Sustainability: On-site renewables

The OpTIC office in Denbighshire, north Wales, has a large array of photovoltaic panels

Introduction

2006 is likely to be a watershed year for sustainable construction. As well as the introduction of the revised Part L, project teams may have to consider ways they can meet the rule that, where feasible, 10% of the heat and power needs of large projects be generated on site using renewable technologies. Then there is the first review of the London Plan, likely to require a growing proportion of on-site generation from 2008 onwards, which is also likely to be published this year.

The promotion of on-site renewables originates with the ODPM’s Planning Policy Statement 22, primarily aimed at large-scale renewables such as wind farms, but which also promotes the three strategies of energy efficiency, combined heat and power and on-site renewables. The requirement to encourage small-scale renewable energy generation has been adopted readily by local authorities who see it as an effective and visible means of contributing towards sustainable development.

On-site renewable options

Wind generators

In a suitable location, building-integrated wind energy can be an effective source of renewable power generation. Installed costs range from £2500 to £5000 per kW of generator capacity. Wind speeds of 4 m/s can produce useful energy from a small generator up to, say, 3 kW, but larger generators require at least 7 m/s. Wind speed increases with height, making turbines on tall buildings an option, but their effectiveness may be affected by neighbouring buildings.

A small 2.5 kW generator costing about £12,000 will produce enough power at an average wind speed of 6 m/s to provide electrical energy equivalent to the demand for four two-bedroom flats. Site constraints and planning are likely to limit the extent to which wind power can be used on denser schemes with higher power requirements.

Building integrated photovoltaics

Photovoltaic panels generate direct current electrical power when exposed to light. Conversion efficiency of solar energy to electrical power is improving and ranges of between 7% to 18% have been achieved under laboratory conditions. In practice, however, allowing for typical UK weather conditions, an installation of at least 7 m2 of the latest modules is needed to produce 1000 W peak output. The 800 kWh that these panels should generate in a year would supply half the electricity for a two-bedroom flat. Costs range from £300 to £450/m2 for roof coverings, and from £850 to £1300/m2 for laminated glass.

Photovoltaics are sensitive to location in terms of orientation, elevation and potential shading. PV panels are increasingly common but present significant affordability and scalability problems.

Ground source heat pumps

Ground source heat pumps employ the compressor technology used in refrigerators to take heat from a source, increase it and emit it in a building. Ground temperature is used as the source because it is stable all year round. Heat is extracted using water circulated in a pipe network and an electrically powered heat pump raises the temperature to a usable level.

Most ground heat systems consist of a cluster of pipes inserted into vertical holes 50-100 m deep. Ground source heating systems rely on electricity and are therefore only partially renewable, but can achieve a high coefficient of performance (the ratio of heat output to electrical energy input) of between three and four, making them a good option for larger projects. Installed costs are relatively expensive, at £800-1200/kW, depending on system size and complexity.

Borehole cooling

Ground temperature is well below ambient air temperature during the summer, so “coolth” can be extracted and used to supplement building cooling systems. Borehole systems may be either “open” – discharging ground water to river or sewer after passing it through a heat exchanger – or “closed” – circulating water or another fluid through vertical pipes extending below the water table and a heat exchanger.

Like heat pumps, ground source cooling systems are only partially renewable because they rely in part on electrical power. Indicative system costs are £200-250/kW.

Solar water heating

The technology ranges from simple, flat plate water-based collector panels to more complex evacuated tube models. Solar water heating is particularly suitable for buildings that have year-round, day-time hot water needs. A typical 4 m2 system will cost £2500-4000 depending on its complexity and the length of pipe runs, and it could produce an energy saving of 2000 kWh a year, equivalent to 40% of the hot water and heating needs of an average house. Commercial systems are larger and may be more complex because of longer pipe runs and so on. Indicative costs are about £700/m2.

Biomass boilers

Wood chips or pellets derived from waste or agriculture are considered carbon neutral, having absorbed carbon dioxide prior to harvest. Biomass boilers can replace conventional boilers but depend on the existence of a local fuel source, and require ash disposal and maintenance. In larger installations, biomass boilers typically form part of a modular system to ensure reliability. There is a cost premium for the boiler system, although the cost of the fuel is comparable with other solid fuels. The major constraint on the adoption of biomass is the reliable availability of local fuel materials.

Feasibility of on-site renewables

Key issues associated with the application of integrated renewables include:

• Requirements for energy generation are intended to be dealt with flexibly, because of technical and financial constraints. However, planning authorities are adopting a tough negotiating stance, for example by assuming that the cost of on-site renewables will have been factored into recent land purchase prices.
• Most renewable energy systems are much more expensive to install than either passive energy efficiency measures or, where appropriate, gas-fired CHP plants. The cost of the renewable component could reduce funds available for other carbon-reducing initiatives.
• The use of on-site renewables may be constrained by issues of noise, odour, traffic or visual impact even before the issues of cost and technical feasibility have been considered.
• It is difficult to meet energy generation targets for renewables on large and dense schemes and as a result more expensive technologies are often required to meet targets.
• The use of low and zero-carbon technologies is promoted in the 2006 Part L through calculations that reduce a building’s overall carbon reduction target. This is a key aspect of the promotion of low and zero carbon technologies, but potentially contradicts the widely promoted hierarchy of using less energy first, before switching to renewable generation.

Comparison of on-site renewables

The key factors affecting the selection of systems are physical constraints, affordability and effectiveness. Scalability of systems and the requirement for back-up supply sources are particular challenges, as is the high cost and low payback from many technologies. The following table sets out the technologies, suitable applications and the power/carbon savings achieved with a nominal £100,000 investment. It can be seen that large-scale wind power is potentially the most effective option in terms of power delivery and carbon reduction, but is difficult to use in urban locations. Passive solar water heating is relatively efficient but suffers from space constraints. Technologies such as heat pumps that involve some carbon emissions in the generation process are also relatively efficient in energy production and are less subject to space constraint, but are less effective in reducing carbon emissions.