Water, earth, and air – three of the four ancient Greek elements are suitable as heat sources for electric heat pumps. Depending on site conditions, installers must recommend a suitable heat pump based on the most appropriate source. German company Mefa Befestigungs- und Montagetechnik now wants to make things easier, developing absorbers that are suitable for use with all three heat sources, along with ice storage.
With a capable absorber, an installer should find themselves able to cover almost every situation with a brine heat pump. At a press conference, the company's managing partner, Martin Schneider, presented the product, which is also used to heat the company's own production site in Kupferzell in the winter and to cool it in summer.
Called multiQ, the new absorbers are made of polypropylene polymer and have almost the same configuration for uses in the ground, on rooftops, or on a facade, in combination with PV, or for ice storage and in bodies of water.
Matters of size
A ground absorber is 7m long by 1.20m high, and a few millimeters thick. Inside, a water-glycol mixture circulates in capillaries and transports the heat to the distributor and onto the heat pump. Several of these absorber blades can be buried in a narrow trench at a depth of 2.50 m to save space. Usually, no permits are required for this in Germany, except in water protection areas. As long as the air temperature is still higher than the temperature in the ground, the heat pump can also draw the ambient heat from the roof.
The absorber for PV applications is 1.20 m wide and 5 m long. However, shorter variants are also available, for example with a length of 3.50 m. “A 5m absorber weighs 27.7 kg empty and can be lifted from the scaffolding onto the roof by two people,” said Peter Kömmelt, technical manager at Mefa Energy Systems.
On pitched roofs, vertical rails are fastened with roof hooks, on which the absorber is placed horizontally. Horizontal rails can then be screwed above and below the absorber, on which most commercially available framed photovoltaic modules can then be fastened in portrait format with module clamps.
For flat roofs, Mefa offers an east-west mounting structure on which either the absorber is covered with photovoltaics or two absorbers can be stacked. When filled, the absorber weighs 44 kg and covers six square meters of the roof. Depending on the weather, the absorber produces heat from 250 W per meter square upwards.
According to Kömmelt, measurements by the company on customer systems showed an average heat output of 500 to 750 kWh per square meter from November to March in Germany. When using two sources – geothermal and ambient air – the roof absorber is used until its heat output falls below 500 W per square meter and then switches to the geothermal-absorber.
According to the company, its absorbers have passed the Solar Keymark Certification and are both UV-resistant and hail-proof. The same glycol medium flows through all multiQ absorbers and thus allows a wide variety of operating modes. Depending on the weather, a brine that is too warm can be cooled back from the roof in the ground and regenerate the soil. It is possible to air-condition the building passively using geothermal energy or to actively emit heat via the surfaces on the roof at night.
Return on investment
At the company headquarters in Kupferzell, which was newly built in 2017, the absorber is visible as a large-scale façade system on a high-bay warehouse. In addition, there are absorbers under the photovoltaic system on the roof, under the floor slab of the building and also in an ice storage facility. Together with three heat pumps, they cover the required 500 kW of heat output and 1,200,000 kWh of heat at the site.
The ice store is primarily used to cool the machines and server rooms. In summer, the offices are passively cooled and the soil is regenerated. Thanks to the sophisticated control of the cheapest source in each case, the heat pumps achieved an astounding annual performance factor of seven, according to Kömmelt from Mefa. This means that with one part of electrical energy, mostly from the roof, seven parts of heat are produced.
With its system, Mefa wants to fight against the supremacy of the easy-to-install air heat pump. A brine heat pump with geo-absorbers is quiet, completely protected from the weather and is expected to last much longer. In addition, the lower power consumption and the higher annual performance factor of the brine heat pump would compensate for the higher investment costs after just a few years, says Schneider. In the example presented of a system in a single-family house with 8 kW of heat output and 16,000 kWh of heat consumption per year, the additional investment in geo-absorbers and underground installation would have paid for itself after six years.
In the future, Mefa also wants to enter the utility-scale market. A solar farm that stands on ground absorbers and also accommodates sun absorbers under the modules can serve as a heating center for a district's local heating network. A first project is already in the planning stage.
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