CPD PROGRAMME
paying CCL on the gas used to fuel the scheme, while also benefiting from not having to pay CCL on the electricity generated. For larger consumers (with total electricity consumption under 6,000 MWh per year), CHP can reduce the liabilities under the Carbon Reduction Commitment (CRC).8 ‘Good quality CHP’ can also attract business
rates exceptions.9 Additionally, the CHP can enhance the environmental rating of a building. For example, a properly installed, commissioned and monitored system can attract BREEAM credits for areas including emissions, energy monitoring, low and zero carbon (LZC) technologies and innovation.
Sizing CHP schemes This requires historical or projected data about heat and electricity use. From this data, an annual profile may be established for heat usage and concurrent power demand that will assist in determining the base load for the building. As indicated in Figure 3, the profiles of heat load will be highly dependent on the building type, and this will strongly influence the installed capacity of CHP (keeping in mind the ‘rule of thumb’ average 12 hours-a-day, 4,500 hours-per- annum breakpoint for economic viability). However, proper consideration requires half-hourly or hourly data of concurrent heat and power usage, as using time frequency data (see Figure 3) may conceal periods of mismatch between the power and heat loads. Similarly, temporal data will enable spreadsheet models to be created that can include the vagaries of fuel tariffs. A suitably sized CHP can then be matched to the profile, which typically may be under 20% of the peak heat load. It is possible to have electrically-led operation,
where the CHP is operated to primarily meet an electric load, but this will inevitably lead to the wasteful rejection of heat. Although there could be occasions where this might be economical
Figure 4: Simplified CHP hydraulic loop (Source: Bosch)
(for example, in off-grid locations), the total efficiency of the scheme will drop dramatically.
Hydraulic integration Figure 4 shows the typical water circulation around a CHP unit. A pump circulates return water first through the oil cooler, followed by the engine block and on to the heat exchanger around the exhaust system, before finally transferring the collected heat via a plate heat exchanger to the heating circuit. With this configuration, the return temperature to the CHP must not fall below 70°C, as this can damage the engine. Some CHP units have an internal ‘back end protection’ system fitted to maintain the return temperature or, if it is not incorporated, an external (but closely coupled) back end protection system is used. The CHP units will operate in conjunction
with other heat sources, such as condensing boilers. To help meet peaks in demand, a thermal store is often incorporated into the system and, for mini-CHP, this is typically sized10
reducing the call on the more carbon-intensive heat sources, even at times when the load is below the point where the CHP plant can operate effectively. This adds further complexity to the economic modelling of the CHP system, as it allows heat to be supplied from that generated by the CHP even at times, such as at night, when the electrical tariff would make it uneconomic to operate the engine. Using a predictive control strategy, the charging of the thermal store can be optimised to maximise the operational hours of the CHP. A CHP installation can offer a sound financial investment, but this is dependent on a rigorous site evaluation and pre-selection, appropriate sizing, holistic design, installation, and a fully monitored and maintained operation. © Tim Dwyer, 2013.
to store the heat from one hour run time
of the CHP. The thermal store and CHP should be connected into the system so that the heat from the CHP is used in preference to any other non-renewable heat source, acting as the ‘lead boiler’. This will extend the period during which the CHP may generate electricity, as well as
Further reading: There are two particular documents that provide extensive detail on the appropriate application of CHP. CTV044 – Introducing CHP, published by the Carbon Trust, is available on the web, and the recently revised CIBSE AM12 Combined Heat and Power for Buildings provides much excellent material and is available through the CIBSE Knowledge Portal – freely downloadable for all CIBSE members.
References 1 AM12 Combined Heat and Power for Buildings, CIBSE, 2013.
2 CTV044 – Introducing CHP, Carbon Trust, 2010, http://
www.carbontrust.com/media/19529/ctv044_introducing_ combined_heat_and_power.pdf accessed 5 May 2013.
3
FITs Quarterly Statistics, 1 October – 31 December 2012, Ofgem, 2013.
4 CTC788 – Micro-CHP Accelerator – Final Report, Carbon Trust, 2011,
http://www.carbontrust.com/media/77260/ ctc788_micro-chp_accelerator.pdf –accessed 5 May 2013.
5 7
https://www.gov.uk/feed-in-tariffs – accessed 5 May 2013.
6
https://www.gov.uk/renewableheatincentive – accessed 5 May 2013.
Use of CHPQA to Obtain Enhanced Capital Allowances,
https://www.chpqa.com/guidance_notes/GUIDANCE_ NOTE_42.pdf – accessed 5 May 2013.
8 DECC – CRC Energy Efficiency Scheme,
https://www.gov. uk/crc-energy-efficiency-scheme – accessed 5 May 2013.
9
http://chp.decc.gov.uk/cms/business-rating-exemption/ – accessed 5 May 2013.
Figure 3: Typical heat load profile examples for different building types (Source: Bosch) 10 Bosch internal document.
www.cibsejournal.com
June 2013 CIBSE Journal
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