Beata Blachut, CHP product manager with SAV Systems, explains that to achieve maximum performance from a CHP installation it pays to use a modular approach that takes advantage of the latest technologies
There is often a perception that combined heat and power (CHP) is unsuitable for smaller projects, as the technology is often associated with large scale plant designed to meet high but steady power and heat loads.
However, the energy and cost saving benefits of CHP can be enjoyed by a wide range of building types by utilising modular CHP (configured rather like modular boilers) that can track loads and modulate outputs accordingly.
The challenge with CHP is that when it’s running it is generating both heat and electrical power – you can’t have one without the other. Therefore, the key to making CHP sufficiently flexible to cater for a wider range of projects is to ensure it manages both of these efficiently.
For example, conventional fixed output, non-modulating CHP is usually sized to match the site’s base electrical load. The result is that the CHP does not contribute to the site’s power usage beyond that base electrical load. Clearly this limits the energy savings that can be achieved while also increasing the amount of expensive grid electricity that has to be purchased. It costs less to use gas to generate electrical power from a CHP unit than to buy it from the electricity company.
Alternatively, the CHP may be sized to meet a base heat load and then the electrical power output is determined by the heating requirements of the site. This can often result in selling electricity to the grid at unfavourable rates.
With CHP units that are able to modulate their output and track site demand, the power output of the CHP is aligned to changing site requirements, providing effective management of the most expensive utility (the electrical power), to deliver maximum cost savings (see Figure 1 – right). Any excess heat is typically transferred to water in a storage vessel to ensure it is not wasted.
This performance is further optimised by the ability of modulating CHP to ‘self learn’ that building’s loads and adapt to changing conditions (see Figure 2 – below left). This also ensures that if any other power saving measures are implemented, such as upgrading the lighting, the CHP will adapt. There are many instances where CHP has had to be turned off following such upgrades because the site’s electrical demand becomes too low for the CHP to be viable.
With modular CHP, up to five units can be combined to provide a range from 15kWe/30kWth to 100kWe/ 200kWth. So, on a small site, one unit may be sufficient, while several small units can be combined to meet the needs of larger sites.
This principle was demonstrated recently when SAV Systems evaluated the options for a small leisure centre. With conventional CHP, sized to cover base electrical demand only, the CHP would only provide 39% of site electricity usage, resulting in energy and carbon savings of around ten percent. In contrast, by using a modular configuration of modulating CHP units, 80% of the site’s electrical demand could be met.
Constant flow temperatures
Another challenge is that traditional CHP operates with a constant temperature differential, resulting in variable flow temperature and inconsistent performance of the system.
The solution to this is to incorporate a heat distributor that maintains a constant flow temperature, corresponding to the design flow temperature, irrespective of the return water temperature. The flow controller in the heat distributor can be set to deliver a heating flow temperature in the range 40-80°C.
As a result, the CHP always produces high grade heat that can be used on-site without ‘topping-up’ from boilers. In fact, as long as the heat loads are within the CHP’s capacity, there will be no need to use the boilers.
Furthermore, any surplus heat is stored at 80-85°C and this stored heat helps to optimise CHP operating times and further reduce the likelihood of back-up boilers being operated.
Load tracking, modular CHP can be incorporated into a heating system either in series or in parallel with the boilers. In either case, the key criteria for achieving the best possible performance are laid out in the requirements of CIBSE Guide AM12, which identifies the following key objectives:
- The CHP unit should operate in preference to the boilers at all times.
- When boilers are in service, they should not prevent the CHP from operating at maximum output.
- Heat recovery from the CHP unit should be optimised.
- The CHP unit should always be able to generate heat even under part-load heating conditions.
- The building heating system should be designed so that return temperatures do not result in the CHP unit shutting down unnecessarily.
Given the potential for CHP to help reduce energy consumption and carbon emissions, it’s clearly important to ensure the system delivers maximum performance. Selecting modular CHP that is able to adapt to changing conditions is an obvious way to achieve this.
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