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Getting smarter with combined heat & power

When specifying CHP it's essential to ensure that the system is aligned with, and responsive to, the needs of the building. Beata Blachut explains how optimum flexibility and efficiency can be achieved
Now that sustainability is near the top of the agenda for most organisations, there is a strong desire, often reinforced by local planning requirements, to make wider use of low or zero carbon (LZC) technologies. Of the various LZC options available, combined heat and power (CHP) is proving to be increasingly popular. And, crucially, CHP is now viable for many smaller projects that might not have been considered in the past.

Typically, CHP will be a core component of a central energy centre that also incorporates other heat sources, which together serve a large or small district heating system with hot water for space heating and domestic hot water (DHW). Power generated by the CHP may be used locally, exported to the grid or a combination of the two.

According to the Carbon Trust, CHP technology can potentially reduce carbon emissions and energy costs by 30 per cent, compared to using gas-fired heating plant and mains electricity. There are two key reasons for this. Firstly, CHP captures the heat generated by power generation and uses it for heating water. Secondly, a proportion of the site's electricity requirements are met using mains gas, which is more cost efficient than mains electricity.


Configured to maximise efficiency
The key word in the paragraph above is 'potentially', as the maximum carbon and energy savings will only be achieved if the CHP system is designed and configured to deliver maximum efficiency.

This is in terms of the efficiency with which the CHP generates heat and power, how well its operation is aligned to the needs of the building and how the electricity and hot water are used in the building.

Beginning with the central CHP plant, one of the significant limitations of large-scale fixed-output CHP systems is that they need to be carefully matched to the anticipated electricity and heating loads.

As a result of this inflexibility, they are usually sized to match the site base electrical load and therefore do not contribute to site usage beyond the base load. Consequently they may only achieve relatively small reductions in overall energy usage at the site.

An alternative is to use CHP units that are able to modulate their output, using small-scale CHP units in a modular configuration.

For example, a unit that can modulate down to 40 per cent of its maximum electrical power output will ensure that the electricity generated never exceeds demand, so there is no need to 'dump' heat or sell electricity back to the grid at unfavourable rates.

To put this into perspective, we recently evaluated the options for a small leisure centre.

With conventional CHP sized to cover base electrical demand only the CHP would only provide 39 per cent of site electricity usage, resulting in energy and carbon savings of around 10 per cent.

In contrast, using a modulating CHP system to track site demand and modulate output accordingly, 80 per cent of the site's electrical demand could be met.

At the heart of such a system is monitoring and control, tracking the requirements of the load demand in real time and 'learning' as the system operates to ensure consistent optimal performance.

Sophisticated controls also allow the overall operating strategy to be aligned to the building's most significant demands to maximise savings. So, in the example above the system was optimised for electricity demand, but systems may also be optimised for heating/DHW demand.

They may also be 'economy' led, based on actual fuel and electricity prices to gain maximum economic benefits.

Modulating CHP
Experience shows that a modulating CHP system can deliver energy savings 15 per cent over and above those delivered by a fixed-output CHP system.

In parallel with optimising plant efficiency, the hot water for space heating or DHW also needs to be optimised. Here, use of heat interface units will enable both heating and DHW needs to be met directly from the hot water generated by the CHP, without the need for separate hot water storage in the spaces requiring DHW.

In this configuration the heating circuit is designed for direct generation of heat, using differential pressure controllers to enable individual temperature control in each room. A zone valve with actuator and a room thermostat can also be included for time-dependent temperature control.

DHW is heated in the heat exchanger in the heat interface unit and the temperature is regulated with a flow-compensated temperature controller with integrated differential pressure controller.

In this way, the heat exchanger cools the flow water from the central heat source to a safe temperature for DHW, while compensating for variable loads, supply temperatures and differential pressures.

Heat interface units can also be fitted with integrated idle temperature controllers to ensure that DHW is responsive at times when space heating loads are low.

It is this attention to detail that will enable CHP to deliver maximum benefits to both the end user and the environment.

// The author is CHP product manager with SAV Systems //
1 February 2012

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