the first week of the contest. This chart was extended for simulate a yearly consumption, but increasing winter and summer consumptions for simulate the heating a cooling system usage. The demand chart shows a repeatability in the consumption peaks. This is because of the ADSM used during the contest, where all the loads could be scheduled, which was a part of the contest strategy. So, for this analysis, the extended chart of figure 6 –with the yearly variation added– is considered as the 0 phase condition of the analysis –origin of the consumptions–, showed as the x axis of figure 7. 12
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Roof- Load Balance AC AC PAC XW = PR − PL
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Yes PAC XW > 0
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No
Fac¸ade Balance
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AC AC PAC D = PF + PXW
Yes
PAC D >0
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No
Charging
Yes Q E BQ ≤ E B−MAX
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State of Charge:
Discharging State of Charge:
Power to Grid
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No
AC PGAC = PAC XW + PF
Charging power
Yes Q E BQ ≤ E B−MIN
Power from Grid
No
PGAC = |PAC D |
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DC DC PDC D = PXW + PF
Discharging power
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Yes
DC DC PDC D = PXW + PF
DC PDC D > P B−MAX
Power to grid:
No
Yes
DC PGDC = |PDC D − P B−MAX |
DC |PDC D | > P B−MAX
Power to batteries: DC PDC B = P B−MAX
No Power from Batteries DC PDC B = P B−MAX
Power to batteries: DC PDC B = PD
Power from Grid
Power from Batteries
DC PGDC = |PDC D | − P B−MAX
DC PDC B = |PD |
Figure 5: Functioning process
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5
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3
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Power (kW)
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350 400 Measures
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Figure 6: Demand chart in 1st Competition week
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In other words –to understand what the phase means–, the value of the phase is an hour displacement from the origin of the consumptions. A value of the phase of 2 hours means a displacement of all the consumption values 2 hours yearly.
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The two charts of the figure 7 are the self-sufficiency ratio, calculated as equation (4), and the yearly energy balance. A positive balance means injected energy to the grid –achieved in the three cases–. The energy produced in the three different configurations is the same, the produced energy is the value of the
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energy outputted by the solar panels, added as an only generator or as multiple ones. So, the main difference is the performance of the conversions. Better the global performance, higher the yearly energy balance.
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As mentioned earlier, the use of ADSM during the contest makes the extended demand chart as a quasi-controlled demand-side. So, the phase 0 is the better correlated ratio achieved. Never is 100% because the energy produced depends on the meteorology and not always can be produced the energy than is needed. The battery bank is just a back up for a daily consumption.
When the consumption change the phase from the 0 one, correlation ratios decrease because the demand peak change the time position. For the systems including batteries, the correlation ratio losses are softer than for the pure AC ones because of the buffering of the batteries. That can be shown in the yearly energy balance: when the use of the batteries raise the global performance decrease, so, more energy is wasted in the conversions. The self-sufficiency achieved with the two systems with batteries is quite similar, both systems are more independent of the demand chart than the pure AC, and the performance of the mixed AC-DC is greater than the pure DC one, but the cost too. In other words, the use of a mixed AC-DC system was a good decision for the SDE contest, because 14
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Self-Sufficiency ratio 100 80
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ξ (%)
60 40
−10
−8
−4
−6
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0 phase (h)
2
6
8
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Pure AC coupling system
Pure DC coupling system
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Mixed AC-DC coupling system
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4000
2000 −12
−10
−8
−6
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0 phase (h)
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Yearly Energy Balance (kWh)
Yearly Energy Balance 6000
4
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0 −12
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Figure 7: Year simulations of the three models
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high self-sufficiency ratios and high performance energy surplus were achieved.
4. Competitions results
A real test of the system took place at Madrid between 17th and 28th of September in the Solar Decathlon Europe 2012, international competition of solar houses, that means is not a laboratory test with controlled conditions. So, the measures were limited to four data from three meters installed as figure 8 shows. According with the nomenclature of the figure 4, the energy measures are: electrical energy to the grid (EGAC = ES ell ) and from the grid (−EGAC = EGrid ); energy consumed in the whole system (−E LAC = E L ); and the energy produced by the PV Roof Generator (−ERAC = E PV ). The entire competition was 14 days, but just only two days are shown in figures 9 and 10 as the most representative ones. The data of the Roof PV, Grid and Consumption are direct measures of the meters. The DC bus data is the result of a balance between the measures in the AC-Loads side of the XW4548 joint. 15
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Figure 8: SMLsystem electrical system diagram
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Dashed line is the instantaneously power of the PV Roof Generator. It shows at the right side the abrupt finish of the generation, that is the close of generation frame, when the power suddenly goes to zero. Unfortunately, the functioning the of the XW4548 was not be the expected, the energy surplus of the PV Roof Generator was transferred to the electrical grid, because of a configuration problem that couldn’t be solved. In figure 9 the Grid line is the energy surplus of the PV Roof Generator after the loads were feed. In the same figure, around 15 hours there was a energy peak demand that the PV Roof Generator could not supply but rapidly the system inject energy from the DC bus to the AC side. The same happens when the generation must shut down (17 hours). The control system extract the energy from the batteries until they can not supply the demand and it had to be feed with energy from the grid. The Battery Bank could not supply that demand peak by two main reasons: (1) there was a extremely high peak, so the internal resistance of the batteries made the energy performance lower than in a normal situation –softly demand–
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wasting more energy; (2) and the energy stored in the batteries was not enough to supply, because just the energy produced in the PV Fac¸ade Generator was stored. Power charts-4th day of competition Roof PV Grid Consumption DC bus
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Figure 9: Power charts - 4th day in competition
The solution to those problems during the contest were not easy. More power was directly connected to the
PV Roof Generator (Roof PV line).
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batteries, in order to increase the self-sufficiency. It can be seen in figure 10 as reduction of the power produced in the
of the competition.
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In table 6 can be seen the results of the 14 days of competition and in table 7 the simulations done before the start
Energy Measurement
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Energy Measurement
Consumptions
197,7 kWh
Roof Generator
174,0 kWh
Total production
211,4 kWh
Sold to Grid
13,7 kWh
Yearly consumption
4903 kWh
Yearly PV production
8758 kWh
Competition PV production
449 kWh
Competition consumption
178 kWh
Sold to grid yearly
271 kWh
Table 6: Total energy during contest Table 7: Simulations and analysis done before contest
There is a significant difference between the competition results and the simulations in the PV generation, because the weather was not so good during the test and the modifications in the PV generator made worst the whole energy performance, wasting more energy for the conversions.
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Power charts-6th day of competition 3000 Roof PV Grid Consumption DC bus
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Figure 10: Power charts - 6th day in competition
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5. Conclusions and future improvement
SMLsystem was awarded with the Second Place in the Electrical Energy Balance Contest, which was a very good
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result for the CEU Valencia Team.
During the competition period some problems appeared and were solved successfully, showing some hardware problems in the energy management of the main joint of the system, but also the SMLsystem Generator was working
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very well, showing a hardy behaviour, even in a non controlled demand. All the analysis done show that for achieve a high self-sufficiency ratio (greater than 60%), with an stochastic
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demand, a battery bank is need.
As the current situation impose, the use of the Pure AC part in the Mixed AC-DC system has to be according with a good ADSM, because to store that energy the cost are higher than for a pure DC. This Pure AC sub-system was a requirement to get score in the Autonomy sub-contest (section 1.2.1), this subsystem raise the energy injection to the electrical grid. In the currently situation, without a remuneration with feed-in tariffs, the whole energy from the pure AC has to be consumed, in order to increase the global performance, but not be injected to the grid that could be even forbidden. Nowadays, the prototype is been modified to implement an hydrogen storage system, using the electrolysis to produce hydrogen for use it in a 3 kW High Temperature PEM Fuel Cell. The use of a High Temperature Fuel Cell can work as a cogenerator providing electrical energy and some hot domestic water.
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Acknowledgements All the CEU Valencia Team want to thanks to the Government of Spain the support to the Solar Decathlon Europe 2012. Thanks to all the sponsors and partners of the Team. An special acknowledge to Schneider Electric which technical
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and economical support made the electrical generator proposed possible.
And, of course, thanks to the CEU Cardenal Herrera University and CEU San Pablo Foundation, who done all of
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this possible. Helping and supporting the whole CEU Valencia Team.
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References
[1] E. Recast, Directive 2010/31/eu of the european parliament and of the council of 19 may 2010 on the energy performance of buildings (recast), Official Journal of the European Union 18 (06).
[2] D. Ilic, P. G. Da Silva, S. Karnouskos, M. Griesemer, An energy market for trading electricity in smart grid neighbourhoods, in: Digital
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Ecosystems Technologies (DEST), 2012 6th IEEE International Conference on, IEEE, 2012, pp. 1–6.
[3] R. Velik, Battery storage versus neighbourhood energy exchange to maximize local photovoltaics energy consumption in grid-connected residential neighbourhoods, IJARER International Journal of Advanced Renewable Energy Research 2 (6).
[5] Solar Decathlon Europe 2012 Rules for the competition.
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[4] G. Strbac, Demand side management: Benefits and challenges, Energy Policy 36 (12) (2008) 4419–4426.
[6] A. Scognamiglio, H. N. Røstvik, Photovoltaics and zero energy buildings: a new opportunity and challenge for design, Progress in Photovoltaics: Research and Applications 21 (6) (2013) 1319–1336.
(2012) 90–103.
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[7] H. Hu, G. Augenbroe, A stochastic model based energy management system for off-grid solar houses, Building and Environment 50 (0)
[8] J. Weniger, T. Tjaden, V. Quaschning, Sizing of residential pv battery systems, Energy Procedia 46 (2014) 78–87. [9] C. Breyer, A. Gerlach, J. Mueller, H. Behacker, A. Milner, Grid-parity analysis for eu and us regions and market segments–dynamics of
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grid-parity and dependence on solar irradiance, local electricity prices and pv progress ratio, in: 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany, 2009, pp. 21–25.
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[10] K. Moutawakkil, S. Elster, Re hybrid systems: Coupling of renewable energy sources on the ac and dc side of the inverter, Refocus 7 (5) (2006) 46 – 48.
[11] M. Castillo-Cagigal, E. Caama˜no-Mart´ın, E. Matallanas, D. Masa-Bote, A. Guti´errez, F. Monasterio-Huelin, J. Jim´enez-Leube, Pv selfconsumption optimization with storage and active dsm for the residential sector, Solar Energy 85 (9) (2011) 2338–2348.
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Highlights Electrical system developed for the Solar Decathlon Europe 2012. A Mixed AC-DC coupling system for an electrical grid-tie installation. Analysis of different self-sufficiency systems for nZEH with low capacity batteries. How appropriate Active Demand-Side Management saves electrical energy.
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