Choosing cogeneration for energy efficiency in food and beverage

Energy-efficient manufacturing is critical as changing economic and environmental demands force manufacturers and plant developers to consider alternative forms of production. Cogeneration is an increasingly important energy option to consider when designing food and beverage manufacturing facilities, providing an attractive long-term management choice.

Typical centralised electricity generation plants are high energy generators but produce a lot of heat and expend energy to get rid of it. This heat is not used, making production inefficient. There is also a high degree of energy loss in the transmission lines to the plant. Cogeneration basically takes the generation of electricity to the site and allows heat to be used rather than expending energy getting rid of it. This heat would have previously been paid for in the plant’s boilers, but with cogeneration done correctly it comes at a reduced cost.

In food and beverage processing, trigeneration is also useful as it uses heat to drive refrigeration - making it well placed in the efficiency cycle. Trigeneration using absorption chillers needs low-grade heat and utilises what is left over after steam and hot water production. This option is energy efficient and good for the environment - using less energy from traditional fuels means fewer greenhouse gas emissions.

Cogeneration’s biggest drawback is that energy supply from centralised power plants is cheap, and to make cogeneration economically viable requires efficient design, efficient usage and the right energy prices. To design an efficient plant, the first element is making use of all of the incoming energy.

For example, a milk powder plant may have little need for refrigeration, while a cheese plant is expected to require a high level of temperature control and refrigeration usage. Conversely, the powder plant will have higher relative heat demand. If refrigeration is removed, each may use about the same amount of electricity relative to milk inputs. Balance is extremely important, and the developer must ensure the cogeneration plant considered is the right match to achieve balance.

A large range of plant sizes exists, from the engines and turbines generating the electricity through to heat recovery steam generators, heat exchangers and absorption chillers. Matching a plant to these varying energy demands is a relatively simple yet detailed process, with the primary aim being not to waste anything.

It does not always work, however - if a plant is too small it is difficult to achieve economies of scale through cogeneration or trigeneration. An unfortunate reality, but possibly a rule of thumb, is that plants under 1 MW will struggle, as larger machines perform more economically.

Added realities are that the solution needs to cater for usage peaks and technical requirements around the way plants operate - they perform better, cleaner and with less maintenance if run consistently. These details need to be modelled and optimised to find the most appropriate solution for the particular application. It is clear, in this era of rising energy costs, that there are real gains to be made, both in economics and the environment, by an effective cogeneration or trigeneration project.

*Dr Steve Edwards, Principal Consultant Energy and Water, pitt&sherry, has 25 years’ experience as a scientist with a background in organic and natural products chemistry, and in aquaculture. Since joining pitt&sherry in 2007, Steve has gained extensive experience in process optimisation and process modelling, with a particular focus on carbon accounting and energy and water efficiency.