1.1. General conditions

Before explaining simulation procedure, some general terms are detailed:

• Meteorological data from a weather station located in Madrid were used.

• The simulation period is from November 2005 to April 2006.

• The DX-SAHP system is assumed to operate 24 hours per day.

• A time step of 3600 seconds was selected for the calculations.

1.2. Simulation procedure

Numerical simulation of the system consists of the stages showed in Figure 3:

1st International Congress on Heating, Cooling, and Buildings " ‘ 7th to 10th October, Lisbon — Portugal * Fig. 3. Diagram flow of simulation model Energy balance for the building

The energy balance carried out in this stage enable us to estimate heat demand of the whole building depending on climatic data and constructive characteristics of the building. A 120 m2 detached house on two floors, designed according to Spanish current building regulations, was selected for this study. Thermal load is calculated as difference between heat losses and heat gains, as ASHRAE Handbook indicates. Heat losses include both loss through exterior surfaces of the building and those due to ventilation. The selected detached house presents an overall heat loss coefficient equal to 335.58 W/K. As heat gains we consider lighting and occupancy loads. Once known heat losses and gains, total heat demand is obtained easily.

Energy balance for the heat pump system

In knowing the heat demand of the building, the energy balance for the heat pump system gives the mass flow rate of refrigerant, the heat that evaporator must collect (Qe in figure 1), the electrical energy consumed by the system and its COP. As a 24 hour a day operation is considered, one can assume that the energy rejected by condenser (Qc) is equal to heat demand of the building (Qt) plus the heat loss from condenser to heat exchangers. According to past experience with heat pump systems, it was assumed that this heat loss represents the 10% of building heat demand. Then, knowing Qc, evaluation of aforementioned parameters is made according to the thermodynamic cycle shown in Figure 2 and considering the following assumptions:

• A hermetic compressor with a mechanical efficiency of 0.90 and an electrical one of 0.95 is supposed.

• Representing average conditions during the heating season, the following condensation temperatures (Tc) were considered: 52 °C in using fan coil heating and 48 °C in underfloor heating.

• The degree of superheat is assumed to be 10 °C and the degree of sub-cooling 5 °С.

• The evaporation temperature (Te) is -12 °С during the whole heating season. Thus, we ensure the panel collects heat from environment during the whole heating season in Madrid

• The energy consumption of auxiliary equipments (Wp and Wp’ in Figure 1) is the 8% of the total input electric power to the compressor.

• The efficiency reduction of the compressor in operating at low capacity is taken into account by the coefficient ф, which is calculated as a function of its nominal power (Wcnom) and the instantaneous one (Wc) as indicated in Figure 4.

1 -­0.8 Ф

0.6 -­0.4 -­0 0.2 0.4 0.6 0.8 1

r=Wc/Wcnom

Fig. 4. Low capacity operation coefficient Energy balance for an individual collector

Thermodynamic solar panels may be installed in many different ways, but one of the most typical is that one shown in Figure 5. Although this is the configuration studied in present work, other configurations can be also simulated by correcting some initial assumptions in the proposed model.

This thermodynamic solar collector absorbs heat from both its two faces. The exposed face receives energy from both solar radiation and environment, while the non-exposed one is supposed not to receive solar radiation. Thereby, the heat transfer occurred on the exposed face is due to radiation and convection; on the other face, convention is the only heat transfer process that takes place. As evaporation temperature is -12 °C a thin ice layer is formed over panel surfaces, making more difficult the heat transfer process. The total heat collected by the panel is obtained by making an energy balance for each face separately. This energy balance was derived from diagram shown in Figure 6, where Rs represents the net solar radiation, Ta is the ambient temperature, t is the thickness of aluminium plate (1.5 mm), Tr is the refrigerant temperature inside the collector tube,

Ts is the surface temperature at point x, h represents the heat transfer coefficient from ambient to collector and 2L is the distance between two consecutive tubes. Neglecting any frictional pressure drop in the collector, the refrigerant temperature inside the collector tube remains constant at the saturation value corresponding to the saturation pressure in the collector. This energy balance permits us to know not only the heat gained by the panel, but also its surface temperature.

The number of panels needed to cover the total heat demand is obtained by comparing the energy absorbed by an individual collector and the energy demanded in the evaporator. Then, an estimation of CO2 emissions associated to energy consumed by DX-SAHP is made. Next, these emissions are compared with those ones corresponding to natural gas and gasoil heating systems. It was assumed the seasonal performance of natural gas heating system is 0.85 and that one of gasoil system 0.81 [4]. The following ratios of carbon dioxide emissions were considered: 0.358 kg per kWh of electrical energy [1], 0.203 kg per kWh of natural gas and 0.266 kg per kWh of gasoil [4].

2. Results

As above mentioned, the comparison of energy demanded in evaporator with energy absorbed by an individual collector gives the number of panels needed to install. Figure 7 shows this comparison, made for a fan coil installation using R134a. As showed in Table 1, seasonal results for the other studied configurations are not too different.

Fig. 7. Comparison of energy demanded and collected in evaporator

Table 1. Energy balance for evaporator during heating season Heat exchanger Refrigerant Energy demanded (kWh) Energy absorbed by one panel (kWh) Number of panels to cover 100% Fan-coil R134a 13 172 3 111 16 Fan-coil R407C 12 658 3 111 15 Underfloor heating R134a 13 621 3 111 16 Underfloor heating R407C 12 915 3 111 15

One should note that installing the number of panels given in Table 1, the whole seasonal heat demand is provided by the DX-SAHP system. However, when installing 8 panels 95% of seasonal heat demand will be covered in any case. The electrical energy consumed by the different configurations of DX-SAHP in order to cover the whole heat demand is given in Table 2, where the results of seasonal COP are also shown. It is clearly seen that energy consumption is lower when using underfloor heating systems. This is because, in operating with fan-coil, condensation temperature is higher and therefore compressor needs more input energy. Likewise, it should be noted that energy consumption decreases when using R134a as refrigerant.

Table 2. Energy consumed by DX-SAHP and seasonal COP Heat exchanger Refrigerant Seasonal COP Energy consumption (kWh) Daily averaged consumption (kWh) Fan-coil R134a 2.24 8 002 44.21 Fan-coil R407C 2.08 8 627 47.66 Underfloor heating R134a 2.41 7 455 41.11 Underfloor heating R407C 2.16 8 314 45.94

Finally, Figure 8 shows the comparison of CO2 emissions associated to DX-SAHP using fan-coil and R134a with aforementioned conventional heating systems. The results corresponding to other configurations are indicated in table 3.

Table 3. CO2 emissions associated to different heating systems during heating season Heating system Kg of CO2 Gasoil boiler 5 896 Atmospheric natural gas boiler 4 288 DX-SAHP + fan-coil + R134a 2 865 DX-SAHP + fan-coil + R407C 3 089 DX-SAHP + underfloor heating + R134a 2 669 DX-SAHP + underfloor heating + R407C 2 977

It is clearly seen that any configuration of DX-SAHP can be an available solution to reduce CO2 emissions associated to building heating. DX-SAHP operating with underfloor heating and R134a

is the most efficient system. Thus, it is able to save roughly 55% of CO2 emissions with regard to conventional gasoil system and 38% with regard to atmospheric natural gas one. Using DX-SAHP with fan coil and R407C, CO2 emissions are 16% higher than using underfloor heating and R134a.

3. Conclusions

A new simulation model was developed to study DX-SAHP in space heating applications. After applying this model to four configurations of DX-SAHP, it is concluded that any of them is an available solution to save energy and reduce CO2 emissions when comparing with conventional heating systems. Likewise, the simulation carried out indicates that installations using underfloor heating and working with R134a as refrigerant are more efficient. On the other hand, simulation results for Madrid indicate that an individual panel exposed to solar radiation may absorb roughly 15 kWh per day. Furthermore, the predicted results for this region indicate that installing eight panels it could be covered 95 % of seasonal heat demand corresponding to a typical detached house.

References

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