How to generate an accurate load profile for CHP

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Courtesy of ENER-G

If you fail to accurately work out your site's energy demands, you'll give rise to a cogeneration system that underwhelms when served up to your customer.

That's why load profiling is the key ingredient in successfully specifying your cogeneration project and ensuring that you meet your clients' expectations.

Generating an accurate load profile

In order to generate a load profile for an optimal combined heat and power (CHP) system, you need to know exactly what the existing heat and power consumption and pattern of the building is and how you can measure it.

Understanding precisely how a site’s buildings and processes consume energy is crucial to deciding whether there is scope for a CHP project, and then determining its size and design.

Energy load profiles are a representation of the site’s existing energy consumption, or load. When the site’s electrical and thermal energy loads are plotted graphically against time, they visually portray a pattern of how much energy is required on site at a specific time.

Why are load profiles important for CHP sizing?

For CHP to be economically viable, its operation must be optimised. If the full benefits of energy cost savings and a reduction in carbon emissions are to be achieved, a CHP system needs to be sized correctly to operate at its optimal performance output. It, therefore, needs to match the site’s actual energy load profiles as closely as possible.

Data collection

In order to calculate load profiles, data needs to be collected to show how and when energy is actually required. The more information gathered, the better the calculations and the load profiles produced.

Data can be gathered by:

  • Monitoring information from a site’s building energy management system (BEMS) – this might even generate the various profiles required.
  • Analysing the current year’s bills for electricity and the fuel used to generate on-site space heating and hot water. These are generally monthly or quarterly. Ideally, previous years’ bills will also provide invaluable information to explain any annual variances.
  • Contacting your energy suppliers to request hourly, or even better, half hourly meter readings. This will ultimately provide a better understanding of patterns of energy use and better load profiles.
  • If hourly readings are unavailable, short-term monitoring may be used to measure actual energy consumption.

Energy audits, like those carried out under the Energy Savings Opportunity Scheme (ESOS) and/or by special third party assessors, will provide a basis for acquiring good, reliable information.

Checking data reliability

If in recent years there have been noticeable variations in seasonal temperatures, it will be beneficial to also check the information from previous years, make any adjustments and check if this provides a more representative profile. The more detailed the analysis carried out at this stage, the more the CHP system’s performance can be optimised in design.

It is also essential to establish if the data is, and will continue to be, representative of the site’s actual energy loads going forward. The following needs to be checked and confirmed:

  • Are there any proposed changes to the site’s energy requirements in the future that could increase or decrease the energy load, i.e. have energy saving measures been implemented, such as energy-efficient lighting and/or insulation, or are they likely to be?
  • Don't forget the flip side. The site might be about to be redeveloped or expanded, in which case a bigger CHP unit might be beneficial.
  • Can operational savings be made by making processes and equipment operate more efficiently, i.e. upgrading/replacement of inefficient boilers?
  • Have cooling loads been accounted for?

Compiling the data

The data gathered should be compiled and plotted graphically against time to show how energy consumption varies:

  • At different times of day (morning/afternoon/evening peaks and troughs).
  • On different days of the week (e.g. is energy load constant at the weekend?).
  • With seasonal variations (summer loads versus winter loads – heating may be greater in winter, but is cooling required in the summer?).

Plotting the actual energy load against annual total hours will also produce a load duration curve, which is very useful for visualising how a CHP system (operating as lead boiler) can operate with support from a secondary boiler (for peak loads), or with multiple CHP units to accommodate a widely varying load.

Read ENER-G's technical guide to using load profiling in specifying cogeneration projects and building the economic case for investment: How building consultants can complete an end-to-end Combined Heat and Power (CHP) economic feasibility study

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