Modelling The Performance Of A Domestic Low-Temperature Heating System Based On A Heat Pump

Low temperature heating systems for single-family houses, based on heat pumps are becoming progressively more preferable in the global market. The possibility of such systems to optimise the energy utilisation and to achieve a high annual energy efficiency make them more promising for future low energy houses. In this study we model and analyse the performance of a low temperature heating system based on a heat pump. A system model is developed in two parallel simulation programs: in EES (Engineering Equation Solver) for simulating the operation of the heat pump and in TRNSYS for studying the performance of the heat distribution system and the building zone. The efficiency factors for the various components in different case studies are calculated. The computation seeks to highlight possibilities for energy savings and for increasing the overall efficiency. The simulation results indicate the need to study the performance of the heat pump and the heat distribution system together. NOMENCLATURE Aj: internal overall surface of the J wall or window Cap: room air and furniture effective capacitance hj: total heat transfer coefficient of the J surface inf m& : mass flow rate of air infiltration inf Q& : infiltration energy gains int Q& : internal energy gains lat Q& : latent energy requirement sens Q& : sensible energy requirement speop Q& : sensible gains from people v Q& : energy gains due to ventilation z Q& : convection energy gains j s T , : temperature on the J surface z T : zone temperature ∆hvap: heat of vaporisation of water a ω : humidity ratio of ambient air I ω& : rate of internal moisture gains to zone z ω : humidity ratio of zone air INTRODUCTION The possibility of optimising the performance of a heating system and minimising the energy requirement for heating a house is of great importance. Especially in a Nordic country like Sweden, where 60% of the annual energy use in the building sector accounts for space heating and domestic hot water production, energy efficient heating systems are concepts with increasing value (Nutek, 1997). Recently, there has been growing interest in using low-temperature heating systems combined with heat pumps in single-family houses. Considering the heat distribution system, the general trend is to move in the direction of lower inlet temperatures and large heat transfer surfaces. A floor heating system is an interesting solution and has been rather extensively studied in recent years. Furthermore, installers show an increasing preference for it. Taking the heat-generation side into account, the main technical trend is to choose a system that is environmentally friendly, requires low power input to produce the heat, and eliminates the heat losses. A welldesigned and properly dimensioned heat pump fulfils the previously mentioned prerequisites. In Sweden, which is a market leader in manufacturing heat pumps among the European countries, heat pump is a considerable option when selecting a residential heating system (Bouma, 1999). Nevertheless, using a heat pump is more a technoeconomic issue than a simple question of performance (Granryd, 1998). System design is probably the most crucial part in the route to successful implementation of heat pumps in dwellings (Traversari et al, 1999). For future low-energy dwellings, there is an urgent requirement for low-cost heat pump heating systems with a highannual energy-efficiency (Afjei, 1997). Furthermore, when selecting a heating system, great attention should also been devoted to the building construction and its characteristics (Norén et al, 1999). The critical point for every heating system is to achieve the optimum operation after its installation. So the optimisation phase must include the building as well. The performance of a heating system based on a heat pump has been individually studied at length. However, to achieve the objective of high comfort in a cost efficient and environmentally acceptable manner, the heat pump, the heat distribution system and the building have to be studied as a complete system (Afjei et al, 1999). The aim of the present paper is to study the operation of the heating system and the building as a whole. The results show a comparison of the performance of the heating process based on different cases for the heat generation, the heat distribution system and the building envelope. The significance of the building construction and its behaviour as a dynamic system is extensively highlighted when selecting the appropriate thermal energy system. CASE STUDY DESCRIPTION Reference model and system description The main objective of this paper is to investigate the performance of a low-temperature heating system based on a small-size heat pump for a single-family house. The system analysis focuses particularly on the importance of the building’s characteristics when a low-temperature heating system is chosen to serve its thermal requirements. Therefore, a reference system is developed in two parallel simulation programs: one for the building zone and another for the heating system. The simulation tools allow the link between the building zone and the heating system by a method of exchanging information between the programs. Hence, previously set outputs from the heating system can be inputs in the building and vice versa. In this way a unique system is obtained where the simulation can be run in iterative loops between the building zone and the heating system. The system boundary includes everything from the heat source to the evaporator of the heat pump up to the climatic conditions of the geographic location where the building is situated. Description of the building zone model The building zone is developed in TRNSYS, a package of stand-alone utility programs that enables a system dependent on time to be built and analysed. The building zone is built by components included in the standard TRNSYS library. The main component in the simulation program describes the building zone. The system includes component routines that handle the input of the weather data and the output of the simulation results. The zone is specified by separate sets of parameters and inputs describing the internal space, the external weather conditions, the walls, the floor, the ceiling, the windows and doors (ASHRAE, 1993). The ASHRAE transfer function approach is followed for modelling the zone structure. The convection and radiation losses and gains are calculated based on the transfer function coefficients that are determined after selecting the zone construction. The internal energy input is based on the radiant and convective gains due to lights, equipment, presence of people and the level of their activity, as well as any other instantaneous heat gains to space. The simulation program based on the reference model permits the heating load of the building zone to be estimated. By setting the desired zone temperature and the desired zone humidity ratio, the thermal requirement of the building zone can be calculated. The mathematical description for the sensible and the latent energy requirement of the zone are the two basic equations governing the heat transfer through and between all elements in the zone: ∑ = − − ⋅ ⋅ + + + + + = N j z z j s j j speop v z sens T Cap f T T A h Q Q Q Q Q Q 1 , inf int ) , ( ) ( & & & & & & ) ) ( ( inf I z a vap lat m h Q ω ω ω & & & + − ⋅ ∆ = The sensible energy requirement for maintaining the desired indoor temperature is obtained from an energy balance of the zone air and furnishings considered as a lumped capacitance system. The simulation model takes into account both convection and radiation heat transfer when estimating the sensible load. The latent energy requirement is based on a moisture balance of the room air at any instant. The latent load is the energy required to maintain the zone humidity ratio within the desired humidity comfort zone. The key inputs to the simulation program are the climatic conditions for a whole year provided on an hourstep basis and the main building characteristics (Table 1). At present, it is worthwhile mentioning that the program allows changes to the window total heat transfer coefficient. Hence, variable window transmittance of solar radiation and thermal energy transfer across the window are achievable. Table 1. Main input information for the building zone model on the climatic conditions and the building location and construction. Climatic hourly-provided data Building location and construction • Beam radiation on horizontal surface (kJ/m2hr) • Diffuse radiation on horizontal surface (kJ/m2hr) • Dry bulb temperature of ambient air (oC) • Relative humidity of ambient air (%) • Wind velocity (m/s) • Malmö, Sweden, 55.8o north latitude • One-storey dwelling, no cellar • Floor area of 120 m2 • Window area corresponding to 15% of the total floor area • Variable gain windows as an option 1 Variable gain window is a component in the standard TRNSYS library that allows the window total heat transfer coefficient to be changed due to insulating curtains on the window. Description of the heating system model The heating system is modelled in the Engineering Equation Solver (EES). EES is an equation-solving program with built-in functions for thermodynamic and transport properties of many substances including refrigerants. Since refrigerant properties can be obtained in EES, it is rather convenient to model the performance of a heating system based on a heat pump. The development of the heating system depends on the building’s characteristics and dynamic behaviour. Despite the fact that the simulation programs of the heating system and the building zone are built in two separate environments, the simulation runs simultaneously, connecting the models as if they were one. Therefore, the heating process in the chosen building zone and the interaction between them can be attained. The heating system model is based on assumptions corresponding to a realistic approach. The model pertains to a ground coupled heat pump unit connected to a low temperature, hydronic distribution system. The heat pump unit is sized so that it covers a certain amount of the maximum energy demand of the chosen building zone. Nevertheless, in Sweden it is common to have an auxiliary heating system supporting the heat pump during the coldest period of the year when the maximum load occurs. The climatic conditions are the same as the weather input to the building zone. The ground conditions are taken for the location of Malmö in Sweden. For buildings in Malmö the outdoor design temperature (ODT) is set to –17.5oC. With respect to the heat source, a single borehole with a U-shape tube is placed in rock. An indirect system is chosen with a brine of ethylene glycol circulating between the heat source and the evaporator. For the sake of simplicity, a constant amount of energy extracted from the ground is assumed equal to 35 W/m. This is typical for a single borehole presupposing sufficient installation in rock with proper backfilling (Granryd, 1998). R410A is chosen as the working fluid. However, performances from using different refrigerants are given later on in this study. The rest of the heat pump components are modelled based on technical details of current components with sufficient performance. For the heat distribution system, a hydronic floor heating assembly is selected. The floor is directly placed on the ground and the ground is considered in the building zone model as a separate zone attached to the building zone with a constant temperature of 9C. The total thermal conductivity of the floor depends on its construction and the thickness of the insulation placed on the ground. A major prerequisite is that the ground is properly insulated in order to minimise energy transfer to the ground and the risk for moisture transfer in the construction. A suspended floor heating system is considered in this study with a rather low capacity to absorb large changes of loads in the building. The operation of the heat distribution system is included in the building zone model developed in TRNSYS. However, the flow regulation and control mode are included in the heating system model developed in EES. With respect to the heat pump, the governing equations in the simulation program enable the calculation of the heating power and the coefficient of performance for the heat pump. The isentropic and volumetric efficiencies are estimated depending on the compressor’s pressure ratio and the refrigerant’s molar mass (Pierre, 1979). The transmission and electric motor efficiencies are also taken into consideration. Regarding the floor heating system, its operation is regulated on an ON-Off mode. The design supply-water temperature is 28C, which gives an overall heat transfer coefficient of 4.8 W/mC. Heat transfer due to radiation has a portion of around 65% and the remaining 35% corresponds to convection heat transfer. The main set variables for the heating system are given in table 2. The heating process is controlled from the temperature of the building zone. A single two-stage thermostat regulates the operation process. When the zone temperature exceeds 20oC, the brine pump and the compressor are stopped. Table 2. Main input information for the heating system. Heat pump unit • Refrigerant R410A • Inlet brine temperature –0.5C • Capacity of the borehole 35*x W, where x borehole length (m) Suspended floor heating system • Heat flux upwards 40 W/m • No sub-cooling • Steady superheating 5C • Temperature difference between the condensation and the supply water 5C • ON-OFF compressor’s function mode RESULTS AND DISCUSSION One of the main purposes of this paper is to investigate the energy requirement of different building constructions, taking the building’s characteristics into account. For this reason, three building zones with different construction tightness are selected. Table 3 provides information on the chosen building types. As we can see, the variation in the wall and roof heat transfer coefficient is rather small. However, a larger effect factor is taken when the floor heat transfer coefficients are selected. There is also a possibility of varying the windowradiant gains. Three cases are considered. In the first case, it is assumed that there is no internal shading and all the radiation entering the window strikes the floor and is then captured by the internal space of the building zone. In the second case, there is internal shading corresponding to 50% of the total window area and in the third, there is internal shading corresponding to 80%. A time-schedule is used for applying the internal shading only in hours of high solar radiation. The option of a variable gain window affects the transmittance of solar radiation and consequently the thermal energy transfer to the building zone. At this point, it is worthwhile mentioning that all windows are of the same type with a conduction transfer coefficient of 1.2 W/m2K and an average window transmittance of 0.7. Table 3. Information on the characteristics of the chosen building zones. Super-insulated Well-insulated Med-insulated Wall heat transfer coefficient (W/m2K) 0.213 0.245 0.294 Roof heat transfer coefficient (W/m2K) 0.195 0.226 0.258 Floor heat transfer coefficient (W/m2K) 0.278 0.361 0.417 The wall and roof heat transfer coefficients are increased by a rate of 15% and 30% from super to well-insulated and med-insulated construction respectively. The floor heat transfer coefficient is increased by a rate of 30% and 50% from super to well-insulated and medinsulated construction respectively. The fraction of the incoming beam radiation is set to 100%, 50% and 20% respectively, in hours of high solar radiation. The results, shown in figures 1 and 2, highlight the influence of the chosen building’s characteristics on both the heating and cooling requirements of the house. Although the heating requirement is kept low, the cooling requirement might rise considerably in a tight construction especially when big fenestration is applied with the latest types of glazing that allow for high radiation gains but low heat transfer losses. However, the use of the variable gain window mostly influences a very well insulated construction, considerably decreasing the cooling requirement. Nevertheless, due to aspects of aesthetics and thermal comfort, internal shading on windows in building zones is commonly applied, so that using a variable gain window is closer to a more realistic approach. Figure 1. Heating load for different building types and different cases for internal shading. Figure 2. Cooling load for different building types and different cases for internal shading. Table 4 shows the change in the heating and cooling requirement of the three building types when internal shading is not applied and 80% of internal shading is applied respectively. Table 4. Change in the thermal energy loads when 0% and 80% of internal shading is applied respectively. Super-insulated Well-insulated Med-insulated Increase in the heating requirement (kWh) 87