D DAVID PUBLISHING. Efficiency of Combined Solar Thermal Heat Pump Systems. 1. Introduction. 2. Solar Thermal and Heat Pump Systems

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1 Journal of Civil Engineering and Architecture () - doi:./-/.. D DAVID PUBLISHING Efficiency of Combined Solar Thermal Heat Pump Systems Werner Lerch, Andreas Heinz, Richard Heimrath and Christoph Hochenauer Institute of Thermal Engineering, Graz University of Technology, Graz, Austria Abstract: In this project, different combinations of solar energy and heat pump systems for preparation of DHW (domestic hot water) and space heating of buildings are analyzed through dynamic system simulations in TRNSYS (Transient System Simulation Program). In such systems, solar rmal energy can be used, on one hand, directly to charge buffer storage and, on or hand, as heat source for evaporator of HP (heat pump). In this work systems, in which solar heat is only used directly (parallel operation of solar and HP), systems using collectors also as a heat source for HP are analyzed and compared to conventional air HP systems. With a combined parallel solar rmal HP system, system performance compared to a conventional HP system can be significantly increased. Unglazed selectively coated collectors as source for HP have advantage that collector can be used as an air heat exchanger. If solar radiation is available and collector is used as source for HP, higher temperatures at evaporator of HP can be achieved than with a conventional air HP system. Key words: Solar heat pump, solar system, heat pump, low energy buildings.. Introduction In Austria and also in or European countries, several companies have developed solar air HP (heat pump) systems. In many of se systems, solar energy is not only used to charge a buffer storage (parallel system) but also as heat source for evaporator of an HP. In some configurations, unglazed collectors are used as a heat source []. In IEA SHC (International Energy Agency Solar Heating and Cooling Programme) Task ( performance and relevance of combined systems using solar rmal and heat pumps are evaluated, a common definition of performance of such systems and contributions to a successful market penetration of this new promising combination of renewable technologies are provided []. In task, more than systems from participating countries were surveyed during and. The results of survey showed that % of systems were found to be parallel solutions, % serial Corresponding author: Werner Lerch, Dipl.-Ing., research field: renewable energy. werner.lerch@tugraz.at. systems, % complex systems and very few regenerative solutions. The number of air and ground source systems were about in same range and some systems were operated only with solar collectors as source for evaporator and many in several modes of operation [].. Solar Thermal and Heat Pump Systems In this paper, different heating systems are analyzed and compared through dynamic system simulations in TRNSYS (Transient System Simulation Program) []. On one hand, a conventional air HP system with and without solar rmal system with covered selectively coated collectors (parallel system) is considered in project, and on or hand, serial solar air HP systems with covered and uncovered selectively coated collectors have been defined and evaluated. Additionally, a system with ice storage has been taken into account.. Concept Fig. shows hydraulic layout of Concept, which is an air HP system without any solar rmal

2 Efficiency of Combined Solar Thermal Heat Pump Systems system. The HP provides heat to buffer storage but also directly to heating system of building. The buffer storage is divided into a DHW (domestic hot water) and an SH (space heating) volume. As a backup, an electrical heater is placed in buffer storage (space heat volume), which provides heat at times when performance of HP is not sufficient to cover heating demand. In this system without solar collectors, buffer storage has a volume of. m. The DHW preparation takes place through an external heat exchanger. system supplies energy to ice storage at same time, n energy for evaporator of HP comes directly from solar rmal system. The advantage is that in this case higher source temperatures can be achieved at evaporator of HP.. Concept The difference between Concepts and is that, in Concept, a solar system is installed in a parallel way. For solar system, glazed selectively coated collectors are used. The buffer storage volume is increased to m. In Fig., integration of solar rmal system is shown. The solar loop is divided into a brine (solar side) and a water loop (buffer storage), whereby heat transfer between m takes place via a plate heat exchanger (Fig. ). Solar energy is only used to charge buffer storage. As a second energy source, an air HP system is installed. Fig. Concept.. Concept As shown in Fig., Concept is a serial solar HP system. As heat source for HP, only unglazed selectively coated collectors are used. When HP is operated, unglazed collectors are used as heat source for HP. Solar energy can eir be used to charge buffer storage or as heat source for HP, a simultaneous operation is not possible. Fig. Concept.. Concept Concept is similar to Concept with difference of an additional ice storage (Fig. ), which is used as source for evaporator of HP. As in Concept, unglazed selectively coated collectors are used. The buffer storage (hot water storage) is charged with priority. If HP is operated and solar rmal Fig. Concept.

3 Efficiency of Combined Solar Thermal Heat Pump Systems Fig. Concept.. Basic Conditions Strasbourg has been calculated for design ambient temperature of - C. For this calculation, solar gains have not been considered, but internal loads of building, present persons and electronic equipment. With heat preparation system, heat demand of building and DHW demand shall be covered. For dimensioning of HP, heat load of building increased by. kw was used, which is to be covered by HP at operation point AW (HP air source temperature C and water temperature on sink side C). Table shows heat loads of three buildings used.. Domestic Hot Water The domestic hot water demand is used as defined in Task. The DHW preparation is done via a plate heat. Building exchanger which is charged from buffer storage in all systems. The hot water set temperature is defined as For system simulations, boundary conditions of IEA SHC Task are used [, ]. In Task, C. From annual DHW heat demand, a mean daily heat demand of. kwh/d has been calculated. three different buildings (single family house, heated floor area m ) have been defined. For climate in Strasbourg, buildings have a specific heat demand of, and kwh/m²a []. For SFH and SFH, a floor heating system, and for SFH, a radiator heating system is assumed. The flow. Simulation Models The dynamic system simulations have been performed using software TRNSYS []. The most important component models (types) which were used in se simulations are listed in Table. temperature is controlled depending on ambient air temperature (for SFH and SFH, / C at Table Calculated heat loads of considered buildings. SFH SFH SFH design ambient temperature, radiator exponent.; for Heat load (kw)... SFH, / C, radiator exponent.). Heat load including DHW... The heat load of building for location preparation (kw) Table Used models in TRNSYS. Type number Description Documentation Description/parameters Type Heat pump model [] Air HP, power: SFH,. kw; SFH,. kw; SFH,. kw (COP (coefficient of performance) at A/W =.) Model glazed collector: a (linear heat loss coefficient) =. W/(m K), a Type Collector model (glazed) [] (quadratic heat loss coefficient) =. W/(m K ), η (optical efficiency) =. Type Collector model (unglazed) [] Model unglazed PVT (photovoltaic rmal) collector with condensation: a = W/(m K), a = W/(m K ), η =. Type Storage model [] Buffer storage; volume: with solar m, without solar. m Type Ice storage model [] Ice storage: internal heat exchanger (pipe diameter mm, distance between pipes mm Type Building model [] Standard type; more zone model Type Radiator model [] SFH and SFH: flow-/return flow temperature = / C; SFH: flow-/return flow temperature = / C

4 Efficiency of Combined Solar Thermal Heat Pump Systems. Evaluation Figures For evaluation of system performance, different indicators were defined. These indicators are SPF (seasonal performance factor) of system (SPF System ), seasonal performance factor of HP (SPF HP ) and solar fraction (SF). The SPF of entire system with penalties is defined as: ( Q SH Q DHW )dt SPFSystem () ( P P P )dt el,tot el,sh,pen el,dhw,pen The definition of SPF of HP is: Q HP,Cond dt SPFHP P d t el,hp,tot () The Solar fraction as: ( Q HP,Cond Pel,heater )dt SF () ( Q SH Q DHW )dt where: Q SH is heat power space heat; Q DHW is heat power domestic hot water; Q HP, Cond is heat power HP condenser; P el, tot is electric power of all components (HP, controller, electric heater and all pumps); P el HP, tot, is electric power of entire HP (compressor, controller, fan of outdoor unit); P el, SH, pen is electric power penalty space heating; P el, DHW, pen is electric power penalty DHW; P, is electric power electrical heater. el heater. System Comparison Different systems evaluated using SFH are shown in Table.. System Comparison The comparison of Concepts and shows that, by integration of a solar rmal system ( m ), SPF System for SFH can be increased from. to.. The total electricity consumption of system is reduced by about %. The SPF HP is reduced, as solar collectors are providing heat at times, when HP would orwise be operated with a high efficiency. However, SPF System is more important figure from an overall system point of view. The simulation results for Concepts and show that through integration of an ice storage with selected boundary conditions, SPF HP can be Table Simulation results (Concepts -). Concept different performance indicators. Simulation results for Building Collector area (m ) Buffer storage volume (m ) Ice storage volume (m ) SPF System SPF HP SF (%) Q el,tot (kwh) , -..., SFH -...,...., , -..., SFH -...,...., , -..., SFH -..,...., Q el,heater (kwh)

5 Efficiency of Combined Solar Thermal Heat Pump Systems Temperature ( C) Temperature ( C) Q HP, evaporator (MWh) Q HP, evaporator r (MWh) (a) (b) Fig. Heat provided at evaporator of HP with different evaporator inlet- and outlet-temperatures for SFH: (a) Concept, m unglazed collector; (b) Concept, m unglazed collector,. m ice storage. increased, but SPF Sy ystem decreases. This is due to lower solar energy input directly to buffer storage. For SFH and for Concept, solar fraction amounts..% and for Concept with an ice storage of about. m it is.% %.. Evaporator Inlet Temperature Compared to Ambient Air Temperaturee Fig. shows cumulative diagrams, in whichh ambient air temperature and evaporator inletfunction of and outlet temperature is shown as a supplied heat at evaporator of HP. With Concept (Fig. ),, kwh is supplied by unglazed collectors. About, kwh is provided to evaporator with a brine inlet temperaturee higher than ambient air temperature. This happens, when solar radiation is available, which lifts brine temperature in collector to a level higher than ambient. With Concept (Fig. b),,, kwh is supplied by unglazed collector and ice storage of solar system. The phase change (liquid/solid) in ice storage can be clearly seen in course of evaporator inlet temperature. A comparison of Figs. a and b shows that integration of an ice storage causes an increase of evaporator inlet temperature, which results in a higher efficiency of HP (SPFF HP ). The heat pump efficiency (SPF HP ) only increases slightly with chosen boundary conditions (Concept : SPF HP.; Concept : SPF HP.).. Monthly Simulation Results In Figs. and, monthly heat balances for SFH for Concepts - are shown. On input side (Figs. a and a) ), energy flows into system and, on output side (Figs. b and b), energy demand of system is shown. Additionally, averagee monthly performance figures are depicted (SPF System, SPF HP and solar fraction). Fig. a showss simulation results for air HP system for SFH. On input side (Figs. and ) of energy flow, air source for HP (Q air ), electricity consumption of compressor of HP (Wel,comp,HP) and electricity consumption of electrical heater (W el,heater ) are shown. On consumption side (Figs. b and b), energy for DHW (Q DHW ) and SH demand (Q SH ) and also heat losses of storage (Q heatlos ss storage,hp), pipes (Q heatloss s pipes) and HP can be seen. The average monthly SPF HP decreases in summer months because HP is only in DHW mode. In DHW mode, temperaturee difference between heat source and sink is greater than in SH mode and, refore in DHW mode, HP is lesss efficient than in SH mode. Fig. b showss simulation results for solar air HP system with m glazed selective coated collectors

6 Efficiency of Combined Solar Thermal Heat Pump Systems,,,, Wel,comp,HP SPFsystem Wel,heater Qheatloss storage,hp Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. (a) (b) Fig. Monthly heat balances SFH: (a) Concept, air HP; (b) Concept, air HP and m glazed selective coated collectors (parallel). Qsolar Q storage Wel,heater W Qheatloss Q storage,hp,,,, Solar fraction Wel,comp,,HP SPFsystem Qsolar storage Wel,comp,HP Qsolar storage Wel,comp,HP Wel,heater Wel,heater Qheatloss storage,hp SPFSystem Qheatloss storage,hp SPFSystem Solar fraction Solar fraction,,,, Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. (a) (b) Fig. Monthly heat balances SFH: (a) Concept, HP and m unglazed collector; (b) Concept, HP and m unglazed collector,. m ice storage.,,,, Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. (parallel) for SFH. Comparing Figs. a and b, it can be observed that, through integration of solar rmal system, system performance can be significantly increased. From May to September, total heat demand can be covered through solar rmal system. In Fig., simulation results for SFH for Concepts and are shown. In both figures (Figs. and ) on input side, solar energy, which is directly used to load buffer storage, is shown in Figs. and as Q solar stora age, and solar energy which is used as heat source for HP (or ice storage) is shown in as Q sola ar HP. In Concepts and, unglazed collectors are used as heat source for HP. In Concept, ree is an additional ice storage. From May to September, solar energy in Concept is only used to charge buffer storage, because heat demand can be covered through solar rmal system and re is no operating time of HP. In Concept, solar energy is additionally used to charge ice storage. In summer months, re is also no operation time for HP.

7 Efficiency of Combined Solar Thermal Heat Pump Systems. Conclusions Highly efficient combinations of solar rmal and heat pump systems (SolPumpEff) within this work, different combinations of solar energy and heat pump (HP) systems are analyzed through dynamic system simulations in TRNSYS. In such systems, solar rmal energy can be used, on one hand, directly to charge a buffer storage and, on or hand, as heat source for evaporator of HP. In this work systems, in which solar heat is only used directly (parallel operation of solar and HP), systems using collectors also as a heat source for HP are simulated and compared to conventional air HP systems. For solar rmal system, glazed and unglazed selectively coated collectors have been considered. Using latter type of collector as source for HP offers advantage that collector can be used as an air heat exchanger. With a combined parallel solar rmal HP system, SPF System compared to a conventional HP system can be increased significantly. With unglazed selectively coated collectors as source of HP, evaporation temperatures of HP can be increased compared to a conventional air HP system if solar radiation is available. With such collectors, a collector area of m must be available to achieve same SPF System than with a conventional air HP system with used boundary conditions and assumptions. Acknowledgments This work was performed within project Highly Efficient Combinations of Solar Thermal and Heat Pump Systems (SolPumpEff) and financed through research and technology program NeueEnergien by Climate and Energy Fund in Austria. References [] Bertram, E, Glembin, J, and Rockendorf, G. Unglazed PVT (Photovoltaic-Thermal) Collectors as Additional Heat Source in Heat Pump Systems with Borehole Heat Exchanger. Energy Procedia : -. [] Hadorn, J. C.. IEA Solar and Heat Pump Systems, Solar Heating and Cooling Task & Heat Pump Programme Annex. Energy Procedia : -. [] Solar Energy Lab, University of Wisconsin.. TRNSYS. A Transient System Simulation Program: V... Madison: Solar Energy Lab, University of Wisconsin. [] Dott, R., Haller, M., Ruschenburg, J., Ochs, F., and Bony, J.. Reference Buildings Description of IEA SHC Task /HPP Annex. Subtask C Working Group Boundary Conditions Draft, IEA SHC. [] Haller, M., Dott, R., Ruschenburg, J., Ochs, F., and Bony, J.. The Reference Framework for System Simulations of IEA SHC Task / HPP Annex. A preliminary report of subtask C. Deliverable C. Draft, IEA SHC. [] Heinz, A., and Haller, M.. Appendix A Description of TRNSYS Type by IWT (Institute of Thermal Engineering) and SPF. In Models of Sub-components and Validation for IEA SHC Task /HPP Annex Part C: Heat Pump Models, edited by IEA SHC. A technical report of subtask C Deliverable C. Part C. [] Haller, M., Paavilainen, J., Dalibard, A., and Perers, B.. TRNSYS Type v. Dynamic Collector Model by Bengt Perers. Updated Input-Output Reference. Graz: IWT. [] Stegman, M., and Bertram, E.. Model of an Unglazed Photovoltaic Thermal Collector Based on Standard Test Procedures. Presented at ISES (International Solar Energy Society)-Solar World Congress, Kassel. [] Drück, H.. Multiport Store-Model for TRNSYS, Type version.f. Graz: IWT. [] Lerch, W., and Heinz, A.. Solar Thermal HP Systems Inkl. Waste Water Heat Recovery: Energy Assessment through Dynamic System Simulations in TRNSYS. Presented at Gleisdorf Solar, Gleisdorf. (in German) [] Transsolar Energietechnik GmbH.. Type Multizone Building Modelingwith Type and TRNBuild. Stuttgard: Transsolar Energietechnik GmbH. [] Holst, S.. Type Dynamic Radiator Model with Pipes (Type ). München: Bayrisches Zentrum für Angewandte Energieforschung e.v..