Heat pumps as a viable renewable-energy source

Published:  21 February, 2005

buffer store
Heat pumps are best suited to generating heat over many hours — using buffer storage to smooth out demand fluctuations.

The ability of heat pumps to extract renewable energy from the ground continuously for relatively little energy input gives them a key role to play in the future of space heating. STEWART PURCHASE explains.

2004 was the year when heat pumps moved from being mainly project-based one offs to a viable renewable-energy source. Not surprising really, as ground-source heat pumps offer probably the best return per £ invested of the alternative-heating approaches in terms of reduced carbon-dioxide emissions and energy consumption. Furthermore, the equipment is essentially simple with proven low service costs. How many times have you had a service engineer out to your refrigerator?

Design

Where heat-pump installations do require a different approach is in their design. Heat pumps challenge us to think holistically about the building process, the energy supply, the capital cost and the client’s requirement — as well as system design and installation. The architect, building-services designer, contractor, client, supplier of the heat pump and geothermal specialist have to talk to on another — a true Egan approach.

Why is this approach necessary for a heat-pump project? Because a heat pump project is a complete energy-production and utilisation system in its own right. Also, the capital cost of the heat pump and, especially, the ground works are directly related to the required energy output and form a large part of the total cost. Every kilowatt of heat output required adds proportionately to the capital cost of the heat pump and ground works.

System design is another area where new thinking is required.

We design heating systems with boilers that are hares — to get the system up to temperature and then turn off.

Heat pumps are tortoises. They work slowly and steadily to generate heat over many hours and use a buffer store to smooth out demand fluctuations. This steady base-load generation also works well with other alternative energy inputs — especially solar thermal panels.

We are now seeing more and more projects moving through to a successful conclusion by this teamwork approach.

What is clear is that there is a learning curve to go through on this integrated approach It is not complicated, just different and to judge by the numbers of designers, contractors and others who have joined our heat-pump training sessions at Viessmann, something that many in the industry now recognise as essential.

Ideally the project might run as follows.

The client would like to consider a heat-pump approach for environmental reasons and long-term cost savings.

From the initial building plan, a likely heat loss is worked out, and the geothermal specialist assesses the required ground loop or bore hole.

Standby

This iteration may suggest that the building heat loss is further reduced by additional insulation.

In parallel, the heating system is considered. Often the economical case for a standby fuel for peak loads can reduce the heat pump capital costs at minimal addition to the annual running costs. A flow boiler or immersion heater are possibilities.

A firm system design is produced, with a control system and buffer tank to allow input from other sources ( perhaps solar panels) and gives a heat reserve for peak demand

The final design then settles the heat pump sizing and the ground extraction pipe.

Stewart Purchase is managing director of Viessmann UK, Hortonwood 30 Telford Shropshire TF1 7YP.
info-uk@viessmann.com



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