The risk of condensation of water vapour is the most important factor limiting the use of cooling devices. To prevent this from happening, ensuring that the temperature of their surface is at least 1 K above the dew point of the ambient air is crucial. For this purpose, the technology is able to monitor the value of the ambient temperature, humidity and dew point temperature. Technology is set to keep the temperature of the thermal panels as low as possible but still above the dew point. Simultaneously, technology is able to compared energy gained from the PV and consumed by TEC modules (or HP)—in case of low energy gains are turning off some TEC. Of course, this automatic control mode can be by-passed and the technology can be operated manually without all restriction.
The heating and cooling performance of the accumulation device is also important. For this purpose, it is important to know the specific power, which is given by the value of the heat transfer coefficient and the active area. The average heat transfer coefficient is about 6.4 Wm−2K−1 and the active area is 5.2 m2. So, the specific heating/cooling power is around 33.2 WK−1. It is, therefore, clear that the cooling power is dependent on the temperature difference between the surface of the panel and the ambient temperature. The higher temperature difference means higher the power. As previously mentioned, the used PCM has a temperature range of phase change between 18 °C and 22 °C. When active cooling is on, the sensible heat is removing as first (above 22 °C), then the phase change occurs (between 22 °C and 18 °C) and then the sensible heat is removing again (under 18 °C). The surface of the panels can be cooled to the dew point, but the most amount of energy is able to accumulate during the phase change. If the panel temperature is around 18 °C and the ambient air temperature is 25 °C, the cooling power is approximately 230 W. For example, at a temperature difference of 15 °C, the cooling power is about 500 W.
As previously mentioned, the proposed technology is designed to be able to use the power produced by the PV. Energy production covers the consumption of TEC modules with an average cooling output of about 550 W. This power is higher than the average cooling power of the accumulation panel. However, TEC modules lower the latent heat of the PCM panel, i.e., the surface temperature does not change much, but accumulation panel is able to accumulate much more energy for later utilisation. If the phase change limit value is reached, the surface temperature of the panel starts to decrease rapidly. The lower temperature increases cooling output, but when the technology is turned off, the temperature returns quite fast back to the phase change range.
During designing and initial measurements was discovered a technical problem with overheating in cooling mode. This is described in the following subsection. After this, there are presented results of measurements with solved the problem out.
3.1. Technical Problem
As we already know, heating can be provided by the electric heating foils inside the panels or by heating the panels by hot water from the hot water tank—heating by the heat pump or an electric boiler. The technology has been tested and measured in all heating modes. These measurements were successful and the technology was effective without obvious problems. After that, test measurements were performed in cooling mode. As mentioned earlier, cooling can be done by the heat pump or by thermoelectric modules. The cooling mode with the heat pump was made and measured without any problems.
There were some difficulties in measuring of the thermoelectric cooling. Above all, cooling is ineffective. Instead of decreasing the indoor temperature, it was increasing. Measurement of the original design of the technology and the thermoelectric cooling mode is shown in
Figure 5 below. As can be seen, even when the room was cooling by the technology, the temperature was growing. The temperature got to start growing up very fast when the cooling by TEC was turned on. The indoor temperature was about 27.2 °C at the beginning of the measurement. After active cooling by TEC, it reached up to 30.8 °C. The result was: the technology transferred more heat to the room than it removed.
After some time, it has been found out why this problem occurs. A clear indicator was the thermographic diagnostics of the accumulation device and all parts of the thermoelectric modules with coolers. In the thermographic images below, see
Figure 6, it can be seen all six thermoelectric assemblies and their temperatures. The image captures the state of the device with active thermoelectric cooling. The lowest temperature on the surface of the hot side is about 20 °C and the highest temperature is about 45 °C. This growing up the temperature is caused by connection of the liquid coolers in series. In this place, it can be said, it is also a little mistake of design. Maybe, it could be better to use the parallel connection. On the other hand, it is possible to get lower outtake temperature of chilled water.
In figures below, see
Figure 7, it is possible to see details and temperatures of both sides of the coldest (first in the row) and hottest assembly (last in the row).
The problem with the overheating came from the waste heat of the TEC modules. The thermoelectric modules are fitted with the liquid heat exchangers on both sides. These are for the extraction of cold and waste heat. The surface of the heat exchanger (cooler) on the warm side is very high. Therefore, the waste heat is radiated to the surroundings from the surface of the cooler. The problem can be solved by additional insulation of TEC modules and hot-water pipelines.
The problem has been solved by separating the space with thermocouples from the monitored space. Around the thermoelectric system, it was enough to build additional insulation to keep the heat in the enclosed space. However, there is still a freestanding cold water tank for absorbing the waste heat from the thermoelectric modules. On the other hand, the water tank is additionally cooled by the heat pump at a minimum temperature of 10 °C. Therefore, it was not necessary to separate the cold water tank from the monitored space. So, the final improvement was additional hot water pipes insulation. All of the following measurements in this article were performed with this adjustment.
3.2. Measurement
In the following paragraphs, there are presented measurements and results of the technology in different cooling modes.
The first measurement is focused on common passive mode—the most common use of the PCM. In this mode, the PCM-based panels reduced the temperature fluctuation and also stabilised the indoor temperature during passage of a few days of warm weather. An example of the system’s behaviour can be seen in
Figure 8. This passive mode managed to stabilise the indoor temperature between 23.2 °C and 24.0 °C when the outdoor temperature oscillated between 5.3 °C and 22.7 °C.
Another experiments focus is the active cooling mode. The cooling cycle of the heat pump was used in this measurement. Results are shown in
Figure 9.
Every measurement is made of two cooling cycles. One cycle lasted two days. Active cooling was turned on during the first day. During the next day, the panels were left to just accumulate heat from the room. The cycle was then repeated.
During the first cooling cycle, the time it took to cool down of the surface temperatures from 21.4 °C to 17.9 °C for the unmodified surface and from 22.6 °C to 20.0 °C for the modified surface was around 7.5 h. The indoor temperature lowered from 24.3 °C to 23.1 °C during the day with the active cooling turned on. The indoor temperature continued to decrease until the next morning. The following day the technology was still left unpowered and the indoor temperature was increasing, conforming with the outdoor temperature.
In the next cycle, the measurement followed the previous one very closely. The outdoor temperature was a little bit higher in this cycle and the active cooling was in operation longer. The time it took to cool down from 21.8 °C to 17.6 °C and from 22.4 °C to 19.3 °C amounted to about 8.5 h. The indoor temperature lowered from 23.8 °C to 22.6 °C. The second half of this cycle was very similar to the second half of the first cycle. During these two cooling cycles, the sum of energy removed from thermal panels was approximately 7.9 kWh.
The following measured mode was focused on thermoelectric coolers. Thermoelectric assemblies were powered by the installed photovoltaic system. As was mentioned earlier, this measurement was also done in two cycles. Both cycles were measured during days with the outdoor temperature reaching up to 25 °C. In this cooling mode, the reduction in the indoor temperature by the thermal panels was from 24.0 °C to 22.9 °C and from 24.1 °C to 23.2 °C, respectively. Measured parameters are shown in
Figure 10.
In
Figure 10, it is visible that the first cooling cycle was activated before 9 o’clock. Deactivation was at 15 o’clock. At first, the indoor temperature was rising in accordance with the outdoor temperature. Once the technology was activated, the indoor temperature began to fall. The temperature continued to fall all the time when the technology was turned on. After deactivation, the outdoor temperature continued to steadily fall, up to the next morning. Following that, the indoor temperature has risen again in accordance with the outdoor temperature.
The subsequent cooling cycle took the similar course and had similar results. The technology was activated after 9 o’clock. Deactivation was before 18 o’clock. The indoor temperature was falling as in the previous cycle, at first. But during the last measured day it started to rise fast. This fast increasing was caused by the higher outdoor temperature and also by the fact that the temperature of the PCM panels has gone over the range of phase change. So, the accumulation device absorbed just sensible heat, and it was not able to store latent heat.
From the obtained results, it can be determined that area of the thermal panels, compared to the total area of the measured room, is low. This can be solved by increasing heat flux. Forced convection is one option. The thermoelectric cooling (also heat pump) was powered by the photovoltaic system. This system delivered over 35.5 kWh throughot both cycles of measurement (four days, cooling by TEC). The heat which was removed from accumulation panels was totalled over 8.7 kWh. Consumed of power by the thermoelectric assemblies was over 18.5 kWh. The energy balance that is the production of energy and the consumption of it is shown in
Figure 11.
When comparing and evaluating the results, it is necessary to take into consideration that the operation and efficiency are very dependent on outdoor conditions. The comparison of both measurements is quite difficult because it is not possible to ensure the same conditions for each of them. Moreover, each mode has different conditions, for example, the speed of cooling of liquid and minimum liquid temperature. In the real conditions and operation, the temperature of the liquid from TEC could be lower than from the heat pump. In every case, it must be above the dew point temperature of course. In our both measurements was set the same temperature, about 11 °C. Measurements are shown in this article were also made when the outdoor conditions were similar, i.e., sunny days and outdoor temperatures between 7 °C and 25 °C.
Table 1 shows the balance of energy produced by the photovoltaic system, energy consumed by the heat pump or by the thermocouples and energy removed from the thermal panels. The table shows values measured during one cooling cycle in the previous year (2016) and values measured during two cycles in this year (2017). In both cases, the one cooling cycle was about two days long. During the first day, the technology was turned on, and the rest of time was turned off.
The installed heat pump has average Coefficient of Performance (COP) over 3.5 and Energy Efficiency Rating (EER) over 2.7. The table shows energies removed from the thermal panels (PCM), the energy produced by the photovoltaic system (PV), energy consumed by the heat pump (HP) or by the thermoelectric coolers (TEC), and the last one is the balance of these energies (SUM).
Energy balance remains possitive in all of these examples. Performance and efficiency of the system are, however, dependent on both weather condition and more importantly on the energy source being applied in proper range of temperature. The biggest disadvantage of the thermoelectric cooling is production of large quantities of waste heat on the thermocouples opposite side. This waste heat caused also our technical issue mentioned earlier in the manuscript. The hot side of TEC is cooled by a liquid cooler connected to the cold water tank. This waste heat can be used to save energy of another system. For example, using the waste heat to pre-heating water for another utilisation.
The waste heat from the TEC was up to 35 MJ (9.76 kWh) during both cooling cycles (four days). This energy could be used for preheating domestic hot water (DHW). The amount of heat needed for preparing DHW (
QDHW) can be calculated by the equation below.
where
z | system heat loss coefficient | (-), |
| intake water temperature | (°C), |
| hot water temperature | (°C), |
| density | (kg·m−3), |
| specific heat capacity | (J kg−1 K−1). |
The following example is very simplified. However, it can show some of the possibilities of using the waste heat. We want to know the average volume flow during four days of these conditions: intake water temperature is about 15 °C, outtake temperature is about 40 °C and the heat loss coefficient of the ideal system is 0.
The results show that the waste heat could be used for preheating DHW from 15 °C up to 40 °C with volume flow about 84 L per day. The value corresponds to the amount of DHW consumed by two persons in the ordinary household per day.
Another possibility is using the waste heat for heating of a swimming pool. In case of the standard swimming pool with volume of about 10 m3, the theoretical maximum temperature difference is about 0.7 °C. This value is quite small, but it is better from the point of view of the almost constant temperature and flow.