Energy sources for heat pumps 
(10/01/2007)
Energy sources for heat pumps can be varied:
Heat source air
Air heated by the sun is universally available. Even at temperatures as low as -20 °C, heat pumps yield sufficient heat from the outside air. However, air as heat source has the disadvantage that it is coldest when the highest heat demand persists. Although it is still possible to extract heat from air as cold as -20 °C, the heat pump coefficient of performance is, however, regressive in line with the outside temperature. It is for that reason, that in most cases a combination with a second heat source is desired that boosts the heating system, particularly during the colder season. One particular benefit is the ease of installation of air/water heat pumps, as no extensive ground work or well drilling is required.
Heat source water
Groundwater is a good store of solar energy. Even on the coldest days in winter, temperatures of +7 °C to +12 °C are achieved. And that is where its advantage lies.The almost constant temperature level of this heat source enables the heat pump to achieve a favourable coefficient of performance all the year round. Regrettably, groundwater of adequate quality is not universally available. Where it is available, its application is certainly worthwhile.
The utilisation of groundwater requires the approval of your local water board (check local regulations). Utilising this heat source requires the drilling of a delivery and return well. Lakes and rivers too are suitable heat sources, as they also act as heat store. Your local water board will advise you about the possibility of utilising these waterways.
Energy sources for heat pumps
Heat source ground with ground collector
At a depth of 1.2 to 1.5 m, the ground remains warm enough, even on colder days, to enable an economical heat pump operation. However, this requires the availability of a property large enough to accommodate a pipe system for collecting the heat from the ground. In dry, sandy soil, the collector can extract between 10 and 15 W/m² and up to 40 W/m² in ground that carries groundwater.
An environmentally-friendly brine mixture that cannot freeze up and transports the yielded energy to the heat pump evaporator courses through the pipes. As a rule of thumb, you would need approximately two to three times as much ground area as area to be heated. If your property is large enough, you have an inexhaustible reserve of energy and ideal conditions for a brine/water heat pump from STIEBEL ELTRON.
Heat source ground with ground probe
Ground probes that are set up to 100 metres deep into the ground by specialist drilling equipment require less space. Ground probes comprise a probe foot and endless, vertical probe pipes made from PE. As with ground collectors, a brine mixture that extracts heat from the ground circulates through the plastic pipe system.
The extraction rate is subject to the ground characteristics, and generally lies between 30 and 100 W per metre ground probe. Subject to heat pump and ground conditions, several ground probes can be linked up in a single system. These systems must be notified to and possibly approved by your local water board.
Heat pump operating modes
For the type of heat pump operation, the heating technology world uses the following terminology:
Mono-mode: The heat pump is the sole provider of heating in the building. This operating mode is suitable for all low temperature heating systems up to 60 °C flow temperature.
Mono-energetic: The heating system uses no second form of energy. The air/water heat pump operates down to an outside air temperature of -20 °C. Upon demand, an electric booster heater is started at very low outside air temperatures.
Dual-mode – alternative: Down to a fixed outside temperature (e.g. 0 °C), the heat pump delivers the entire heating energy. When the temperature falls below that value, the heat pump switches itself OFF and the second heat source takes over the heating operation. This operating mode is suitable for all heating systems up to 90 °C.
Dual-mode – parallel: Down to a certain outside temperature, the heat pump alone delivers the required heating energy. A second heat source starts at low temperatures. However, contrary to the dual-mode alternative operation, the heat pump proportion of the annual output is higher. This operating mode is suitable for underfloor heating systems and radiators up to 60 °C flow temperature.
Dual-mode – partially parallel: Down to a certain outside temperature, the heat pump alone delivers the required heating energy. The second heat source starts, if the temperature falls below that value. The heat pump is stopped if the heat pump flow temperature is inadequate. The second heat source supplies the entire heating output. This operating mode is suitable for all heating systems above 60 °C flow temperature.
Read the first part of this article: Guide to heat pumps
Related categories: Heating, ventilation, refrigeration
