The Pros and Cons of Heat Pumps

The Pros and Cons of Heat Pumps

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:

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). 

A air source heat pump uses outside air as the heat source for the refrigerant (It’s sucked over the evaporator in those big fan units you see outside the building). 

Split units move the refrigerant in and out to the outside unit- these use smaller lines/ less heat loss than the ‘mono blocks’ where the heating water is moved in and out of the building, but can a bit more complex to install as you need to be an F-gas engineer. 

The outside air temperatures are highly variable in Scotland- so although the ASHP can still extract heat at minus temperatures (indeed even at -25 to 30’C) it will be less efficient than at warmer temperatures. Fortunately Scotland’s climate is pretty good for air source as winter temperatures are between 0-5’C on average. 

Also although the physics of heat pumps suggests, if given a free choice, you would use a ground collector, in my experience air source manufacturers are innovating faster than ground source. Its likely as a consequence of them selling 90% air source to 10% ground source in the UK market. This means we are seeing higher and higher COPs all the time in ASHPs.

These exhaust air heat pumps are becoming very popular in ultra low energy housing such as Passiv Haus.  You need very careful design with these systems as the heat balance in the property is critical and they are not designed for heat loads more than say a few kW. 

 

Also they can be used to capture say the exhaust air from chiller fans in a supermarket and feed this ‘waste energy’ back to the building’s hot water demand say.

These are essentially ground source heat pumps but using collector loops in lakes, rivers, aquifers. This has the double advantage of higher on average mean temperatures for the collector (thus increasing efficiency) and potentially reducing the size of the collector (as energy density and conductivity of water is higher than ground). It also reduces the cost of the install. 

Foundations etc can be used- if you have a new build and short of space or you may be using pile foundations already. Some great projects have been done her with diurnal or seasonal storage using just thermal mass of concrete or PCM (Phase change Materials). When coupled with summer recharge (dumps free cooling back into the slab or foundation) or solar thermal recharge you can see great efficiencies.  

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

There is a short video by Home Energy Scotland below

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The cost of a proper design is generally 'won' back pretty quickly within the project costs

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