Ecobuild and renewable technologies

Billed as the world's biggest event for sustainable design, construction and the built environment, Ecobuild 2009 was held at Earls Court in early March. NBS Technical Author, Bill Clark, tested his energy levels at the event and now looks at one particular area, 'making micro renewables work', discussed during a number of seminars. Micro-renewable technologies include wind, solar heating, photovoltaic, heat pump and biomass technologies.

Such was the level of interest, most of the Ecobuild seminars over the three days were over-subscribed. Speakers represented trade organizations, developers, architects, consultant engineers and equipment manufacturers.

One particularly interesting presentation gave an excellent overview of the benefits and pitfalls of a range of micro-renewable technologies, including detailed case studies. The studies looked at the selection process for the different technologies including issues such as capital and life cycle costs. Micro renewable technology is not often a low cost solution. The message was very much 'reduce energy consumption prior to considering ways of generating energy on site'. In terms of comparing 'cost per reduction in tonnes of CO₂ emitted', the most effective measures are:

• Replacement of direct electric heating with gas
• Constructing to 'good' airtightness levels, not exceeding 5 m³/hr/m²
• Use of 'advanced' glazing with U-values not exceeding 0.8 W/m².

Having established minimum energy requirements, the various methods of generating it can then be considered.

Many of the design decisions that lead to the selection, or rejection, of a particular technology will be based on the location of a development. Typically, these include:

• Planning requirements and restrictions
• Is the scheme refurbishment or new build?
• The impact of surrounding buildings - small scale wind turbines could be ineffective in urban areas – see the Encraft Warwick wind trials project report
• The availability of a gas supply
• Continuity of fuel delivery and related storage concerns (particularly relevant to biomass).

Several of the sessions within the seminars were presented by manufacturers who, unsurprisingly, were keen to promote their products as being 'green'. Caution is needed here of course because some claims with regard to CO₂ emission savings were highly subjective, often with little or no supporting evidence. The same caution applied to some of the manufacturers exhibiting products in the main hall – it would not be difficult to make an inappropriate selection of technology based on some of their claims.

One of the common themes running throughout Ecobuild was the urgency the Government are placing on reducing CO₂ emissions from existing buildings. Perhaps seeing an opportunity, several heat pump manufacturers were making claims regarding coefficients of performance (COP) which at first glance appeared excellent – but all was not necessarily as it first seemed! One quoted their product as having a COP of nine, yet when asked for technical information to substantiate this claim, they were unable to provide any. It should be made clear that the use of heat pumps can offer extremely valuable CO₂ emission savings, especially when compared to alternative fuel use such as direct electric heating, LPG or oil.

Air source heat pumps in particular are easy to install since they do not rely on boreholes required by ground source units.

Historically, most heat pumps manufacturers have suggested that best use can be made of their products on systems which have low operating temperatures (i.e. underfloor systems), but they are increasingly advocating the use of air source units on conventional wet heating systems consisting of radiators designed for higher flow temperatures. At first glance this can appear attractive but it is not without its potential problems and it was worrying that some if not most of manufacturers exhibiting failed to mention this.

Whilst it is true to suggest that heat pumps offer 'free units of energy', if they are fed by grid electricity they may not offer much, if anything, by way of reduction in CO₂ emission. This is due to the relative inefficiency of the generation, transmission and distribution through the national grid. In particular, in the case of air source heat pumps, coefficients of performance will be at their lowest when heat is most needed, Ground source heat pumps do not suffer as much from this problem since their heat source is relatively constant in temperature all year round.

Heat pump COPs are commonly stated at a set source temperature; perhaps what is needed is a figure based on performance assessed over a full season. The addition of direct electrically heated elements to supplement temperatures for hot water, or for defrosting, only reduces CO₂ emissions-saving further still.

The overriding message is that there is not one single solution when considering the use of micro-renewable technologies and that each has a part to play. The key issue seems to be – do sufficient research, seek independent advice and be prepared to question manufacturer's claims.

Related NBS information:

Articles:

• Photovoltaics for dwellings - an update
• Photovoltaic modules
• Biomass

Selected links:

• Renewable energy
• Energy efficiency

Written May 2009

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