Little Marvels: on-site renewable energy

Microgenerators are, as the name suggests, very small power stations. They produce heat and power using low-carbon fuels, and they do so much more efficiently than their larger cousins. As a result of these two facts, they have become a centrepiece of the government's climate change strategy. This means that public money is to be made available for installing them, and if that doesn't tempt developers, regulations are being drawn up to force them to.

One incentive is provided by a grant scheme called the Low Carbon Buildings Programme, which should be in place by April. Then there are the toughed-up energy targets of Part L of the Building Regulations, which come into force at the same time.

At the same time, we have the introduction of Planning Policy Statement 22, which states that councils may require a percentage of energy for any new residential, commercial or industrial development to come from renewable sources generated on site. This percentage may vary from area to area, but in the London borough of Merton it is already as high as 10%. That figure is also the one that Ken Livingstone is pushing to be included in the London Plan.

For many, the combination of meeting strict targets and getting to grips with new technology will be a daunting prospect - but help is at hand. This guide, compiled by consultant ESD, takes a look at the relative merits of the nine technologies that will be eligible for grants from the DTI.

1. Solar photovoltaics

What is it? Photovoltaic panels convert sunlight into electricity. A PV cell consists of two layers of a semi-conductor, often silicon. When light shines on these, electrons flow between the layers, creating an electrical current. PV cells are joined to form panels.

Environmental impact Reduces high carbon emissions from electricity consumption and has a low visual impact.

How much does it cost? At more than £4000-8000/kWp (the peak kilowatt input or output of a device) this is one of the more expensive technologies, but costs can be offset against alternative cladding or roofing systems, which would have to be specified for new buildings anyway.

Reliability Twenty-year lifespan for electricity generation but longer as a weatherproof building element. Electricity is produced throughout the year although output is lower during the winter months. PV has no moving parts and so has good reliability.

Ease of installation Can be installed on angled metal stands or as part of a roof or facade. Can also be used as a form of shading and encapsulated in glazing components. Has to be connected to the landlord power supply or individual apartments.

2. Small wind turbine

What is it? A propeller-type blade attached to an electrical generator, which is attached to a small mast. When the wind blows, the blade spins the generator, producing electricity.

Environmental impact Reduces high carbon emissions from electricity consumption. Visual impact in urban areas is less than in rural locations as the generators tend to blend in to that setting.

How much does it cost? At £2000/kWp, this technology is financially more attractive than solar PV.

Reliability Intermittent power production dependent on wind. Design life is 20 years and annual inspections are recommended. Can be affected by turbulence caused by adjacent buildings.

Ease of installation A number of roof-mounted wind turbines are available on the market. Some need little or no reinforcement of the building structure, unlike larger turbines.

Credit: solarcentury.com

3. Solar hot water

What is it? The thermal solar collector consists of tubes containing water. When the temperature of this water exceeds that of the water in the main hot water tank, a pump switches on to circulate the hot water.

Environmental impact A very effective means of reducing up to 50% of the carbon emissions associated with domestic hot water consumption.

How much does it cost? A 2-3 m2 panel for a home costs up to £2700 for a high-efficiency evacuated tube system or less than £2000 for a less efficient flat plate collector. Cheaper systems are expected to be available in the UK over the next few years.

Reliability Has a design life of 20 years. An annual system maintenance is recommended, as with any pumped water system.

Ease of installation Roof or facade-mounted systems are available, either for individual homes or for a communal system.

4. Ground source heat pump

What is it? A refrigerant is circulated through buried pipes to collect heat in the ground. The compressor in the pump concentrates this heat so it can be used to heat buildings.

Environmental impact The environmental performance of this technology is comparable with solar hot water if the heat pump is powered using electricity generated from a renewable source. Reversible systems can also be used for cooling in summer months.

How much does it cost? Approximate costs of £1000/kW. However, the additional cost of electricity consumed by the heat pump needs to be taken into consideration.

Reliability The key benefit is reliability of supply. Because this works on the principle that the ground is always at a constant temperature, it isn't intermittent, unlike many other renewable technologies. The compressor is probably the hardest working component of this system and will require periodic maintenance.

Ease of installation Involves laying plastic pipes in either 50 m or 100 m deep boreholes or in shallower 2 m deep trenches. The acidity of the soil is an important consideration to avoid corrosion. The system should be used in combination with a low temperature wet heating system such as underfloor heating.

5. Air source heat pump

What is it? It work on the same principle as the ground source heat pump - the difference being that low-level heat is extracted from external air rather than the ground.

Environmental impact The environmental benefits of this system are still open to debate.

How much does it cost? Air source heat pumps are essentially an air-conditioning system working in reverse so there are no abnormal costs unless the power requirement for the compressor is met from solar PV or wind.

Reliability This does not have the supply predictability of ground source heat pumps, as it is dependent on air conditions. But the technology should be familiar to maintenance teams.

Ease of installation As the technology uses air as its low-grade heat source, the system can be easily located and does not require underground pipes.

6. Bio energy

What is it? More a fuel than a technology, the idea is to generate heat or power by burning bio fuels such as wood or methane gas in order to power boilers or a combined heat and power plant.

Environmental impact This offers significant carbon benefits as carbon is absorbed during the growth of the biomass material and released on combustion, providing a near-neutral carbon balance if the fuel is not transported over a large distance.

How much does it cost? £450-£1000/kWth (thermal capacity) for biomass boilers and about £4000/kWe (electric capacity) for biomass CHP.

Reliability This is becoming increasingly commonplace in the form of biomass heating boilers. However, there is still not a fully commissioned biomass CHP plant up and running in the UK. The security of supply of biomass is still an uncertainty. Boilers have a life of up to 20 years, compared with 12-15 years for CHP.

Ease of installation The main challenge is the storage space needed for the biomass and the transport access for regular deliveries. Flue arrangements also need careful consideration.

7. Combined heat and power

What is it? A CHP plant is an electricity generator situated at the point of use. It consists of an engine that generates electricity with the waste heat used for heating. They range from power station-sized units to washing machine-sized domestic units.

Environmental impact CHP provides carbon dioxide savings because both heat and electricity are produced at the point of use. It typically uses 35% less primary energy than traditional generation because the waste heat is not dumped and there are no transmission losses.

How much does it cost? That depends on the scale of the plant. One 8kWth/1.2kWe floor-mounted household CHP system is retailing for £3000 installed.

Reliability Micro-CHP for individual homes is an innovative technology that is likely to become more widely available to the general public over the next few years. Building and community CHP has a proven reliability record of 12-15 years.

Ease of installation A CHP engine requires the installation of a heat and power distribution network. A grid connection provides a back-up electrical supply.

8. Hydrogen fuel cell

What is it? Like CHP, it produces both electricity and heat for local use. But it does this chemically using a catalyst rather than mechanically as in CHP. Also it does this using hydrogen, which produces water as a by-product.

Environmental impact Because the hydrogen needed to power the fuel cell has to be manufactured, the environmental benefits are dependent on which technology is used. If wind or solar power is used, there is no environmental impact, but if coal is used the impact is very damaging.

How much does it cost? The commercial development of fuel cells is relatively new and therefore the cost varies greatly. It is currently one of the most expensive technologies.

Reliability There are a broad variety of fuel cells and only a very few widely used, so information on reliability is difficult to find.

Ease of installation Similar to CHP.

9. Small hydroelectric system

What is it? This uses fast-flowing water to turn a water turbine, which is connected to an electricity generator. There has to be sufficient head of water to spin the turbine so small hydro schemes are usually sited near a fast-flowing stream or a slow-moving river with a weir.

Environmental impact Reduces high carbon emissions from electricity consumption and has a low local impact.

How much does it cost? Very site specific, typically £500/kWe and upwards. Energy output is a function of head (the drop) and the water flow rate.

Reliability Output is site specific and depends on the water resource. The equipment has long lifetime - 40+years with regular maintenance.

Ease of installation Can either be with an impoundment (such as a weir), to create the necessary head, or a pipeline. Legislation for the use of water is very tight - early liaison with the Environment Agency is recommended.