Solar Thermal Collectors Connected to Heat Pumps

There can be some advantages gained by connecting Solar Thermal Collectors to Heat Pump water heaters, but there are some important points to assess before this combination can be recommended.  The first point to consider is the economics of the project.   In other words will the addition of Solar Thermal Panels provide sufficient reduction in traditional energy use, which can then be calculated as a cost reduction when compared to the actual initial cost of supplying and fitting the Solar Thermal Collector Panels?

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There is an unwritten expectation for energy efficient products such as Solar Thermal and Heat Pump systems to provide a reasonable “payback” period, and this can sometimes be linked to the expected life cycle of the system or probably more closely linked to the warranty period offered for the products involved.   If the capital cost can be recovered within the warranty period then it is worth being given further consideration. However, when assessing the value of adding Solar Thermal Collector Panels to a Heat Pump installation it must also be remembered that during the daylight hours when the Solar Thermal Collector Panels could be contributing to the energy input of the system, that is also the period of each day that the Heat Pump units would be operating at their highest efficiency as well so the amount of purchased (input) energy for the Heat Pump units during the day time period could be between 3½ to 4.0 times less than the actual output that is produced in the form of hot water depending upon geographical location, ambient temperature and humidity.

The thermal contribution to a Heat Pump system from Solar Thermal Collector Panels is governed by the amount of radiation that is available at any given time and this can be intermittent because of varying weather conditions.   Therefore it is important to recognize that the Heat Pump units must be considered as the primary heat source and sized accordingly to be able to meet the total hot water requirement of the project due to the reliability of Heat Pumps being able to produce energy efficient hot water in all conditions including daytime, night time, as well as cloudy or rainy periods.  The Solar Thermal collectors will therefore only provide supplementary energy input during periods when there is a favorable level of radiation available for collection.  A number of factors can influence the decision to add Solar Thermal Collectors to a Heat Pump installation or not, but generally it has been found that in Regions where Government or Utility incentives are offered for the inclusion of Solar Thermal Collector Panels and the value of that incentive covers the cost of the Solar Thermal Collector Panels then obviously the economic equation is positive however, when no incentives or subsidies are available then there are some questions that need to be asked as to the added value of the Solar Thermal Collector Panels fitted to Heat Pump Units.

By Mel Peatey