COMBINED HEAT & POWER Making the most of CHP T
CHP is an obvious choice for backing-up and supplementing renewable energy sources. However, optimum performance requires close integration with other technologies, for both heating and power, says Ian Dagley, general manager of Hoval
he continued growth of renewable energy sources in the UK is clearly a positive development when it comes to meeting decarbonisation objectives. However, the most common of these – wind and solar – are subject to the vagaries of the weather and therefore still need to be backed-up. Unfortunately, the current situation is that most backup is from fossil-fuelled, highly inefficient power stations. These discharge useful heat to the atmosphere, thus wasting it, whilst generating electricity that is subject to major distribution losses because of the distance between generation and use. A more sensible option is to use
CHP, sited locally to the point of use, to provide users with both heat and electrical power. At the moment, CHP still uses natural gas, albeit considerably more efficiently than power stations, but the latest CHPs are already ‘hydrogen ready’ to take advantage of hydrogen as it becomes available (see below). The electrical aspects of using CHP are also important, especially as demand for low carbon electricity for electric vehicles and heat pumps increases. It’s vital, therefore, the CHP units selected can be connected to the electricity distribution infrastructure through compliance with Engineering Recommendation (EREC) G99. Issued by the Energy Networks Association (ENA), EREC G99 replaced the G59 standard and ensures harmonised network standards (Requirements for Generators) are incorporated into Great Britain’s distribution and grid codes for CHP and other power generators with capacities between 0.8kW and 1MW. CHP plant that has been tested to ensure its design meets the technical and compliance requirements of EREC G99 is eligible for inclusion on the ENA
type test register. This is a valuable resource for specifiers as it gives access to a list of products that hold fully compliant status to G99, providing reassurance that the correct product is selected. Fully compliant products do not require on-site witness testing by domestic network operators.
Hydrogen-ready
There has been considerable discussion around the potential for replacing gas and oil fired heating plant with heat pumps, ideally making use of renewable electricity. As a company that designs and manufactures both we can certainly see considerable scope for the latter, but there are also issues that will limit the breadth and speed of their application. In particular, heat pumps operate at maximum efficiency at lower water temperatures than gas fired heating plant. This is because the condenser operating temperature is largely a function of the pressure to which the refrigerant can be elevated. This sets its dew point and in most heat pumps this is simply not high enough to lift the system heating water to the same level of temperatures as can be achieved by a conventional combustion-based boiler. Heat pumps, therefore, generally must operate with a lower maximum heating system flow temperature. This means existing radiators and other heat emitters will not emit as much heat when served by a heat pump. This makes it difficult to introduce heat pumps as the sole heat source for space heating to many existing buildings without considerable investment in improved thermal insulation. It has also been estimated that between 600,000 and 1 million homes would require upgrades to their electrical network to make them suitable for running heat pumps. There is also the consideration of how much renewable electricity will be available as both heat
pumps and electric vehicles are rolled out.
It’s for these reasons that we see a major role for hydrogen as an alternative to natural gas in CHP and other heating technologies, and field trials blending hydrogen with natural gas are already underway. The proposed timetable is to begin blending – up to 20% hydrogen - this year. Consequently, there are already CHP units on the market that are ‘hydrogen-ready’. The ‘hydrogen- ready’ industry classification means they can run on a natural gas blend with up to 20% hydrogen and, crucially, can be quickly and cheaply converted to 100% hydrogen in the future. Clearly it makes sense to specify hydrogen- ready CHP and other heating plant in the future to help future-proof the end customer.
Designing CHP for optimum efficiency
A major efficiency benefit of CHP is that it generates both heat and electricity, though you can’t have one without the other. The design
of the system should therefore ensure maximum run-times for the CHP to maximise power production – as well as optimising heat production.
CHP is typically used alongside other heat sources, possibly serving a heat network from an energy centre. Here, the different technologies need to be integrated to ensure maximum run- times for the CHP.
For example, the CHP could be sized on thermal output to achieve a constant base load throughout the year, with the power generated by a G99-compliant CHP unit being used onsite, exported to the grid or both. Surplus heat in summer can be used to drive an absorption chiller to generate chilled water for air conditioning. Sizing the CHP to meet the base heat load through the year is often a good way to maximise run times. As noted earlier, excess electrical power can be exported to the grid but there is growing interest in storing it locally to increase system resilience and security of supply. Battery storage is one option, or it may be more cost-effective to use surplus electricity to pre- heat domestic hot water via an immersion heater in a cylinder. In this way, less heating is required by other heat sources to bring it up to the required storage temperature. With so many options available the control of this mixture of technologies becomes increasingly important. yet experience has shown that controlling multiple heat sources with different controllers is extremely difficult. Specification of controls able to effectively handle a range of heat sources as single units or in cascades is as important as the specification of the plant itself.
Summary
With its capabilities to generate low carbon heat (or zero carbon on 100% hydrogen eventually) – and to supplement local power networks - there’s no question that CHP has an important role to play in the UK’s decarbonisation. Such installations will need to be EREC G99 compliant and, ideally, ready to make use of hydrogen as a fuel in the years to come.
20 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2023 Read the latest at:
www.bsee.co.uk
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