As social housing makes increasing use of CHP and district heating to address rising energy costs and associated fuel poverty, it's important to take effective control at all levels. Beata Blachut and Silas Flytkjaer explain
Social housing landlords and developers of other multi-residential accommodation are increasingly turning to centralised energy centres, rather than individual heating appliances in each apartment. Typically these serve small-scale district heating systems and, when using combined heat and power (CHP), are capable of delivering energy efficient, low carbon space heating, domestic hot water (DHW) and electrical power.

Given the inherent variation in demand for power and hot water in such cases, it is essential that the CHP can track loads and modulate accordingly to ensure maximum run time for the CHP. Equally, it is important to take effective control of the heating and hot water in individual apartments.

CHP and temperature control

Indeed, it is this combination of modulating CHP and precise temperature control that enables small scale CHP and district heating schemes to deliver maximum cost-saving and environmental benefits in both new build and refurbishment projects. The underlying principles are best illustrated through examples of both new and refurbishment projects.

A new build development at Westfield Avenue in Edinburgh, providing 192 affordable flats and eight business units, is making use of four modular 15kWe/ 30kWth LoadTracker CHP units. These supply 74 per cent of site heating and hot water demand and 54 per cent of electrical demand - helping to meet the brief from developers Dunedin Canmore Group to reduce CO2 emissions by at least 20 per cent, compared to a traditional design.

Also, unlike traditional CHP, the LoadTracker CHP incorporates a 'heat distributor' to ensure that the actual flow temperature consistently corresponds to the design flow temperature. This arrangement ensures the CHP always produces high grade heat. Any surplus heat can be stored to help optimise CHP operating times and reduce the likelihood of back-up boilers being required.

Control of water temperature within the apartments is also critical, so it was important to avoid the lack of control that has blighted many residential district heating schemes in the past. Such schemes are known to have suffered swings of +/- 15 deg C at the tap.

To prevent this occurring at Westfield Avenue, each apartment/business unit is fitted with a FlatStation heat interface unit. These incorporate control to ensure that differential pressure between flow and return remains at around 35kPa in every unit, irrespective of location within the distribution system.

The result is that DHW temperature is controlled to within +/- 2.5 deg C. Also, an integral idle temperature control function will ensure that water in the supply pipe remains warm. This enables the DHW to be highly responsive, even at times when space heating loads are low.

The system at Westfield Avenue was designed by the Edinburgh-based Keenan Consultancy and installed by the Emtec Group of Cambuslang.

The multi-residential development by Link Group Housing Association at West Bridge Mill in Kirkcaldy involved refurbishment of flats in a former B-listed rope mill. It comprises 16 separate flats housing vulnerable young people, as well as several office units used by family service charities.

Until May 2010 the social housing flats were heated by electric storage heaters with hot water from immersion heaters, while the office units were heated with gas-fired boilers. In the flats, the combination of inefficient heating and rising energy prices exacerbated problems of fuel poverty while failing to deliver adequate heating.

Central energy centre

The solution was to install a 15kWe/30kWth LoadTracker CHP in a central energy centre linked to a heating system serving the apartments and offices via FlatStation heat interface units.

CHP displaces grid electricity and replaces it with gas, which is about a third more carbon efficient. In the case of West Bridge Mill, installing CHP was also more cost-efficient than extensive renovation of the building fabric so there are commercial benefits in addition to the sustainability benefits.

If this had been a fixed output 15kWe unit, it would have been switched off at times when site electrical demand was less than 15kW. Consequently, the run-hours would have been greatly reduced and the site would have used considerably more expensive and carbon-intensive mains electricity - as well as running the back-up boilers more frequently.

Since July 2010 the CHP has run for 15,792 hours and produced 171,668kWh of electricity and 373,938kWh of heat, with average electrical power generation of 10.8kW. It has met 71 per cent of the site's electrical demand during this time, reducing carbon emissions by around 70 per cent compared to the previous heating and hot water system.

It has been estimated that there are currently around 6 million homes in the UK that are subject to fuel poverty and some believe this could rise to 9 million by 2016. Taking measures to deliver affordable hot water and electricity is clearly an effective way to tackle this growing problem, and flexible CHP serving a district heating system offers the ideal solution in many cases.

//The authors work at SAV Systems //