Solar energy, as either direct or diffuse radiation, is similar to air in its characteristics. A solar-source heat pump or a combined solar/heat pump heating system has all the disadvantages of the air-source heat pump, low performance and extreme variability with the additional disadvantage of high capital cost, particularly as in all cases a heat-store or back-up system is required. In areas with high daily irradiation, this may not be the case.
Each of the above mentioned heat sources for heat pumps presents some drawbacks. Presently considerable research is devoted to the technical problems involved and alternative heat sources. Also solar energy may provide a suitable heat source. Unfortunately, solar systems presently are very costly. Furthermore, the intermittent character of solar energy requires the use of large and costly storage volumes.