A number of challenges may be encountered when sizing a CHP project. Here are the six main challenges and how to tackle them.
Probably the most common question is: ‘What size Combined Heat and Power (CHP) plant is required for the best energy savings?’ A relatively simple question; but unfortunately not one which has a simple, straightforward answer. Even the concept suggests a certain complexity: the generation of both heat and power as the output from one system.
The challenges of CHP sizing require a logical approach, which if correctly followed will provide a reliable and highly efficient system. Here are some of the challenges that most frequently arise.
I just want a quick idea if CHP will work?
There is a rule of thumb which will give an initial indication whether CHP is viable. CHP requires a minimum 4,500 hours of operation per year to be cost-effective – it needs to be able to generate both an electricity and heat output for this minimum duration. A site’s energy consumption data will therefore need to confirm this minimum energy usage profile..
Another principle is the site’s spark spread. This is the difference between the electricity sell price and the cost of the gas available to generate electricity with a CHP. A spark spread of between 4 and 8 is a further indication that CHP is worth consideration.
What’s the most important information I need to start sizing CHP?
Understanding a site’s energy demands is critical for correctly sized CHP. A site’s energy consumption data should be collected to produce energy load profiles – usually plotted against time to provide a pattern showing how much energy the site uses and at what times.
CHP will be sized to supply this energy and so it is essential this data is accurate if the CHP is to run continuously and achieve the minimum 4,500 hours of operation per year. Undersized CHPs will not achieve maximum energy savings, and oversized CHPs will operate at part-loads, which will be an uneconomic and inefficient way of using CHP and is unlikely to deliver the desired return investment.
My energy bills don’t have half-hourly data readings?
For data accuracy, half-hourly readings are preferable. The basis for correctly sized CHP is reliable load profiles – the whole-life operation depends on it. If normal quarterly bills do not contain such a detailed breakdown, then contact the energy suppliers as they should be able to provide this information.
Alternatively, if the site has a Building Energy Management System (BEMS), the required data may already be recorded. A further option is short-term monitoring – installing temporary metering to measure the data over a designated period, from which the load profiles can be estimated.
What’s more important for CHP – electricity or heat demand?
Both are important. CHP generates electricity and heat simultaneously and will have a specific heat-to-power ratio requirement, so this needs to be compared with the site’s required heat-to-power ratio which should be calculated from the site’s actual load profiles, where possible.
CHP units in buildings are often sized to operate at minimum heat demand, known as the baseload when operating as the site’s ‘lead boiler’. However, CHPs can also be designed to have a maximum or electricity-led or heat-led output.
If I design for baseload, how do I get any extra heat or electricity?
When the CHP operates at baseload demand, extra or peak heat will need to be supplied by a secondary boiler, and additional electricity supplied from the national grid. In some cases, there may be an option to supply peak demands by installing a second smaller CHP unit, but the economics of this will need to be carefully and thoroughly considered.
Do I need to think about any practical issues on site?
Site issues that could affect CHP sizing include available space on site for the equipment, including any auxiliary and cooling equipment; access for maintenance activities; the proximity of the CHP’s proposed location to the site’s infrastructure – electrical and heating connections; and the fuel supply needs to be sufficient for the CHP selection.