Sunday, February 28, 2010

Renewable domestic heating

On a domestic level, the Government has committed to support domestic micro-CHP with £2,300* cash support. But this does not apply to district level heating schemes.

This article looks at both ideas, carbon impacts and support in the UK.

District heating

District heating is more carbon-efficient than heating individual homes where the density of accommodation is high enough. The example in Southampton is often cited.

The idea was part of last year's DECC Heat and Energy Saving Strategy Consultation. It did suggest support for these schemes.

The conclusions were published in August 2009. We're still waiting to see what the Government decides to do.... and will probably wait for some time as there is an election on.

District heating is recommended to the Government by this month's report from the Green Building Council. This says:

1. Public sector buildings should be required, where available and viable, to connect to existing or planned community heat networks, to provide an ‘anchor load’ of demand, and large businesses should be encouraged to do the same.

2. The ‘allowable solutions’ mechanism should be used as a way of providing additional ring fenced capital to support the delivery of heat infrastructure. Government has said that developers will be able to invest in so-called 'allowable solutions' in order to meet the required standard when constructing new zero carbon buildings.

It says nothing about existing non-public buildings though.

Neither is district heating part of the current renewable heat consultation. This scheme, which is due to start in April 2011, will subsidise a rapid increase in the number of homes and offices heated by woodfuel, biogas, solar thermal, heat pumps and waste-to-energy technologies. The deadline for responses to this consultation is Monday 26 April so do have your say.

District heating systems are ideal if a whole street, area or block of flats is to be renovated. Economies of scale make this form of heat and power delivery the cheapest on a per-household basis, and by far the most carbon-efficient, if low carbon fuel sources are specified.

A district heating scheme in Southampton, England, serves many residential developments from gas-fired CHP and geothermal energy, saving 11,000 tonnes of carbon a year and benefiting residents with a service price 5 per cent less than the market rate.

Systems are most efficient when servicing both homes and businesses or premises used during the day, as the two heat loads throughout a 24 hour period suit the continuous running required of a large plant.

District CHP plants may utilize fuel sources from waste to biomass, as well as geothermal where it is available. They work best where buildings are close together. A not-for-profit energy service company is usually formed to manage the system.


Micro-CHP – combined heat and power – is a nascent technology of small units for individual homes, typically the size of a fridge. They run on natural gas to produce up to about 10kW of power.

The current crop of models are based on the Stirling engine, Organic Rankine Cycle (ORC) or internal combustion engine. The first two have high thermal efficiency and output but low electrical efficiency (10 per cent) – and this is a sticking point.

Electricity output is around 1.1kW, enough to maintain back-up power in the event of a power cut or boil a kettle. A 1kWe (1kW electrical power) model from Honda called Ecowill has sold well in Japan.

A 2007 trial by the UK’s Carbon Trust concluded that micro-CHP can cut electricity bills and overall CO2 emissions by 15–20 per cent when they’re the lead boiler in larger contexts like care homes, district schemes, apartment blocks and leisure centres.

The best individual home for them therefore is a medium-to-large, moderately well-insulated one, maybe with solid walls, solid floors and no loft space that is harder to insulate well and has a relatively large heat demand.

Here, micro-CHP units can potentially deliver carbon savings of 5–10 per cent – fewer than a condensing boiler, since capacity is likely to be best matched to demand, for both heat and power.

Payback can be as little as five years. But they offer limited benefits for smaller and newer dwellings.

The key to success is matching the thermal output to the building’s pattern of use so that they operate not intermittently but for many hours at a time, making the value of electricity generated pay for the marginal investment in as little as three years in a typical family home.

It therefore works best with a buffer storage tank to save the surplus heat for later.

Grid connection for electricity export is going to be crucial to micro-CHP’s widespread acceptance. On average, half of all electricity generated by a typical 1kWe micro-CHP device is exported to the grid as it’s not needed at the time.

Reliability is also a key issue – service agreements will be essential. So homeowners shouldn’t yet trade in their condensing boilers, which have about the same overall heating efficiency – 90 per cent – without also producing electricity, but they might keep an eye on developments.

Superinsulated homes will have to wait until the next generation of machines, based on fuel cells. These generally come in two types – proton exchange membrane fuel cells (PEMFCs) and solid oxide fuel cells (SOFCs).

They have a heat to power ratio that is approximately equal so for example they could produce 5kW of heat and 5kW of electricity.

Support for micro-CHP

Under the Feed-in Tarriff scheme, from the 1st April 2010, microCHP units with a capacity below 2kW will receive 10p per kW hour generated, for a period of ten years. This tariff is available for the first 30,000 microCHP installations. A review will take place when 12,000 units have been installed.

However the Government has not followed through on commitments made in the Energy Act to support miniCHP units of up to 50 kW capacity.

* - based on 10p generation tariff and assuming a 3p pence export rate. Assuming annual generation of 2000 kWh and 50% export. Assuming import electricity price of 14p kWh-1. The total income paid to a generator over a 10 year FiT period would be £2,300 over full period of 10 years. Annual figure therefore of £230.

The Feed-in Tariff (FiT) scheme is the first phase of the Government’s Clean Energy Cashback programme - see the Energy Saving Trust website for details.

What is the most carbon efficient heating?

An independent survey conducted by the UK Energy Efficiency Partnership for Homes which looked at the carbon impact of different domestic heating and hot water systems in both houses and flats concluded that the following performed best, all other things being equal (figures in kgCO2/m2/yr):
• community heating and CHP, fuelled wholly or mainly by biomass - 4.15
• community heating without CHP fuelled wholly or mainly by biomass - 7.11
• wood burning boilers - 10.02
• wood burning boilers with solar water heating panels - 10.09
• ground source heat pumps with low temperature heat distribution/emitters (e.g. underfloor heating) - 20.83
• solar water heating panels in conjunction with gas boiler systems - 21.98

Source: Heating Strategy Group of the Energy Efficiency Partnership for Homes, January 2008

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