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7 considerations when sizing a CHP unit

7 considerations when sizing a cogeneration plant

A number of challenges may be encountered when sizing a cogeneration system. Here are the seven main challenges and how to tackle them.

Probably the most common question is: ‘What size cogeneration 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 sizing a cogeneration system 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.

1. Can you give me a quick idea of how cogeneration will work?

Typically, Cogeneration systems run at 7,884 hours per year. Sometimes these systems load follow, meaning a site doesn’t need 100% of the systems electric and/or thermal output capability – when this is the case the system can run at reduced output. Systems can be designed to run with efficiency as low as 60% of operation per year – however, to be cost-effective, it needs to be able to generate both an electricity and heat output for this minimum duration.

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 cogen system. A spark spread of between 3 and 8 is a further indication that cogeneration is worth consideration.

2. What’s the most important information I need to start sizing my cogeneration system?

Understanding a site’s energy demands is critical for correctly sized system. 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.

The cogeneration plant will be sized to supply this energy and so it is essential this data is accurate if the system is to run continuously and achieve the minimum hours of operation per year. Undersized plant will not achieve maximum energy savings, and oversized cogeneration systems will operate at part-loads, which will be an uneconomic and inefficient way of using the technology and is unlikely to deliver the desired return investment.

It is also critical to understand the client’s goals. For example, when assessing if there are requirements for backup power or if a client has high demand charges, it may be prudent to oversize electrically more than standard acceptable design would call for – exceptions like these should be assessed.

Does the utility allow for exporting and what is the value-add for doing so?

3. Does the utility allow for exporting and what is the value-add for doing so?

The prerequisites for exporting are:

  • A suitable connection to the utility provider
  • An export meter
  • Permission from the local distribution network
  • An agreement with the utility company that buys the surplus

Cogeneration systems that have been sized to meet a large base-level heat demand can produce more electricity than the site requires. If this excess supply is substantial and predictable, an agreement can be reached with a utility company that will purchase the excess, generating additional revenues for the site. Installing the equipment required for exporting electricity can cause disruption, but with detailed planning, the impact can be minimized. Any short-term disruption should be weighed against the longer-term benefits.

It is possible that the surplus electricity available is insufficient to justify the expense and effort required to arrange for exporting. It is also possible that site power demand may increase in the future, therefore consuming any surplus electricity and making the export connection obsolete. Costs may be incurred for supporting infrastructure, such as additional equipment and for the maintenance of export hardware. These factors should be carefully considered in relation to the site demand profile and any planned growth before committing.

4. The energy bills don’t have half-hourly data readings/Interval Use data

For data accuracy, half-hourly readings are preferable. The basis for correctly sized cogeneration plant is reliable 24 x 7 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?

5. What’s more important for cogeneration – electricity or heat demand?

Both are important. Cogeneration systems generate 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.

Cogeneration 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, systems can also be designed to have a maximum or electricity-led or heat-led output.

6. If I design for baseload, how do I get any extra heat or electricity?

When the cogeneration plant operates at baseload demand, extra or peak heat will need to be supplied by a secondary boiler, and additional electricity supplied from the utility grid. In some cases, there may be an option to supply peak demands by installing a second smaller cogeneration unit, but the economics of this will need to be carefully and thoroughly considered.

7. Do I need to think about any practical issues on site?

Site issues that could affect sizing the equipment include available space on site for the equipment, including any auxiliary and cooling equipment; access for maintenance activities; the proximity of the cogeneration system's proposed location to the site’s infrastructure – electrical and heating connections; ambient air flow availability and the fuel supply needs to be sufficient for the cogeneration unit selection.