When electricity is generated centrally at large power stations, at least 50% of the energy in the fuel is wasted as heat released into the atmosphere. Further losses occur when the electricity is transmitted large distances through high voltage cables. This wasted energy, coupled with the infrastructure and maintainance costs of power stations and transmission networks, is the reason that electrical energy has a higher economic value than heat.
Generating electricity locally at a household or community level in a combined heat and power (CHP) scheme can yield very high overall efficiencies since both electricity and heat can be used at the point of generation. MicroCHP is the co-generaton of electricity and useful heat at the domestic/small business scale. It is equivalent to a household boiler that generates electricity. These systems are grid-connected and the end user can export unused electricity back to the grid or import additional power at peak usage times.
Small scale domestic generation is attracting interest from major utility companies and could offer reduced energy costs to the customer. It also eliminates the need to increase grid capacity and build large power stations. Boiler manufacturers are also heavily involved in microCHP as they seek the next generation products to replace condensing boilers.
Loss Comparison Between MicroCHP and Conventional Distribution
The Recuperated Microturbine for MicroCHP
Microturbine-generators are well suited for producing electricity in a microCHP scheme, due to a high power to volume ratio and long service life. However, in their most simple arrangement, the electrical efficiency is not high enough to justify the additional capital cost of a microCHP system. The electrical efficiency can be improved by the use of a recuperator, which is a heat exchanger that transfers heat from the exhaust stream of a gas turbine engine back to the air before it goes into the combustor. This reduces the amount of fuel burned, doubling the electrical efficiency of the system and making the recuperated microturbine the optimum microCHP power source. However, the mechanical and thermal demands placed on a recuperator are significant, and many recuperator designs have proven to not meet service life requirements.
Recuperated Microturbine Schematic
The Hiflux Recuperator
The core technology at Hiflux is a high temperature recuperator specifically developed for gas turbine applications. Innovative design and state of the art modelling tools have been used to create a patented design that has the right combination of strength and compliance required for high temperature operation. An array of pins, laser welded to plates, form the heat transfer path as well as providing structural integrity.
Pin Array Schematic
Flow in Pin Array
The Hiflux design is made up of a number of identical cells, each cell consisting of two plates joined by pins. The cells are very strong and well suited to withstand the load from a pressurised fluid. Hiflux recuperators heave been tested successfully at up to 800°C. The cells are flexibly connected to the engine using a novel, patented design. With a counterflow arrangement, a high effectiveness can be achieved. Instrumented field tests of a customer prototype installation have shown a peak effectiveness of approximately 90%.
- Reliable operation at turbine exhaust temperatures up to 800°C
- High resistance to creep, oxidation and thermal shock
- Low pressure losses for maximum engine output
- High surface to volume for maximum compactness
- Customers’ exact requirements met with flexible manufacture
- Quality manufacture achieved through automation
- Available in stainless steel and nickel superalloys
The Hiflux high-temperature heat exchanger is a key component in a European microCHP system, doubling the electrical efficiency. A high electrical efficiency is critical, due to the disparity between fuel and electricity unit prices.