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Cutting emissions with modulating CHP

Many small healthcare buildings can benefit from small-scale, modular CHP that tracks loads and modulates output, says Beata Blachut
Combined heat and power (CHP) is often associated with large scale plant designed to meet high but steady power and heat loads - as would be found in large hospitals. In parallel, there is often a perception that CHP is unsuitable for smaller healthcare buildings such as community hospitals, care homes, nurses' accommodation and health centres. However, modular CHP (configured rather like modular boilers) that can track loads and modulate outputs accordingly enables the energy and cost saving benefits of CHP to be applied to a wide range of building types.

As HVR readers will be aware, a challenge with CHP is that when it's running it is generating both heat and electrical power - you can't have one without the other. Therefore the key to making CHP sufficiently flexible to cater for a wider range of projects is to ensure it manages both of these efficiently.

For example, conventional fixed-output, non-modulating CHP is usually sized to match the site's base electrical load. The result is that the CHP does not contribute to the site's power usage beyond that base electrical load. Clearly this limits the energy savings that can be achieved while also increasing the amount of expensive grid electricity that has to be purchased. It costs less to use gas to generate electrical power from a CHP unit than to buy it from the electricity company.

Alternatively, the CHP may be sized to meet a base heat load and then the electrical power output is determined by the heating requirements of the site. This can often result in selling electricity to the grid at unfavourable rates.

With CHP units that are able to modulate their output and track site demand, the power output of the CHP is aligned to changing site requirements, providing effective management of the most expensive utility - the electrical power - to deliver maximum cost savings. Any excess heat is typically transferred to water in a storage vessel to ensure it is not wasted.

This performance is further optimised by the ability of modulating CHP to 'self-learn' the building's loads and adapt to changing conditions. A key benefit of this is that if other power-saving measures are implemented in the building, such as upgrading the lighting, the CHP will adapt accordingly. There are many instances where CHP has had to be turned off following such upgrades because the site's electrical demand becomes too low for the CHP to be viable.

Used in modular configuration
As noted above, CHP can also be used in a modular configuration. With modular CHP, up to five units can be combined to provide a range from 15kWe/30kWth to 100kWe/200kWth. So on a small site one unit may be sufficient, while several small units can be combined to meet the needs of larger sites.

Another engineering challenge with traditional CHP is the need to operate at a constant temperature differential (ÄT). This may result in variable flow temperatures so that the system performs inconsistently.

To overcome this issue, a heat distributor can be incorporated so that a constant flow temperature (corresponding to the design flow temperature) is maintained, irrespective of the return water temperature. A flow controller in the heat distributor allows the heating flow temperature to be set between 40 and 80 deg C.

This arrangement ensures the CHP always produces high grade heat, minimising or even eliminating the need to run back-up boilers. Any surplus heat can be stored at 80-85°C, which helps to optimise CHP operating times, thus further reducing the likelihood of back-up boilers being operated.

Where modular, modulating CHP is used in residential applications such as nurses' accommodation and care homes, it's also important to control heating and domestic hot water (DHW) efficiently in each apartment. Traditional heat interface units (HIUs) may allow as much as +/- 15 deg C temperature in DHW temperature and this is clearly unacceptable. However, in using HIUs with differential pressure control the DHW temperature is controlled to within +/- 2 deg C. Also, an integral idle temperature controller in the control valve 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.

Given the vast numbers of healthcare buildings in the UK, and the fact that small scale CHP is now available, it no longer makes sense to restrict the use of CHP to large hospitals. Modular, modulating CHP provides a highly responsive and efficient option with the potential to greatly reduce the energy consumption of the healthcare estate. Building services engineers can play an important role in facilitating this.

// The author is CHP product manager with SAV Systems //
14 May 2013

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