The mechanics of the heat pump were arguably discovered by Lord Kelvin in Glasgow in 1852. The physics of heat pumps rely on a gas changing ‘phase’ and can thus essentially can be thought of as an extremely effective method of heat transfer.
The heat pump is a machine that follows the ‘Carnot cycle’- essentially the heat pump is strongly tied to the input and output temperatures required. Thus Heat pumps, by their nature, are very sensitive to the design conditions and the heating/ cooling loads for a property.
This means that even a small change in the design conditions has a large influence on say the required number of boreholes, or the size of the air evaporator unit required. Obviously more boreholes or bigger fan units increase your project cost.
The internal distribution (radiators, fan coils or underfloor etc) system effectiveness is much more sensitive at the lower flow temperatures typically associated with efficient heat pump operation.
It can be calculated that 1 to 2° change in the ambient air / ground conditions and/or the flow temperature output can affect efficiency and performance in real life conditions by up to 10%. Hence short cuts or quick ‘rule of thumb’ design tends to oversize the plant requirements leading to significant installation cost, or can undersize and lead to long term running cost and performance issues.
Heat pumps are essentially refrigeration systems whereby the heat ‘rejected’ from the condenser (the heat exchanger) is used usefully as an output into your heating demand for the building. As the heat output is much greater than the input energy from the generator (typically an electrical powered compressor) this means heat pumps have massive advantage in “efficiency” (technically known as coefficient of performance COP) – typically 300% or more. The heat absorbed at the ‘input’ stage can come from any source- some notes are below. Click on each point to expand more info.
In ground source heat pump system, heat is extracted from the fluid in the ground connection by a geothermal heat pump and distributed to the building. The fluid is then re-warmed as it flows through the ground. In cooling mode, the process is reversed -“active” cooling. A quite neat trick is that a GSHP can do ‘passive’ cooling as well if designed for it -i.e. circulating heat from the building back into the bores but with out using compressor energy.
Thus although an expensive way of installing (drilling boreholes or digging trenches for collector pipe costs a lot), it does have the advantage of pretty much stable temperatures (approx 8-11’C depending on where you are in the country and your altitude) year round. It also is heavily dependent of rock type and water saturation level for rate of heat transfer (conductivity).
Conclusion
- Keep the temperature of the input energy as high as possible. Use warmest air temperature – so you may position the unit on a south facing side of the building say, and operate it during day to store energy for use in the evening.
- Keep the temperature of the output energy requirement as low as possible. This means good energy efficency and proper emitter design so the flow temperatures can be dropped.
- Heat pumps tend to use fluorinated hydrocarbons (HFCs) as a refringent. Not that long ago we were working with refrigerants with Global warming potential of 1500-4000 times that of CO2. increasingly we are specifying refrigerants with ultra low global warming potential (0-3 GWP) as in the real world there is leakage at set up and seepage from systems.
- The maximum temperature for refringent are typically 55’C- however in the last few years there are now other refrigerants coming to market that are commercially available. (such as CO2) to give 90’C with careful application.
- Heat pumps are proportionally expensive to install- use cheap fossil fuel appliances or electrical as standby /emergency back up or for a small (say 1-2 days a year) for top up requirements.
- Emission factors are dropping all the time- used to be 519 gCO2/kWh for grid electricity. There also no local NOx, SOx or PM emissions.
- Smart systems can do simultaneous heating and cooling or use ‘passive cooling’ (free cooling) in the summer
