2.1. System Components
This section outlines the whole system and introduces the individual components, noting the science, thermodynamics and specifications that will be relied on in the next section to properly size and cost the integrated package.
The reference solar PV system for this study is modelled by that at the Melbourne home of one of the authors. This 6.5 kW microinverter system consists of 20 panels rated at 325 W each. Eight of these panels face northeast and twelve face northwest. The average energy productions for the coldest and warmest months (July and January [
15]) are 13 kWh and 31 kWh, respectively. Typical daily productions on completely overcast days in July and January are 7 kWh and 10 kWh, respectively. This system cost AUD 7700, after government rebates.
A vapor compression refrigeration cycle moves energy between thermal reservoirs. It is implemented as a heat pump (HP) consisting of an evaporator, compressor (pump), condenser and expansion valve. A HP provides the mechanism to transfer outdoor heat from the air, also known as the ‘source’, into a ‘sink’, which in this case for this study is conditioned hot water that is ultimately used to heat a building. When the objective is to cool a space, transferring heat from a building to the outdoors, this is called a refrigeration cycle. If the heating or cooling is not required immediately, it can be stored in or extracted from a thermal battery (TB).
For the proposed heating system shown in
Figure 1, an air-to-water HP transfers heat from the outdoor air to water in its buffer tank. This water is then circulated to a bank of TBs to store enough thermal energy to support space heating in winter (e.g., by hydronic heating and radiant panels). This energy is extracted as required by another water loop.
For the cooling system, heat flows in the opposite direction: from the building space to the outdoor air. This could be accomplished by using the same HP system to extract heat from a different TB (storage tank), providing a cold reservoir with which to cool the building via a hydronic loop connected to chilled beams, for example. However, as will be explained later, there is no need for TBs in the case of cooling studied here. It is more straightforward to cool the space directly via the evaporator of an air-to-air HP split system, as shown in
Figure 1.
It is acknowledged that, ideally, the cooling side system could not only cool the room air but also heat the DHW with the room heat rejected by the HP. Such systems are not commonly available in Australia for low-capacity applications, such as the case considered in this paper. Furthermore, the company that manufactures the high COP HP selected for the cooling (noted below) does not offer this option of also providing DHW. The authors thought it preferable to rely on such a highly performing HP even if it could not also provide DHW. Instead, the heating side HP is used to meet the DHW needs during the cooling season as well as the heating season. Regardless, Melbourne is a heating-dominated environment, and the cooling system is really only used intensely for two months of the year. Therefore, the missed opportunity to use the cooling side HP for DHW is not significant. This arrangement is in the interest of simplicity and to ensure the use of available high-performance off-the-shelf components. For the much warmer Brisbane, Australia, a cooling system that also provides DHW would be the preferred choice because Brisbane is in a cooling dominated climate.
The heating and cooling systems operate on solar PV energy passing through a bank of electrical batteries (EBs). Domestic demand can draw more solar energy for loads (e.g., appliances) or it can be exported to the grid, if so configured. Otherwise, no additional energy is collected.
The thermal performance of an HP system is given by the coefficient of performance (COP). When operating in the forward or heating mode:
where:
Q’delivered = the rate of heat delivered to the high temperature reservoir (W);
W’compressor = the power input to the compressor (W).
When operating in the reverse or cooling mode:
where:
Q’removed = the rate of heat removal from the low temperature reservoir (W);
W’compressor = the power input to the compressor (W).
The coefficients of performance for heating and cooling are usually greater than 1. The higher the coefficient, the more effective the HP.
The thermal battery (TB) considered in this study consists of a core of phase change material (PCM) surrounded by water.
Table 1 presents the properties of TBs from Sunamp Ltd. (Tranent, UK) suitable for the heating and cooling processes modelled here, although, as will be explained later in this paper, cooling TBs will not be needed [
16,
17,
18]. The cost per heating TB is approximately AUD 4600 [
19]. A similar cost is assumed for the cooling TB (as of July 2021, 1 AUD~0.76 USD).
In order to provide space heating at a later time, hot water from a HP flows into a TB, heating it from the minimum temperature of 45 °C until it reaches the melting temperature of 58 °C. Subsequent heat addition just melts the PCM at this constant temperature. Once fully melted, the temperature of the PCM rises. Most of the energy stored in the PCM is from the phase change. This leads to a denser energy storage than, for example, heating a tank of water that remains liquid. When space heating is required later, cool water from the house is passed over the PCM to extract the stored energy, eventually returning the PCM to a solid.
If TBs are used for space cooling, cold water from an HP flows over liquid PCM, cooling it until it reaches the freezing temperature of 11 °C, at which point it begins to solidify. Further heat extraction leads to more solidification without a decrease in temperature but with a significant storage of energy. Once the PCM is fully solidified, the temperature continues to drop. When space cooling is required, warm water from the house is circulated over the PCM to cool the water, eventually melting the PCM.
The TB for heating requires a minimum water supply temperature of 65 °C [
17]. This is beyond the range of most conventional HPs using standard refrigerants. An HP with carbon dioxide (CO
2) as the refrigerant allows for higher temperature ranges to be achieved and, conveniently, higher COPs.
A commercial HP was selected to provide realistic specifications. The Sanden Eco
® plus system, which uses carbon dioxide as the refrigerant, can supply water for a DHW system at a maximum temperature of 65 °C, with a COP
HP of 5.6 while operating at 0.9 kW [
20]. For the heating system studied, heated water is passed to the TB and into a DHW tank.
Ideally, the heating and cooling systems should share a reversible HP. However, a commercial reversible CO2 heat pump with air-to-water heating and a high COP could not be found. The most practical solution is to use a proven conventional HP for the entire cooling process.
The HP selected for the cooling process is the 5.1 kW-rated cooling-only (air-to-air) split system from the Pioneer (Aust.) line of air conditioners [
21]. This system has a maximum capacity of 6 kW and uses M20 hydrocarbon refrigerant. The
COPcooling for this system is 6.1, with an average power consumption of 0.83 kW.
Table 2 presents the published COPs for the HPs of the heating and cooling systems.
It is known from the ideal Carnot HP and refrigerator cycles that COPs vary with the temperatures of the hot and cold reservoirs (T
H and T
C) [
22]. For this study, the reservoir temperatures approximately correspond to the indoor set points and outdoor conditions. (There is a small difference across each heat exchanger). Since these temperatures differ from those at which the COPs given in
Table 2 were determined, the COPs should be adjusted according to the actual temperatures.
Table 3 presents T
H and T
C for the test conditions used to measure the COPs of
Table 2, with the test parameters set to the standard conditions to evaluate the performance of HPs [
23]. The operating temperatures for this study that were used to calculate the COPs are also tabulated in the last column of
Table 2 In
Table 3, the operational outdoor temperatures for the heating and cooling processes (6 °C and 32 °C) are the annual heating and cooling design temperatures for Melbourne [
24], each based on a 1% frequency of occurrence. This means that, for 1% of the hours over the year, the air temperature is equal to or less than the heating design temperature—or it is equal to or greater than the cooling design temperature. Relying on the design temperatures helps with sizing the proposed system appropriately. As for the other operating temperatures in
Table 3: the T
H for the heating process is the aforementioned maximum water temperature; and the T
C for the cooling process of 25 °C is within the usual range for the indoor temperature under cooling (though TBs, which would require a colder temperature, will not be used in the proposed system).
For the heating system, the COP is adjusted based on the temperature dependence of the COP for a Sanden CO
2 HP system [
25], and the conditions presented in
Table 3. This reduces the
COPheating by 13% from 5.6 to 4.9. The adjusted coefficient is presented in
Table 2. A similar adjustment of the quoted
COPcooling [
21] based on the test and operating temperatures in
Table 3 changed this coefficient from 6.1 to 6.0, as also noted in
Table 2.
The TB energy storage capacities presented in
Table 1 are based on heating and cooling to the maximum and minimum temperatures of the PCM, respectively (85 °C and 6 °C). While the lower temperature can be met with the chosen HP system (if cooling TBs are used), the water supply temperature to the heating TB is limited to 65 °C. Due to the thermal resistances between the supply water and PCM, it is assumed the PCM will only be heated to 63 °C. A recalculation of the energy storage yields an effective capacity of 8.1 kWh for the heating TB under the operating conditions for this study.
The manufacturer of the TB, Sunamp Ltd., is working with Trina Solar to integrate an HP with its TB system [
26]. Presumably, this could involve a refrigerant-to-water heat exchanger on the condenser side of the HP, with the water circulated directly to the TB.
However, for the heating system proposed here, it is necessary to place a buffer water tank between the HP and TB. The HP heats the water in the buffer tank, which is then circulated to the TB. Sanden, which makes the HP considered here, markets its product with accompanying hot water tanks [
19]. The 160 L size was chosen to serve as the buffer tank for this study. The combined HP and buffer tank system costs about AUD 3000 (the retail price of AUD 4200 less the estimated federal government rebate of AUD 1200).
Another water tank is required for the DHW system, otherwise the heat in the buffer tank could be quickly and completely consumed, for example, by several hot showers. The DHW tank was simulated by another Sanden 160 L water tank, for which the estimated cost is AUD 400.
Electrical batteries are required so that the heating TB can be thermally charged overnight, to run the HP in the early morning and late evening hours and to provide backup electricity on overcast days. The EB unit chosen for this study is the Enphase lithium-ion battery, which can store 1.2 kWh of energy and costs AUD 2000. The battery losses are 4% [
27], i.e., the efficiency of the electrical batteries is 96%. Several of these battery units are combined into an EB bank to meet the electrical needs identified by the analysis in
Section 3.
2.2. Performance and Equations
For the heating season (winter), the total electrical energy (
Etotal) required to meet all of the heating needs of the house is given by:
where:
DHL = design heating load (the maximum load the system must meet);
TBlosses = heat loss from TBs (kWh);
QDHW = heat required for DHW (kWh)
DHWlosses = heat loss from DHW tank (kWh);
BTlosses = heat loss from buffer tank (kWh);
COPheating = coefficient of performance for the heat pump that delivers thermal energy to system;
Epumps = energy required to run pumps (kWh);
εEB = EB efficiency.
The heat required for the DHW is given by:
where:
mwater = mass of water in the DHW tank (kg);
Cp = specific heat capacity of water (4.18 kJ/kg·°C for average of temperature range);
TDHW = water temperature maintained in DHW tank (°C);
Tmains = water temperature at mains pipe going to DHW tank (°C).
For the cooling season(summer), the electrical energy required to provide the cooling,
Ecooling, is given by: the design cooling load,
DCL, in kWh over the COP of the HP for the cooling system (
COPcooling), while taking into account the efficiency of the EBs:
The overall energy required in the cooling season is given by: