Energy load profiling is an essential part of any CHP project, from the initial feasibility study to system installation.
The simultaneous generation of electrical power and thermal energy from a single source such as Combined Heat and Power (CHP), when compared to energy consumption of electricity from the national grid and heating/hot water from conventional boilers, provides energy users with a means to significant energy savings and reductions in greenhouse gas emissions.
However, to achieve its maximum impact, a CHP system needs to operate for as many hours as possible with simultaneous power and heating demands. If the CHP is not running, it will not make savings, and as a result it will not provide the expected financial return on its initial investment. Probably the major cause of underperforming CHP is incorrect sizing.
1. Why is the correct CHP size important?
Electricity supplies and heating systems are normally sized to meet a site’s maximum or peak demand or load, and then operated at reduced levels for much of the time. It will be inefficient to operate CHP in this way since a CHP unit is much less electrically efficient when operating at part load than at maximum load, if the load is too low then the CHP unit will not operate at all.
The best performing CHP schemes are the ones that utilise both the generated power and heat simultaneously, the reason for this is that the value of the electricity generated by the CHP unit is approximately equal to the cost of the gas consumed and the operational and maintenance costs. Therefore it is the utilisation of the ‘free heat’ that ultimately drives CHP economics.
For example, oversized CHP that runs at part-load output will not operate for enough hours and reduce the savings while an oversized CHP that can’t use all of the outputs of the CHP may miss out on savings from green tax incentives available for highly efficient or ‘Good Quality’ CHP.
In contrast, undersized CHPs will operate at full output but will then miss out on potential savings because of a shortfall in energy demand that could have been supplied by a larger unit.
2. Matching energy demands – load profiling
The optimal operating regime of CHP is therefore to find the best match for a site’s energy demands – exactly how much electricity and thermal energy does a site need and, of specific importance, when does it need it?
Energy consumption information (preferably half-hourly data) can be obtained from:
Existing monitoring systems, e.g. BEMS.
This information is used to calculate the site’s energy load profiles. Plotting the data against time will produce a pattern of how and when energy is consumed – on a half-hourly, daily, weekly and monthly basis. This is extremely important as invariably there will be differences in energy demand at different times of the day, week, etc. The more accurate the data, the more accurate will be the sizing of the CHP system.
3. Deciding on the best match
CHP systems are often sized to meet the site’s minimum demand or baseload so to ensure maximum operating hours. The CHP operates as ‘lead boiler’ with secondary boilers supplying the energy at peak times.
It may also be worthwhile considering a larger CHP, especially if the extra heat energy produced could be utilised or thermally stored; otherwise it still may be economic to reject the heat energy into the atmosphere during the summer months.
It may also be economically viable to size CHP systems to:
Load follow or ‘track’ the site’s actual electrical or heat demand.
Operate at a maximum electrical or head load output, exporting any surplus energy when required.
Plotting a site’s specific levels of heat demand against the number of hours annually produces a ‘load duration curve’.
For example, it could be seen that multiple CHP units would be a possible solution, especially useful where energy needs to be cascaded, i.e. fluctuating loads.
CHP systems for new schemes, such as building developments or district heating networks, where energy consumption data isn’t yet available, will need to be sized using comparison data from similar existing schemes or by using modelling software.
4. Avoiding the pitfalls
Inaccurate energy data, if used for load profiling, will inevitably result in a poorly sized and underperforming CHP system. Probably one of the most common pitfalls is making decisions based on incorrect assumptions or out-of-date information. This is particularly prevalent where data has been gathered from meter readings etc. and then energy efficiency measures have been subsequently introduced that have then reduced the site’s electrical and/or thermal energy demands.
5. Performance checks
Historical load profile data can be used to verify the CHP’s performance during the installation’s initial system commissioning, and throughout the first year’s operation, to check the system is matching the required demands.
It can also be used for regular ongoing checks to ensure optimal CHP system efficiency and to highlight if any changes have been made on site that could impact on the CHP’s output.
A load profile gives the shape of the existing energy consumption pattern, which has to be considered prior to designing and installing a CHP system.
The important question is whether the consumption pattern is balanced and the timeframe between generation and consumption needs to be considered too.
The load profile can be calculated from a number of sources, or a model if the building or site hasn’t been constructed yet, but it must take into account seasonal variability.
Be mindful of the three main pitfalls – planning, practicality and timing.