COMBINED HEAT & POWER


BSEE


However varying interpretations of the ‘annual total heating demand’ can produce dramatically different energy calculations – and potentially encourage oversizing of CHP for compliance.


This is a real-life example, calculated for a 395-bed hotel. The estimated floor area for the CHP annual contribution calculation is 15,500 m2.


The estimated annual fossil fuel consumption, based on the CIBSE Guide F Energy Efficiency in Buildings for hotels, is 260 kWh/m2/yr.


By multiplying the two figures, we arrive at the annual energy, which is 4,030,000 kWh/yr.


Our CHP assumption is for two R-Gen 20/44 units providing a total of 88 kW thermal output, operating 18.5 hours a day, or a total of 6,752 hours per year.


To calculate the CHP power output, we multiply the 6,752 hours x 88 kWt which gives us 594,176 kWh/yr. Following this calculation, the percentage of the load supplied by the CHP units would be just 14.7% (594,176/4,030,000 x 100). This could therefore result in a larger CHP unit being specified to achieve perceived compliance.


CHP hot water contribution


But the CHP is not providing total heating within the building. Given that the CHP is providing cold water pre-heat for the domestic hot water requirements, a more accurate approach would be to calculate the CHP hot water contribution in the hotel.


The hot water profile for this hotel can be broken down as follows:


395 showers x 35 litres x 2 = 22,120 litres 20% for baths @ 60 litres x 79 = 4,740 litres Cafe/bar = 6 litres per serving x 395 x 2 = 4,740 litres Total hot water for hotel in 24 hours = 31,600 litres = 2 peaks at 2 hours = 15,800 litres Hourly peak demand = 7,900 litres/hr


We can then calculate the CHP hot water contribution. The two R-Gen 20/44 units produce 1.234 litres/hr @ 50T hot water, or 8,332,585 litres/year.


The hot water profile for the hotel over one day is 31,600 litres/day, or 11,534,000 litres/year.


Following this calculation, the CHP contribution, based on 18.5 hrs/day running, 365 days a year, would in fact be 72% (8,332,585/11,534,00 x 100) – and in full compliance.


Reap the benefits


The efficiency benefits from CHP, boosted by the ability to produce lower cost on-site electricity, make a clear business case for CHP. A further advantage is that CHP schemes that are registered and meet ‘Good Quality CHP’ status are exempt from Climate Change Levy (CCL) payments on the gas they use to generate electricity, helping reduce environmental tax bills and mitigate energy costs. Further guidance is available here:


https://www.gov.uk/guidance/combined-heat-power- quality-assurance-programme


But an oversized CHP will sit in the plant room unable to run. Our recommendation to those looking to use CHP in hotels and other buildings echoes that of the CIBSE Applications Manual AM 12: seek advice from experienced suppliers at an early stage of design. That way, the full economic and environmental benefits can be reaped.


Read the latest at: www.bsee.co.uk


BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2020 17


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