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District energy schemes - lessons learnedJanuary 2009

Building designers are being encouraged to adopt clean sources of power on a building-by-building basis. But does this really make sense, when greater efficiencies can be obtained by thinking beyond the site boundary? James Parker picks up the lessons learned from the district energy scheme.

UEA's campus with the energy centre at the top left

Going green is high on today's agenda. From individual people to whole cities, everyone is trying to do their bit, especially with energy prices rocketing. Clean sources of energy, such as renewables, are high on the energy agenda.

The problem with renewables is that they often don't work very well on a building-by-building basis. The loads are often not high enough to justify large kit with good operating efficiencies, and the skills to operate and maintain the equipment are often absent or unreliable.

District or city-wide schemes work far better. Load profiles from many buildings can be combined, creating a constant energy demand. Operating costs can be shared, and management and maintenance can be centralised. Any spare capacity can be sold to the grid or shared with neighbouring communities cost-effectively.

But what about smaller communities - the provincial town or a cluster of villages. Could district energy work for them?

An example of how this works can be illustrated by university campuses. Universities are often self-contained communities, with a range of commercial and domestic-type premises not untypical of a small town.

A recent conference at the University of Surrey entitled "Sustainable Practice in Universities: Leading and Improving" provided many examples of how universities in the UK are already providing exemplar sustainable developments.

Take the University of East Anglia (UEA). Well known for the ground-breaking Elizabeth Fry Building, UEA is continuing to reduce its carbon footprint with new developments. Many other low-energy buildings have been completed since Elizabeth Fry opened its doors 14 years ago. For example, the Zuckerman Institute for Connective Environmental Research building (ZICER) won the first ever Carbon Trust Low Energy Building of the Year award in 2005.

Data from before and after the installation of combined heat and power at UEA

UEA has also been developing site-wide energy schemes such as district heating and cooling - effectively offsite generation. This is where the real benefits of scale can come into play, with longer utilisation times for technologies such as combined heat and power, and therefore greater efficiencies due to the more constant base loads.

The installation of a chp-based district heating scheme at UEA in 1999 reportedly produced a 33 percent saving in CO2 emissions from the campus (see table 1) and the forthcoming installation of a biomass chp is projected to save another 34 percent (or 6719 tonnes CO2) per year.

Martyn Newton, UEA's risk and sustainability manager, revealed that while biomass was considered due to the increase in gas prices, conventional biomass systems do not work well with CHP (Combined Heat and Power).

To resolve these problems the University considered gasification of biomass as the interim step, with the energy systems running off the resultant biogas. This has the added benefit of not producing NOx, making the system much cleaner.

The economics really stack up as well. Martyn Newton said that the payback period of this £7 million project has reduced already from nine years to nearer seven years since the original concept was developed.

UEA's figures after expansion

UEA's figures are impressive: the University only increased CO2 emissions by 10 percent between 1990 and 2006, at the same time as a 239 percent increase in students and 49 percent increase in the estate's square meterage. That equates to a reduction of 53 percent in CO2 per student, and a 25 percent reduction in overall CO2 emissions. (See table 2.)

Just think what the levels of CO2 savings at UEA could do for meeting government carbon reduction targets if they were achieved across the built environment.

District Heating schemes (or community heating schemes) are nothing new. Battersea Power Station delivered heat to Westminster City Council's largest estate - Churchill Gardens - until the power station closed in 1980 and the district heating was taken over by heat-only boilers.

With the redevelopment of the old power station seemingly back on track, the original district heating scheme will be extended to more dwellings, schools and the National Statistics Office with combined heat and power supplied by two 1.55 MWe chp engines. These will generate enough heat to support the summer baseload, with a further three 8 MWe boilers to cover the winter peak loads and provide some back-up.

This highlights one of the important factors in making a chp-based district heating scheme viable - the summer baseload. If the summer baseload is not large enough the chp system will not run.
Craig Grobety, commercial assistant at supplier Utilicom, the company behind the scheme in Birmingham, says that the key factors for a successful project are getting key partners on board, such as the local authority, planning the extent of the scheme early on, future proofing the energy centre to allow for new technologies, and getting the baseload right.

The absorption chiller used in UEA's tri-generation scheme

The key to the latter is to have the mix of buildings on the network. A load based on 100 percent residential will create a very limited demand for heat in summer. But add industrial premises, and the demand is raised, particularly if absorption chillers are used to convert the heat into cooling.
These sorts of schemes are not just limited to big cities. The use of smaller wood-fired district heating schemes are popping up in smaller communities, especially in the more remote areas of Northern England, Wales and Scotland.

Kielder, a remote forestry village in Northumberland, has installed a 300 kW biomass boiler to provide heat through a district heating network to a range of community and residential buildings. Of course, being a forestry village surrounded by the Kielder Forest, the fuel for the biomass boiler is all around, significantly reducing the transport costs and the associated emissions.

What about other renewable technologies? Well, the Austrians are exploiting a true renewable source for district heating: solar power. The Austrian solar engineering company SOLID has designed and constructed Austria's largest solar thermal plant on the UPC Arena in Graz, commissioned in 2002. Here, 1407 m2 of solar panels capture about 540 MWh of solar energy per annum to be supplied into the district heating network. This gives a reduction of 250,000 kgCO2 per annum over oil-fired boilers.

Having the energy (hot water) feeding directly into the district heating network also removes the need for costly buffer storage vessels. Maximum benefit can be gained from the heat generated from the panels - it all goes into the network and nothing is dumped.

This is where there appears to be a split in the district heating strategy between the UK and the rest of Europe. In the UK the schemes are generally a single source of heat production, while many European schemes have multiple heat-producers supplying heat to the network. The latter can lead to much larger schemes.

Can this be done on a mass scale in the UK? Are the barriers too great? The evidence is that offsite generation for multiple buildings, particularly for heating, can yield significant carbon savings.

The International Energy Agency is pushing the use of district energy, with Pieter Boot (Director of the IEA's Long-Term Office) announcing "a new effort working with cities to capitalize on district energy and energy efficiency to make a major impact in addressing climate change."

Boot highlighted the role cities can play to increase the use of district energy: "Cities like Copenhagen, Helsinki, Seoul and Toronto have taken an important step towards achieving climate and energy-savings benefits for the past several decades through a concerted effort to advance district energy," he said. "Other cities can learn from their example."