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.

Difference between Heat Pumps and Diesel or Gas Fired Boilers

In years gone by heating sources for large volume Commercial/Industrial Water Heaters have mainly been in the form of diesel or gas fired boilers.   These systems have always been afflicted with some disadvantages such as heating inefficiencies and the rising cost and the vulnerability of supply in some cases for these fuel types.  In addition there has also been a need to duplicate these systems with stand-by units to ensure continuous operation in the event of a breakdown or maintenance requirements.   The heating inefficiencies are caused – because only a low percentage of the gross energy that is produced from “burning” the diesel or gas, is actually transferred to the water or other fluid that is being heated.   This percentage of energy input can be between around 75% for new systems down to as low as under 50% for older systems.  The main cause of energy inefficiency in diesel or gas fired systems is that a good proportion of the heat that is generated through the burning process of these fuel sources is dissipated into the atmosphere through the flue pipe or chimney.   Further inefficiencies (up to an additional 30% reduction) can be experienced if the diesel or gas fired systems (in a Hotel for example) are required to heat the water to steam (100ºC) and then that steam is directed to a calorifer type storage tank to produce potable water at 60ºC for circulation to the various fixture points.    
In most cases when a diesel or gas fired boiler is initially installed it has to be duplicated with at least one additional identical system as a stand-by unit that will be able to handle the full hot water load requirement when the primary system requires repair or maintenance.   During these repair or maintenance periods the complete diesel or gas fired system is totally shut down and cannot contribute in any way to the heating requirements of the project; therefore the need for a stand-by unit.   However, the larger Commercial/Industrial Heat Pump products that are now available are normally fitted with multiple Heat Pump modules in the one unit that are digitally controlled in such a way that they can operate individually or together at any given time.   Provided the Heat Pump daily run cycle design is done correctly for each project there is usually no requirement to include any stand-by units thereby contributing positively to the reduction in capital cost and subsequently having a positive effect on the ROI equation.   The design of the system is critical in order to maximize the daily run cycle and at the same time keeping the daily run cycle at a suitable operational percentage that will allow the system to still contribute effectively even if one of the Heat Pump modules is isolated for service or maintenance at any time.  The non requirement of stand-by units for the Heat Pump alternative also reduces the amount of occupied floor space required in the building to house the heating systems.
Finally, a Heat Pump solution also provides a positive effect on the operating cost due to the energy efficiency of its design.   Whilst diesel or gas fired boilers produce a negative Coefficient of Performance (COP) because of the various heat losses that are experienced during their operation, a Heat Pump will actually produce a positive COP that provides a heating output in the form of hot water of somewhere between 360% to 400% more than the energy that is actually consumed by the system depending upon regional ambient and humidity weather conditions.   Heat Pumps are also capable of operating effectively in most areas of the World during the daytime, night time, or rainy and cloudy periods.  
By
Mel Peatey