We are here to investigate if there is a place for cogeneration in the current renewable space.
Combined heat and power (CHP) systems generate electricity and useful thermal energy in a single, integrated system. The heat that is normally wasted in conventional power generation is recovered as useful energy, avoiding losses that would otherwise be incurred from the separate generation of heat and power.
While the conventional method of producing usable heat and power separately has a typical combined efficiency of about 45%-40%, CHP systems can operate at efficiency levels as high as 80%. But a California study found an average CHP efficiency of only 37%.
Cogeneration plant sizing
Engineering inherently promotes contingency into calculations. So we tend to overestimate both the electrical and thermal demand. A typical gas engine needs to run above 70% of the rated capacity. For every hour the engine runs below 70%, it needs to run for 2 hours to remove carbon from the cylinders. So in real life, the engines don’t run as often as the theoretical models and even when they do run, the engines are not running at their optimum point.
Another key is issue is heat and electrical load profiles don’t coincide. If the heat rejection is not working correctly, getting rid of such waste heat has its own costs. Some buildings use cooling towers to reject heat. This results in consuming extra power to run condenser water pumps and tower fans for longer. All this results in power and water bills rise, eating into the overall savings.
Perhaps the single biggest barrier to good cogen design and delivery is the delivery mechanisms used in Australia to create our CHPs. Typical mechanism is: the Builder – Designers – Electrical Contractor – Mechanical Contractor – Hydraulic Contractor – ESD Consultant – Engine supplier share the risk. No single entity is responsible for modelling inputs, plant design, supply, installation and maintenance risk.
Operational Expenditure – Utility Rates and Maintenance Costs
Often people who build the facility and pay the initial costs to build it are not those who pay the on-going energy costs of occupancy. So the utility rates forecasts and maintenance costs associated with the plant need to get the centre stage.
Courtesy of our friends at DC20 below is a Life Cycle Cost of a typical 1MW Generator. So the ongoing maintenance over 25 years is almost twice as much as the initial investment. It is important to do a complete whole of life cost as part of the feasibility study.
First things first – utilities rates need to stack up. ie: is it cheaper to buy electricity from the grid and gas to create hot water than burning gas to create heat and electricity?. Below is a cheat sheet we put together for a 1MWe system. All things being equal, it is cheaper to run the cogen, if the gas purchase price is $5GJ and and electricity $0.04c/kWH.
Just like anything, if it the inputs are correct there is certainly a place for district cogeneration plants. We highly recommend engaging a cogen specialist under a Design, Construction and Operate engagement to review your site requirements.