08 December 2020
Reclaiming more energy output than is put in sounds impossible, but it’s something that, to the layman, heat pumps appear to achieve. And the latest models offer even greater efficiencies. Tim Mitchell, Sales Director of Klima-Therm, looks at what’s hot in heat pump technology.
The science behind heat pumps is no mystery; indeed, it has been known for more than 250 years. William Cullen, a Scottish physician, chemist and agriculturalist, demonstrated artificial refrigeration – which is effectively the foundation of the heat pump’s operating principle – as far back as 1748.
The theory underpinning heat pumps (essentially mechanical devices that extract low-grade heat from one source and transfer it to another) is relatively simple. However, their operational characteristics have come on leaps and bounds since Cullen’s discovery all those years ago.
The latest heat pumps are compact, exceptionally efficient and effective in both cold and warm climates. A typical heat pump system is designed around the concept of the latent heat of vaporisation. Thermal energy from the outside environment is moved into the air or water systems inside a building through the evaporation of a working fluid. The vapour produced is then condensed, releasing thermal energy into the system.
Perhaps the single most important advantage of a heat pump used for heating is that it upgrades the heat and delivers it at a higher temperature than the source from which the heat was extracted.
The biggest benefits, however, revolve around heat pumps’ environmental performance – they are exceptionally efficient and therefore reduce carbon emissions compared to the traditional burning of fossil fuels.
A bonus of heat pumps is that reverse cycle models can also provide cooling in summer. Using heat pumps for heating involves collecting low grade heat from the air, water (for example, lakes or rivers) or from the ground. This heat is then upgraded by using a refrigerant circuit incorporating an electrically driven compressor. It can then be delivered at a suitable temperature for space heating or for the heating of sanitary hot water.
For cooling, this process is simply reversed. So, low-temperature heat is collected from inside the building and then rejected to the atmosphere, water or ground.
They can also go a step further by offering combined dehumidification and process drying and heat recovery:
- Heat pump dehumidifiers are specifically designed to remove water vapour from moist air using an electrically driven refrigeration cycle. They are widely used to improve personal comfort, to protect building fabric and stored goods or materials, and to dry industrial products. They work by circulating the moist air over the evaporator of the refrigeration system. This reduces the temperature of the air, which causes the water vapour to condense. The resulting condensate can be then drained away.
- The latest heat pump technology can use waste heat to produce high temperatures suitable for industrial drying processes, which typically account for 12-25% of the total energy demand in industrial processes. The EU-funded DryFiciency project (https://bit.ly/3an6Aue), for example, is looking into technically and economically viable solutions for turning waste heat into usable heat at temperature levels of up to 160°C.
- Heat recovery is a particularly powerful weapon in the environmental armoury because it saves even more energy than a heat pump can save on its own and reduces emissions still further. In its broadest sense, heat recovery refers to waste heat and cooling recovery and utilisation. It can range from a simple heat exchanger for energy recovery to using a heat pump to translate low grade (low temperature) rejected heat to usable heat in a process.
However, although this last development further improves the output of heat pumps, unleashing their full power can be achieved by combining heat pumps with other technologies to create hybrid devices.
Our own award-winning Rhoss EXP/HT four-pipe heating and cooling system, for example, is a hybrid system that can produce independent cooling and heating, just like a traditional chiller or heat pump, but the innovative part of the system is the use of a third heat exchanger within the machine so that, when cooling, the heating can be essentially free due to heat recovery, or vice versa.
The system comprises two parts. A hybrid four-pipe air or water source heat pump produces simultaneous or independent cooling and heating (using heat recovery wherever possible for ‘free’ heating or cooling), with typical chilled water temperatures of 6/12°C and hot water temperatures, typically up to 55°C.
A high temperature water source heat pump is then used, with the recovered heat being the source to raise the temperature of the domestic hot water or space heating loop to up to 78°C with an impressive CoP of up to 6.0.
Since it is an integrated heat pump solution, there is no need for a separate boiler which results in a significant plant space saving and lower equivalent CO2 emissions in operation.
The Rhoss EXP/HT typically boasts operating energy cost savings of around 22% and 28% reduction in carbon emissions compared to a traditional chiller and boiler four-pipe solution (without water source heat pump). Any additional cost for the hybrid heat solution over a chiller and boiler system is typically recovered within just over two years from the energy cost saving.