Solar Thermal Panels and Heat Pumps Hybrid Systems

Figure 1

The addition of solar thermal panels in conjunction with Heat Pumps to form a hybrid system certainly does have the capability of creating a complete energy efficient solution for any hot water installation regardless of volume requirements.   Solar thermal panels will contribute their maximum free “solar” energy only during the daylight hours when the sun is shining brightly and without any cloud cover.   Air to water Heat Pumps on the other hand will continue to contribute free “solar generated” energy from the air during the daytime, rainy or cloudy periods, as well as night time.  Another point to consider is that air to water Heat Pumps will operate at optimum efficiency and therefore contribute the maximum level of free “solar generated” energy during the same periods of the day as the solar thermal panels which is, during the highest ambient temperature periods of each day.   Therefore when solar thermal panels are used in conjunction with Heat Pumps in a hybrid system the “actual” achievable saving in energy will be reduced because both systems enjoy a “positive” COP.  When the Solar thermal panels are contributing the maximum amount of free “solar” energy, the Heat Pumps will also be contributing the maximum amount of free “solar generated” energy from the air.

Figure 1: above shows a typical layout with solar thermal panels connected with Heat pumps. The daily energy contributed by solar thermal panels can be calculated by using the following formula:

kW = Collector area x Collector efficiency x Average Daily Radiation (Mj/m²) ÷ 3.6

Figure 2

In addition, solar thermal panels actually lose efficiency as the water temperature rises as shown in the chart - Figure 2.  For a good quality solar thermal panel operating in the highest instantaneous solar radiation conditions it can be seen that the efficiency at a point where the temperature difference between the solar panel and the ambient temperature is around 30 degrees Celsius the efficiency is approximately 75%.  If we select as an example the average daily radiation of 17.56 Mj/m² per day for Manila Philippines the equation would be:

 kW      = 1m² x 0.75 x 17.56 Mj/m² ÷ 3.6

            = 3.66 kW/m² per day of available solar collector aperture area.

Therefore, during the maximum energy collection period for the solar thermal panels they will contribute approximately 3.66 kW of energy for every one square metre of collector aperture area that is installed.  In the same location (Manila - Philippines) during the same period & same climatic conditions that has been used to assess the solar thermal panels, the Heat Pumps would also be operating efficiently and producing a COP of approximately 3.8 to 4. 
Then assuming that the solar collector panels were NOT installed as part of the hybrid system and only the Heat Pumps were required to contribute the total energy requirement during that same period.  The 3.66 kW/m² per day of “free” energy contributed by the solar thermal panels would only require approximately 0.96 kW of energy to be “consumed” by the Heat Pump to produce 3.66 kW of output using a COP of 3.8 for the Heat Pumps.  However, in the location of Manila – Philippines during the peak high ambient temperature periods the COP for the Heat Pumps may even be around 4 of higher which would result in even less energy to be “consumed” in order to produce 3.66 kW of heating output.  Therefore, the lower calculated energy consumption, for the Heat Pumps that would be required to produce the same heating output as the solar thermal panels becomes the “actual” or real amount of energy saving that should be used to calculate the ROI or payback in relation to the cost of supplying and installing the solar thermal panels in a hybrid system with Heat Pumps.

By: Mel Peatey.