HIGHLY EFFICIENT AND FLEXIBLE POWER PLANTS IN BUILDINGS

Transcription

1 HIGHLY EFFICIENT AND FLEXIBLE POWER PLANTS IN BUILDINGS Sebastia Stier, Rita Streblow ad Dirk Müller E.ON Eergy Research Ceter, Istitute for Eergy Efficiet Buildigs ad Idoor Climate RWTH Aache, Aache, Germay ABSTRACT This paper ivestigates a ew system for balacig the electrical grid by usig dyamic simulatio. It cosists of a fuel cell that rus cotiuously to achieve high efficiecy. To have the possibility to react to fluctuatios i the electrical grid, the system additioally cotais a certai umber of heat pumps that use the electrical eergy provided by the fuel cell, if it does ot eed to be fed ito the grid. Thus, the system geerates ad cosumes power while keepig the grid i balace. The system ca achieve eergy savigs compared to a combiatio of gas boilers ad a gas turbie of up to 33 %. INTRODUCTION Heat applicatios i buildigs are resposible for a big amout of eergy cosumptio i Germay. 35 % of the fial eergy cosumptio i Germay is spet o space heatig ad domestic hot water supply i buildigs (Ziesig, 2011). If a reductio of the Germa fial eergy cosumptio is aspired, the heat supply of buildigs has to become more efficiet. Besides the heat supply of buildigs, also the electricity geeratio i Germay is chagig towards a more reewable supply of electrical power. Upcomig reewable eergy sources like photovoltaics ad wid fluctuate i their geeratio due to a depedecy o the weather. These fluctuatios lead to a eed for bigger capacities of fast reactig power geerators. They ca deliver eergy if the reewables are ot able to satisfy the demad i the electrical grid. At the momet, basically big cetral power plats work as those fast reactig power plats. This leads to several problems. I those plats, the produced waste heat of the power geeratio process is ofte ot used. Thus, the overall efficiecies of those plats are low compared to a solutio with heat usage. The productio i cetral power plats ca also lead to bottleecks i the electrical grid, as too much power has to be trasported over the trasmissio lies. From the power plats to the cosumers, losses of electricity (e.g. trasformers, cables) have to be cosidered additioally. Due to these aspects, a decetralized possibility for electrical power supply should be aalysed. This paper will aalyse a high-temperature fuel cell (type SOFC) which rus with temperatures of up to 750 C (Huijsmas et al., 1998). Thus, it is ot recommeded to switch it o ad off too ofte as heatig up ad coolig dow take a log time. The cosidered fuel cell has a very high electrical efficiecy of about 60 % (Oberitz, 2012) ad it is also possible to use a part of the waste heat for space heatig ad domestic hot water supply. At the momet, these micro-chp uits based o a fuel cell feed their total productio ito the grid. I times whe there is a high feed-i of reewables i the electrical grid, this could cause problems because of a disbalace betwee geeratio ad cosumptio. To utilize the fuel cell with its high efficiecy for future systems with high rates of reewables i the grid, it is possible to combie the fuel cell with several heat pumps to oe power geeratig uit. The heat pumps are located i viciity to the fuel cell ad ca use some of the geerated electricity to geerate heat. If eough electrical power is geerated by reewables, the fuel cell should ot feed additioal electrical eergy ito the grid. I this case, the heat pumps are tured o. The heat pumps are used to modulate the power output of the total system based o o/off-sigals which come from the electrical grid. The geerated heat ca be stored as well as the thermal eergy which is produced by the fuel cell. Therefore, this is a flexible ad eergy-efficiet power plat which will be aalysed i detail i the followig sectios. This power plat cosists of two types of buildigs. I oe type the fuel cell operates ad i the other type a heat pump is geeratig heat i times whe the elctricity is ot eeded i the grid. With this approach, the problem of storig electrical eergy is partly coverted to a problem of storig thermal eergy. Thermal storages are much cheaper tha electrical storages ad this ca lead to a greater usage of thermal storages i terms of balacig the electrical grid compared to what we have ow. First, the used model of the buildig, the systems egieerig ad the cotrol strategy is preseted. Systems egieerig here describes all heat geeratig uits (fuel cell, heat pump, boiler) ad the storage. A exteral sigal which ca be see as a idicator for the geeratio from reewables is used to cotrol the heat pumps. For the geeratio of the sigal, a methodic approach is preseted. The geerated sigal is used by the cotrol strategy of the heat pumps. The we aalyse to what extet the show system ca supply buildigs with eough heat for a whole year. Due to the fact that it is recommeded to save as much eergy

2 as possible, a aalysis of the used fuel i compariso to a referece system cosistig of decetralized gas boilers ad cetralized electricity geerators is doe. This paper is a first study to evaluate the potetial of such decetralized systems. MODELLING The used model cosists of differet parts. All these parts are modelled i the object-orieted descriptio laguage Modelica (Modelica Associatio, 2010) with the tool Dymola (Dassault Systems, 2013). May parts of the models (especially the oes cocerig the buildig) are take from the Modelica libraries developed at our istitute (Hoh et al., 2005). As stated before, there are two differet models that are used to ivestigate the system. The first model is represetig the electricity-producig part ad the secod model is represetig the electricity-cosumig part of the system. The electricity-producig part icludes the fuel cell, a thermal storage, space heatig ad domestic hot water geeratio. A scheme of this model is show i Figure 1. The fuel cell should operate cotiuously. This meas that it is coected to a thermal storage ad loads it all year roud. The system cosistig of the fuel cell ad the storage is desiged i a way that a peak-load-boiler is ot eeded ad the fuel cell ca supply the buildig with eough heat for space heatig ad domestic hot water. This meas, that a certai part of the heat has to be wasted i times whe the heat demad is ot high eough which ca especially happe i summer. The domestic hot water geeratio is modelled by a tap water statio which is directly coected to the storage. The storage is also the basis for supplyig the buildig with eough heat for space heatig. The space heatig demad is characterized by the mass flow rate, flow ad retur temperature of the total buildig. stored i the thermal storage is ot sufficiet, a peakload-boiler is used to geerate the additioal heat. The sigle parts of the system will be preseted i the followig. Figure 2: Scheme of the system with a heat pump. Buildig Both the fuel cell ad the heat pumps ru i siglefamily houses. As a first approach, all cosidered buildigs are assumed to be the same. This buildig is modelled as detailed as possible. For this, we use Modelica to model a sigle-family house which has a very high isulatio stadard. All walls ad widows are geometrically represeted ad the user behaviour (air exchage rate, iteral gais) is also cosidered with costat values. The model of the buildig is completely equipped with a radiator icludig a thermostatic valve i every room. With this, it is possible to cosider the whole buildig system. More iformatio about the modellig of the buildig is provided i (Müller ad Badakhshai, 2010) ad (Lauster, 2012). The cosidered buildig has a maximum heat load of 4.7 kw, is located i Germay i the TRY-regio 12 (Deutscher Wetterdiest DWD, 2011) ad has a total heat demad for space heatig of about 4900 kwh per year. The aual load duratio curve of the buildig heat demad is show i Figure 3. Figure 1: Scheme of the system with a fuel cell. The electricity-cosumig part is a model cosistig of a heat pump, a peak-load boiler, a storage ad agai the space heatig ad the domestic hot water. A scheme of this model is show i Figure 2. The heat pump is cotrolled by a exteral sigal which is ot cotiuous because of the demad from the electrical grid. If the heat geerated by the heat pump ad Figure 3: Aual load duratio curve of the cosidered buildig. We wat to aalyse systems with more tha oe buildig ad our focus i this paper is ot i the iteractio betwee the buildig ad the systems egieerig. Furthermore, we will aalyse the etire system with differet boudary coditios give by the electrical

3 grid. This meas, we aalyse the behaviour of the systems egieerig with a give respose of the buildig ad the varyig requiremets of the electrical grid. For simulatios with may buildigs, simulatio times are gettig very big if all buildigs are represeted totally. There is a eed for measures to improve the simulatio speed. Thus, we simulate a detailed model of the house with a ideal boiler oly oce. I this model we guratee that the desired room temperatures i the buildig are reached for the whole year ad with this, a certai thermal comfort ca be guarateed. At ight (betwee 22 o clock ad 6 o clock), the set temperature i the buildig is 17 C ad durig the day (6 o clock to 22 o clock), the set temperature is 21 C. For the supply temperature, we use a heatig curve for the flow temperature as a fuctio of the ambiet temperature of the buildig. From this simulatio, we used the flow ad retur temperatures ad the mass flow rate of the whole buildig ad use this as a iput i form of a table for the simulatio of the systems egieerig i every house. So the temperatures ad the mass flow rates which the buildig eeds to satisfy its demad are cosidered ad ifluece the operatio of the storage. This gives us a further advatage besides icreasig the simulatio speed. The room temperatures i the house ca vary with differet systems egieerig if the iteractio of the systems egieerig with the thermal mass of the buildig is also cosidered. Differet flow temperatures ca occur which would result i differet heat flows from the radiators to the buildig ad varyig idoor air temperatures for differet simulatios. This does ot occur i our model. All cosidered variats have exactly the same boudary coditios give by the buildig. As log as we are ot iterested i the buildig as a storage which ca store eergy i its thermal mass or the ifluece of differet user behaviour, this is a appropriate procedure. With a system simulatio with 12 houses, heat pumps ad storages, the simulatio time was about eight hours. I compariso to this, the total simulatio of the sigle buildig with the ideal boiler eeded a time of more tha oe day. Fuel Cell The fuel cell is modelled as a simple heat source that ca load the storage to supply the buildig with heat. Fuel cells with a electrical efficiecy of about 60 % are curretly available o the Germa market (Oberitz, 2012). This fuel cell has a electrical power of 1.5 kw ad a thermal output of about 560 W i its operatig poit. Fuel cells are built up modularly with stacks. Thus, higher capacities are possible if several idetical stacks are combied to a bigger uit. Due to this, we assume the fuel cell to be completely scalable. I this paper, we aalyse a fuel cell which has a electrical power of 12 kw. The electrical power of the regarded fuel cell is eight times the power of the origial cell. The thermal power of the cell is also multiplied by eight (results i a thermal power of 4.48 kw). This fuel cell ca deliver eough heat for the buildig all year roud if it rus cotiuously ad loads the storage. Occasioally, more heat is available tha ca be stored or used. As maximum reachable flow temperature of the fuel cell 70 C is assumed. If the temperature would icrease above this temperature assumig a costat heat source, this surplus heat will leave the fuel cell through the chimey without ay usage. Heat Pump The cosidered heat pump is a speed-cotrolled air-towater heat pump. The reaso for this is that we wat to be able to modulate the electricity-cosumptio whe it is ruig. If the heat pump was operatig with a costat speed, it would ot be possible to guaratee a costat capacity i operatio, because the electrical power would deped o the coditios like source temperature ad sik temperature. The model is adapted from (Huchtema et al., 2013). It is based o a polyomial approximatio for the compressor. Usig polyomial fuctios the model calculates the electrical ad the thermal power. The polyomial fuctios are depedet o the source ad the sik temperature as well as the compressor speed. We took a heat pump which ca guaratee a electrical power of. Thus, we eed four heat pumps ruig at oce relatig to the electrical power of the fuel cell to balace the grid. Storage As storage, a stratified buffer storage is used. The modellig approach is take from the buffer storage model published i the Modelica library Buildigs (Wetter, 2009). I cotrast to this model, i our model o compoets of Modelica Fluid are used. Istead of this, equatios for the heat trasport usig ethalpy flows based o mass flow rates ad temperatures are formulated. A mass flow which eters i the etry layer with a certai temperature ad exits at the exit layer with the temperature of this exit layer trasports ethalpy through the storage betwee etry ad exit. Every layer has a certai heat capacity accordig to its dimesios. The eergy trasfer betwee the layers cosiders the ethalpy flow through loadig/uloadig of the storage, the heat coductace ad the fluid trasfer due to a o-perfect stratificatio. This meas, that a heat flow from a lower to a higher layer is calculated cosiderig bouyacy effects. I additio, the heat trasfer to the eviromet is calculated by a heat trasfer coefficiet. It is possible to have several coectio ports for loadig ad several coectio ports for uloadig. I our simulatios, we use oe coectio pair (top ad bottom of the storage) for loadig. I the first buildig, the fuel cell is coected to the storage, i every other buildig there is oe heat pump coected to the storage. O the demad side of the storage, two uload

4 ig circles are coected, oe for space heatig ad oe for domestic hot water. Both the loadig ad the uloadig cycles are directly coected to the storage without a heat exchager. For the stadard case, a system with a storage of 1000 litres for all heat pump systems is assumed. The buildig with the fuel cell is equipped with a storage of 2500 litres. The storages are bigger tha i actual stadard systems, because the heat caot be geerated at ay time but oly i certai phases. I this time, the system should store as much eergy as possible. The chose volumes of the storages are ot optimized for the system but we assume it to be a adequate approach to aalyse the system. Cosumer Based o the buildig model, the model for spaceheatig is developed. The mass flow rate to the buildig, flow ad retur temperatures are take from the buildig simulatio. With this data, a detailed simulatio of the system is possible. If the temperature i the storage is above the desired flow temperature, we ca calculate a mass flow rate that is eeded from the storage to satisfy the eergy demad of the buildig. For this, we assume that a mixig valve from the retur temperature is preset ad ca be used to regulate the flow temperature exactly. If the temperature is less tha the desired flow temperature, a ideal boiler fills the gap betwee the storage temperature ad the desired flow temperature. The produced heat of the boiler is assumed to be geerated with a average efficiecy of 95 % (Wolff et al., 2004). For the domestic hot water supply, a tap water statio is modelled. It is built up as a couterflow heat exchager. The water is flowig from ad to the storage o oe side ad o the other side, water with a assumed temperature of 10 C eters the heat exchager ad the goes to the cosumer. The temperature of the domestic hot water has to be 45 C at the outlet. The heat exchager is assumed with a heat trasfer coefficiet of 1200 W K. If the temperature comig from the storage is too low, agai the ideal boiler is used to geerate the extra heat o the supply side of the heat exchager with a efficiecy of 95 % (Wolff et al., 2004). The tappig profiles are geerated with a special software called DHWcalc (Jorda ad Vaje, 2005) for a temperature of 60 C. Those profiles are coverted ito profiles with a temperature of 45 C. All cosidered buildigs i this paper have a idetical load profile i heatig as well as i domestic hot water tappig profiles. The domestic hot water demad metioed i this paper is about 4250 kwh per year ad is therefore 46 % of the total heat demad. Cotrol The system we are aalysig is desiged to balace the electrical grid. Thus, we have to use a methodology to aalyse such systems. This paper presets a first approach to implemet a geeric method. This method is based o o/off-sigals which are set to the heat pumps. Figure 4: Scheme of the geeral cotrol of the heat pumps, show for two time periods T. Figure 4 shows the geeral procedure of the geeratio of the sigal profiles for the heat pumps. We assume a time period T which determies a periodical behaviour. The whole year is divided ito time frames with the legth of T. A certai percetage x of this time spa T the sigal is o, after that time period, the sigal switches to off util the ed of T. After T has eded, the process repeats for aother T. The sigal o meas that the heat pump has to be switched o because the fuel cell should ot feed electricity to the grid at that time. 12 kw Figure 5: Total evaluated system with fuel cell ad heat pumps ad their operatio. The fuel cell is desiged with a electrical power of 12 kw ad is ruig cotiuously aroud the year. The cosidered heat pumps should ru with a electrical power of. Thus, we have a eed of four heat pumps ruig at oce if the sigal is o. If oe heat pump has filled up its storage, the sigal is trasmitted to the ext heat pump i the ext house. There is a chai of heat pumps i a row. Those heat pumps share a slot of i our system. This meas we have four rows i total with heat pumps i each row as it is show i Figure 5. If the sigal is o, the 12 kw power are ot eeded i the electrical grid ad we have to switch o all four heat pump slots. I every heat pump slot, there is a cotrol strategy based o the storage temperature of the house. The houses are umbered from 1 to ad whe house i (i 2 1) has a full storage (the temperature rises above a certai temperature T max ), the o-sigal is passed to house i+1 (if i+1=+1, the ext house is 1). The chose storage temperature at which

5 the sigal is trasmitted is at T max = 65 C. The cotrol procedure for each heat pump row is illustrated i Figure 6. If the o-sigal is at a certai house i at the ed of a time period x T, the same house i will get the o-sigal at the time whe the geeral sigal switches to o agai. The sigal is movig aroud betwee the houses several times durig the year. This esures that every house gets a part of the geerated electricity. 1 2 T Storage,1 T Storage,2 T Storage, T Storage,-1 T Storage,-2-1 Figure 6: Cotrol system for oe heat pump row. This will be doe as log as it does ot happe that i a time period of 15 miutes, all houses got a o-sigal. If this happes, all houses are switched to off for aother 15 miutes, because this meas that all storages are completely full ad just little amouts of eergy ca be stored. The excess electricity has to be fed ito the grid which meas i this situtatio that it is fed ito the medium or high-voltage grid. This is i cotrast to the ormal feed-i which is completely cosumed i the low-voltage-grid. RESULTS For a broad aalysis we chose to ru several simulatios for differet umbers of heat pumps. The umber varies betwee 2 ad 12. The periods T are varied betwee four hours, oe day ad five days to determie which ifluece the legth of the period has o the efficiecy ad o the reliability cocerig the balacig of the electrical grid. For every ad every T, we chose differet values for x to get a good isight how the system behaves with differet shares of the o-sigal. A higher x ca be see equivalet to a higher share of reewables i the grid. The questio we wat to aswer is to what extet the heat pumps ca avoid the feed-i of the fuel cell ito the grid i times they are forced to ad how eergy efficiet this will be. These are two aspects which are very importat i the evaluatio of systems with a higher share of reewables. Balacig eergy To determie to what extet the system is able to balace the electrical grid, we have to sum the rutimes of all cosidered heat pumps i oe heat pump row. If the sum is equal to the total desired rutime of the heat pumps (displayed by the o-sigal), we ca obtai that it is possible to completely cotrol the behaviour of the total plat i this case. As a mea to evaluate the balacig potetial of the system, we use the balacig rate which is defied i Equatio 1. = P t r,hp x t year (1) Some of the resultig balacig rates are show i Figure 7. The show values for x are 10 %, 50 % ad 90 % to see the differeces betwee low (x = 10 %), medium (x = 50 %) ad high (x = 90 %) sharigs of reewables i the grid. We show oly the behaviour of the total system for the time periods T of four hours ad five days, because the balacig rates with T chose to oe day are very close to them calclulated for the case with four hours. Balacig rate β i % x=10%,t=4 hours x=10%,t=5 days 50 x=50%,t=4 hours x=50%,t=5 days 40 x=90%,t=4 hours x=90%,t=5 days Number of heat pumps for differet cofigura- Figure 7: Balacig rates tios. We ca see that the balacig rates are icreasig with the umber of heat pumps which is obvious sice with more heat pumps ad their belogig storages we have a higher storage capacity. It is also clear that the balacig rates are higher for lower values of x. This meas that the times i which the heat pumps should ru are shorter ad thus the storages are ot loaded that much. The curves for T = 4 hours ad T = 5 days are differig for a low ad a medium value of x. The cases of T = 5 days deliver lower balacig rates i these cases. This meas that the system caot fill the storages ay more because the period i which the heat pumps have to ru are too log at a time. With a high value of x, we ca obtai that the balacig rates for both time periods are very close to each other. This meas that if x is that high, it does ot matter how low the total period is i terms of the balacig rate. If the value of x is low, with a umber of four heat pumps a balacig rate of 100 % ca be reached. For medium values of x, a umber of te to twelve heat pumps leads to a balacig rate which is 100 % or at least close to 100 %. For high values of x, balacig rates of 100 % caot be reached by ay of the ivestigated variats. Eergy efficiecy Besides the aspect of balacig the electrical grid, it is ecessary to aalyse the eergy efficiecy of the cosidered system. For this, we eed a referece system. Both the referece system ad the cosidered

6 system i this paper are based o a gas supply, so that the amout of used gas is adequate to compare both systems. Our referece system cosists of codesig boilers i every house with a assumed average efficiecy of 95 % (Wolff et al., 2004) ad a gas turbie with a electrical efficiecy of 40 % (Lecher ad Leume, 2010). Those two compoets are chose because they are stadard systems for heat geeratio ad for keepig the electrical balace. Because we are aalysig a system which should react fast to balace the electrical grid, a gas turbie seems to be the actual optio to deliver this service to the grid. Other decetralized systems such as combied heat ad power processes based o iteral combustio egies could also be compared to this referece system. Other factors such as grid or trasformer losses are eglected i this examiatio. For a closer look o the efficiecy, we use Equatio 2 to calculate the theoretical savigs of the ivestigated system compared to a referece system. Our system cosists of the fuel cell, the heat pumps ad the peak-load-boilers. The referece system icludes a gas turbie ad gas boilers i every house. W el,f C,year el,f C + P 4 W gas,boiler =1 (1 x) W el,f C,year el,gt +(4 + 1) W gas,ref (2) For the evaluatio, it is ecessary to cosider the complete amout of gas that is eeded i both cases. I the system aalysed i this paper, the gas demad cosists basically of the gas used by the fuel cell ad the gas used by the peak-load-boilers i all houses equipped with heat pumps. For the referece system, the gas for the electricity eeds (everytime the sigal is off, because this meas that the electricity is eeded) ad the gas for all boilers which were eeded i this case are cosidered. We have 4+1 buildigs i the system (oe with the fuel cell ad 4 heat pumps), so we have to cosider 4+1 boilers for the referece system. If a certai system does ot fulfil the requiremets of the electrical grid as they are metioed before, the surplus electrical eergy has o real value ad has to be fed i to a higher-voltage electrical grid ad we caot determie to what extet it ca be used. Thus, this electrical eergy is ot cosidered i the eergy balace except for the gas which is used to geerate it. The resultig eergy savigs are show i Figure 8. Agai, we chose low, medium ad high values for x ad the shortest ad the logest aalysed time period. I the case of a low value for x, we see a decreasig eergy efficiecy with a icreasig umber of heat pumps. This ca be explaied by the high value of the balacig rate for a low umber of heat pumps. This meas that a system with a low ca store more of the eeded heat i its storages ad a icrease of leads to the problem that more buildigs have to be heated ad the gas cosumptio for this is icreasig because every house is gettig oly a little piece of the excess electrical eergy. The differece betwee the case with a log time period ad a short time period is very small. Eergy savigs ξ i % x=10%,t=4 hours 30 x=10%,t=5 days x=50%,t=4 hours 40 x=50%,t=5 days 50 x=90%,t=4 hours x=90%,t=5 days Number of heat pumps Figure 8: Eergy savigs compared to the referece system for differet cofiguratios. For a medium value for x, the eergy efficiecy icreases up to a umber of heat pumps of six ad decreases after this. This shows that the efficiecy of the system is ot the best if we ca use the total electricity but i a situatio whe the balacig rate is betwee 85 % ad 95 %. This meas that we have to look at the trade-off betwee balacig the grid (which could be importat i terms of security of eergy supply) ad the eergy efficiecy. The values for eergy efficiey are always lower with the loger time period T. The higher the umber of heat pumps is, the more both figures are approximatig. The eergy savigs i the case of the high value for x are below zero for systems with a low umber of heat pumps. For two heat pumps i oe row, the system eeds about 50 % more gas tha the referece system. This is caused by the high amout of electricity i these cases which is geerated by the fuel cell, but which caot be used by the total system. With a icreasig umber of heat pumps, the efficiecy is also icreasig. Agai the results for both aalysed time periods are early idetical. Other aalysed scearios Additioal to the aforemetioed systems, we wated to aalyse some chages i the system ad their ifluece o the basic results. To obtai a easily to iterpret result, we coducted the parameter variatios usig a fixed umber of heat pumps. If it is ot stated differet, we chose the system with = 3 as the stadard system ad aalysed two cases: Oe with x = 10 % ad oe with x = 25 %. The case of = 3 was chose, because there is some potetial to improve the balacig rates.as we saw before, the balacig rate i the stadard system was differet for low ad medium values for x. For the system with 3 heat pumps, x = 25 % is quite high, so we call this value i this sectio the high value. As variatios, we first aalysed a icrease of the storage capacity i each house with a heat pump from

7 1000 litres to 2500 litres. Afterwards, we took the stadard system with a storage of 1000 litres ad reduced the maximum temperature from 65 C to 55 C. As third variatio, we aalysed a combiatio of both. We icreased the volume of the storage to 2500 litres ad decreased the maximum temperature to 55 C. Balacig rate β i % x=10% T=65 C, V=1000 l T=65 C, V=2500 l T=55 C, V=1000 l T=55 C, V=2500 l x=25% Figure 9: Balacig rates for differet cofiguratios with a chose umber of heat pumps (=3) ad a certai time period (T = 5 days). I Figure 9, we ca see the resultig balacig rates for the differet chages ad i Figure 10, we ca see the savigs compared to the referece system for all aalysed variats. The icrease of the storage capacity leads as expected to a higher balacig rate i both cases. I the case with a higher x, we ca see a much bigger rise because i the case with a lower x, the system was already close to a balacig rate of 100 % ad the potetial to get a higher balacig rate was limited. If we look at the eergy savigs, we ca see that the rise of the balacig rate i the case of a lower x does ot lead to a rise i eergy savigs. I the case of the higher x, the eergy savigs ca be rised. Eergy savigs ξ i % x=10% T=65 C, V=1000 l T=65 C, V=2500 l T=55 C, V=1000 l T=55 C, V=2500 l x=25% Figure 10: Eergy savigs compared to the referece system for differet cofiguratios with a chose umber of heat pumps (=3) ad a certai time period (T = 5 days). The decrease of the storage temperature leads as expected to a lower balacig rate tha i the stadard case. This is due to a lower storage capacity which comes with a lower maximum temperature i the storage. The eergy savigs stay i the case of a lower x at a early costat level while they slightly fall for the case with a higher x. If both schemes (icreasig of storage volume ad decreasig maximum temperature) are combied, the balacig rate is 100 % for the lower x ad is therefore equal to the oe with oly a bigger storage. I the case of a higher x, the balacig rate of the combied system is betwee the two system variatios metioed before, but it is higher tha i the stadard case. The eergy savigs are the highest i both variats of x. These variatios of parameters show that several parameters ifluece the ability of the system to balace the electrical grid ad the eergy efficiecy. Especially if the balacig rate is low, varyig the storage capacity ca lead to higher values i terms of efficiecy. Agai, we ca see that higher balacig rates do ot automatically lead to higher efficiecies because the heat pumps ru more ofte i bad operatio poits because of higher storage temperatures i average. This meas that the price for a balacig rate of 100 % compared to a balacig rate of 95 % ca be a fallig eergy efficiecy. If the balacig rate is already close to 100 % i the stadard system, chages i the show system parameters have a lower ifluece. CONCLUSION This paper has show that systems based o decetralized techologies for a supply of buildigs ca play a role i balacig the electrical grid. The preseted system cosists of a fuel cell with a high electrical efficiecy ad several heat pumps that are used to modulate the exported power. The total system ca reach high balacig rates of up to 100 % depedig o the umber of heat pumps. Additioally, the gas cosumptio of this system is up to 33 % lower tha i the referece system. This meas that a cosiderable reductio is possible. The system s efficiecy ca go dow if a too low or a too high umber of heat pumps is used. For every T ad every x, there is ot oly oe umber of heat pumps which delivers good results, but several combiatios are possible. If the umber of times whe the heat pumps have to be switched o icreases (represeted by x), the umber of houses equipped with heat pumps should be icreased too to get a higher efficiecy. This meas that the system ca be desiged to grow with the itegratio of a higher share of reewables i the electrical grid. The umber of heat pumps (participatig households) could be icreased progressively. The ecoomical feasibility will deped o the compesatio which the households get if they participate i a load maagemet for the electrical grid. The developmet of this market seems to be ucertai so that we caot determie if the system preseted i this paper could be ecoomically feasible. This will also deped o the costs for thermal storages ad the costs for alterative storage systems for electrical eergy. We have to esure that all households ca participate best

8 at the load maagemet which meas that we have to guaratee equal rutimes of the heat pumps for every buildig. The ecoomical potetial could be icreased by usig the surplus heat or the surplus electrical eergy i summer for coolig to get the maximum utilizatio of the eergy iput. This will be ivestigated i further work. The heat pumps were simulated separately from the fuel cell because their oly itercoectio would be the electrical grid which is ot represeted physically i our model. Because of this, the same methodology could be used to use the heat pumps to directly cosume eergy from the grid if it is ecessary because of productio peaks, e.g. due to photovoltaics. I total, it ca be determied, that this system is appropriate to be used for load maagemet i low-voltage grids. Further detailed aalyses will follow. These aalyses will iclude the possibility of a usage of the buildig mass as a storage. Additioally, a detailed aalysis of the electrical grid will follow. This is also cocerig the developmet of a method to geerate the o/off-profiles which could be exteded to a method where differet steps are available ot oly the cases of o ad off. Also the aalysis of real photovoltaics ad wid feed-i situatios will be icluded. Differet house types ad user behaviour ad their ifluece o the efficiecy of those systems will be also aalysed i further work. NOMENCLATURE T x i T max W el,f C,year el,f C el,gt W gas,boiler W gas,ref t r,hp t year time period for heat pump sigal share for o-sigal i T umber of heat pumps i every row idex of actual heat pump maximum temperature of heat storage eergy savigs electrical work geerated by the fuel cell for a total year electrical efficiecy of the fuel cell electrical efficiecy of the gas turbie gas demad for every boiler gas demad of the boiler i referece system rutime of each heat pump time of the whole year ACKNOWLEDGEMENT We gratefully ackowledge the fiacial support for this project by BMWi (Germa Federal Miistry of Ecoomics ad Techology) uder promotioal referece 03ET1111B. REFERENCES Dassault Systems Dymola - multiegieerig modellig ad simulatio. Deutscher Wetterdiest DWD Aktualisierte ud erweitere Testreferezjahre (TRY) vo Deutschlad für mittlere ud extreme Witterugsverhältisse. Techical report, Budesistitut für Bau-, Stadtud Raumforschug. Hoh, A., Haase, T., Tschirer, T., ad Müller, D A combied thermo-hydraulic approach to simulatio of active buildig compoets. I The Modelica Associatio ad the Departmet of Thermodyamics, Hamburg Uiversity of Techology, editor, Proceedigs of the 4 th Iteratioal Modelica Coferece 2005, Hamburg. Huchtema, K., Streblow, R., ad Müller, D Adaptive supply temperature cotrol for domestic heat pump systems. I Proceedigs of Clima: 8th Iteratioal Coferece o IAQVEC. Huijsmas, J., va Berkel, F., ad Christie, G Itermediate temperature SOFC a promise for the 21st cetury. Joural of Power Sources, 71: Jorda, U. ad Vaje, K DHWcalc: Program to geerate domestic hot water profiles with statistical meas for user defied coditios. I Proceedigs of the ISES Solar World Cogress, Orlado, USA. Lauster, M Modelica buildig-bibliothek ud gebaeudemodelle. I Symposium: Itegrale Plaug ud Simulatio i Bauphysik ud Gebaeudetechik. TU Dresde, Istitut fr Bauklimatik, Prof. Dr.-Ig. Joh Gruewald. Lecher, C. ad Leume, J., editors Statioäre Gasturbie. Spriger. Modelica Associatio Modelica ad modelica associatio. Available: [Olie]. Müller, D. ad Badakhshai, A Gekoppelte Gebäude- ud Alagesimulatio mit Modelica. I Proc. of BauSim Coferece, Wie, Austria., Wie. Proc. of BauSim Coferece. Oberitz, F Brestoffzelle-Mikro-KWK für die Eergiewede. Eergiewirtschaftliche Tagesfrage, 6: Wetter, M Modelica library for buildig heatig, vetilatio ad air-coditioig systems. I 7th Iteratioal Modelica Coferece, Como, Italy. Wolff, D., Teuber, P., Budde, J., ad Jagow, K Feldutersuchug: Betriebsverhalte vo Heizugsalage mit Gas-Brewertkessel. Techical report, Fachhochschule Brauschweig- Wolfebüttel. Ziesig, H.-J Awedugsbilaze für die Edeergiesektore i Deutschlad i de Jahre 2009 ud Techical report, AG Eergiebilaze e.v