In Germany, heat pumps have been gaining ground in recent years: Since 2011, almost 60,000 heat pumps were newly installed per year (latest data: 58,000 in 2014). As geothermal heat pumps are a costly alternative, air-sourced heat pumps have become the most popular option, particularly for new buildings. They account for 68 % of the newly installed heat pumps for space heating in 2014. Their weak point: Most of the heating energy is needed when the outside air temperature is low. Because the COP (coefficient of performance, ratio of electrical input to thermal output) of a heat pump depends highly on the input temperatures, air-sourced heat pumps work at very low efficiency when the air temperature drops to -10 °C or below. Efficiency even gets close to electric heating then. Including system losses, this can lead to an SPF below 3, considering the complete system. According to the German Energy Saving Ordinance (EnEV 2014), each kWh of electricity requires an estimated 2.4 kWh of primary energy in the current German electricity mix. An SPF below 3 lets one doubt the advantage of air-sourced heat pumps compared to conventional fossil fuel systems.
German company Consolar offers a heating system called SOLAERA, which combines a heat pump with a specially designed solar collector and a heat storage space using the latent heat released when water freezes to ice (see photo). The solar heat pump system with 18 m² of collector area, a 300 litre ice storage space and a 1,000 litre buffer tank provides enough energy to heat a well-insulated building in Germany. In a field test by a local environment group, Lokale Agenda 21, it achieved a Seasonal Performance Factor (SPF) of 5.6, putting performance even above prior simulations. The SPF describes the ratio of delivered heat to electricity consumed over the entire year. Consolar sold about 160 solar heat pump systems in total, mostly in Germany but also in Denmark, the UK, Switzerland, Belgium, France and Italy. The systems there achieved SPFs between 3.5 and 6, depending on collector field size, maximum flow temperature and heat demand.
The Consolar hybrid collector can either produce hot water inside the metallic pipes or warm air below the absorber (circulated by a small ventilator).
Consolar combines a specially designed solar thermal collector with a heat pump. In summer, when there is only need for hot water, the system works in all-solar operation mode most of the time – just like a normal solar water heater. In winter, when the collector cannot collect enough heat at a directly usable temperature level, it provides the energy input for the heat pump. Thus, even outdoor temperature levels around -15 °C can be used in combination with solar irradiation. When there is no sun at all, e.g., at night time or on a very cloudy day, the collector may also work as an external heat exchanger of an air-sourced heat pump: A small ventilator produces an outside airflow through channels beneath the absorber sheet, allowing the collector to gather energy from the air. This is the least efficient operation mode, and it is only used for a few hours each year when it is very cold.
Latent storage based on ice and water
“To secure efficient system operation, e.g., at night-time, we use latent heat store,” says Consolar’s CEO, Andreas Siegemund. The latent heat storage space works based on ice and water. Usually, it operates at temperatures slightly below and above the freezing point of water, therefore using mostly latent heat. But it can also work in temperature ranges of -20 to +20 °C. When the heat pump takes heat from the storage, water will freeze to ice; when it is loaded by solar energy, the ice will melt. According to Siegemund, the ice tank can store about 8 times as much as a sensitive water heating storage unit. Besides the ice storage space, the system comes with buffer tanks from 550 to 2,200 litres to store heat at high temperatures and use it directly for space and water heating. The system covers 100 % of the domestic hot water and heating demand in existing or state-of-the-art, energy-efficient homes up to a total annual heat demand of 13,000 kWh.
Seasonal Heating Performance above 5
In simulations, the system achieved an SPF of 4.9 based on standard ITW conditions for the German town of Würzburg and 12,035 kWh of annual heat demand with a collector surface of 25 m². In a field test in the town of Lahr in Germany’s Black Forest, the environmental group Lokale Agenda 21 measured a COP of 5.6 for a system with 300 litres of ice storage space, a 1,000 litre buffer tank, 18 m² of hybrid collectors and a 7 kWth heat pump. In lab tests with 0 °C source temperature and 35 °C output temperature, the same heat pump achieved a COP of 4.0 on its own. It must be mentioned that the conditions were favourable: The Kaiserstuhl region has very mild winters, and the winter during which measurements were taken (2013/2014) was warmer than average. Additionally, the building in the field test was very well insulated based on the German KfW40 standard. In the winter months November to February, which are responsible for the most of the main heating demand, the system achieved a performance of 3.7. In the shoulder season, it reached its optimal performance, with a still significant energy demand for heating and a very high share of direct solar energy. The peak was an SPF of 5.5, meaning basically an all-solar operation. In summer, the SPF drops again, as energy is only needed to heat water.
The SPF is also the criterion to receive the innovation incentive for heat pumps within the German Market Rebate Programme for Renewable Energies, MAP. Since April 2015, EUR 6,000 grants have been available for geothermal or combined solar and geothermal heat pumps if they are installed in existing buildings and reach an SPF of 4.3 or higher. Solar heating in existing buildings also receives a grant of 140 EUR/m² of collector area. In addition, a bonus of EUR 500 has been available as an incentive for combined heat pump and solar systems. This adds up to EUR 9,300 for a 20 m² system. The same system in a new building would be granted up to EUR 4000 for the heat pump and 500 EUR for a system combination, without any extra grant for the solar collector area.