Getting the best from CHP and heat pumps
Combined heat and power and heat pumps are both viable approaches to reducing carbon emissions. Lars Fabricius of SAV Systems explained to Ken Sharpe how their benefits can be combined.
One of the problems with combined heat and power (CHP) driven by a gas engine is balancing the generation of electricity and heat with the demand for them. Very seldom, if ever, will the demand for electricity and also the demand for heat match the output of the CHP unit.
Too little electricity can be topped up by the grid. Too much electricity can be exported — but you are unlikely to get a very good price for it.
Too little heat can be topped up by other plant, such as a boiler. Too much heat has to be dumped, upsetting the energy and economic balance of the CHP plant.
Lars Fabricius, managing director of SAV Systems has come up with a different approach for smallish-scale CHP that maximises the use of the primary fuel, in this case gas, to achieve a fuel efficiency of around 120% — with no surplus heat or surplus electricity.
So what is the trick?
Quite simply, the answer is that surplus electricity not required for, say, lighting can be used by an air-to-water heat pump integrated into the same system as the CHP unit and the heating system it serves.
A particularly clever, and necessary, technical feature is the ability to modulate electrical output according to demand. At full output, 15 kW of electricity and 30 kW of heat can be produced. Modulating down to 40% of full output produces 6 kW of electricity and 17 kW of heat.
The driving force of LoadTracker, as it is named is a standard 2.2 litre 4-cylinder Toyota engine. These engines are mass produced so they are reliable, readily available and much cheaper than a bespoke engine. The engine turns an electrical generator with a maximum output of 15 kW of electricity and 30 kW of heat. For larger outputs, units can be installed in a modular configuration. A built-in plate heat exchanger provides the interface between engine coolant and water in the heating system.
The electrical efficiency is 30% at full load and still greater than 27% with an electrical output of 9 kW. However, it is, of course, the use of the heat from the engine that makes CHP attractive in its efficient use of primary fuel. Indeed, about 80% of the energy input to a LoadTracker unit emerges as useable energy in the form of electricity and heat.
The relative prices of mains electricity and mains gas quickly indicate why CHP can be an economic option. Typical prices for mains electricity are 11 p/kWh, compared with only 4 p/kWh for mains gas — and, of course, there is lots of heat also available from a CHP unit.
There is also a large impact on carbon emissions, with such a system having the potential to reduce the carbon footprint of many types of building by 30% or more — as is evident from the Sankey diagram. The implication for apartment buildings and social-housing schemes is that the BER can be improved by 50 to 60% over the TER, enabling a Level 3 or 4 rating under the Code for Sustainable Homes to be achieved.
A LoadTracker is quiet in operation, thanks to a specially designed acoustic casing. Noise levels a metre from the unit are less than 49 dB(A).
An overall system for a typical community heating scheme (see diagram) would comprise the CHP unit supplying high-grade heat at 80 to 85°C to a thermal store, with a downstream back-up boiler for occasions when the thermal store is depleted or the output from the CHP needs to be supplemented.
By mixing return water with water from the thermal store/back-up boiler, water can be delivered to the heating system itself at 60 to 70°C as required.
Return water from the heating system at 40°C can be diverted through the heat pump to boost its temperature to up to 55°C. The concept is tried and tested in Denmark, and SAV Systems has been working with EC Power to bring it to the UK in a joint venture to harness the energy-saving potential of CHP and heat pumps to get the best from both technologies. Indeed EC Power won the German mini-CHP of the year award in 2008. Lars Fabricius says that this is the first time the natural synergy between CHP and heat pumps has been exploited on this scale.
A major installation at the Hobro Leisure Centre in Denmark uses three LoadTracker units and five thermal stores in series to replace a single large CHP system. This installation also recovers heat from engine exhausts.
A 24-hour operational profile of this leisure centre showed an total electrical consumption of 1373 kWh, of which 926 kWh was met by the CHP — leaving 447 kWh to be purchased.
An EC Power CHP unit installed in the Royal George hotel in Perth demonstrates the benefit of being able to modulate the electrical and heating output, even though it does not have a heat pump. The system is supported by a condensing boiler.
The hotel has been in business for over 200 years, and the installation has doubled its energy efficiency to 96% and reduced electricity bills by a third, despite rising fuel prices.
This particular machine modulates to deliver 4 to 13 kW of electricity and 17 to 29 kW of heating.
Electricity generation matches the load throughout the day and night, so that the electricity generated never exceeds demand, and all the energy generated is used.
The energy facts are that the CHP system provides 60% of the hotel’s electricity — plus some 20 000 kWh of ‘free’ heat each month. Carbon-dioxide emissions are expected to be reduced by 30 t a year.
In the drive to reduce carbon emissions, there is no single silver bullet. The potential of LoadTracker to reduce carbon emissions by at lest 30% makes it a useful weapon in the armoury.
