This is why Highview has been testing its 300-kilowatt pilot plant for the past nine months, supplying electricity to the UK's National Grid. The process stores excess energy at times of low demand by using it to cool air to around -190 °C. Excess electricity powers refrigerators that chill the air, and the resulting liquid air, or cryogen, is then stored in a tank at ambient pressure (1 bar). When electricity is needed, the cryogen is subjected to a pressure of 70 bars and warmed in a heat exchanger. This produces a high-pressure gas that drives a turbine to generate electricity. The cold air emerging from the turbine is captured and reused to make more cryogen. Using ambient heat to warm it, the process recovers around 50 per cent of the electricity that is fed in, says Highview's chief executive Gareth Brett. The efficiency rises to around 70 per cent if you harness waste heat from a nearby industrial or power plant to heat the cryogen to a higher than ambient temperature, which increases the turbine's force, he says.
Unlike pumped-storage hydropower, which requires large reservoirs, the cryogen plants can be located anywhere, says Brett. Batteries under development in Japan have efficiencies of around 80 to 90 per cent, but cost around $4000 per kilowatt of generating capacity. Cryogenic storage would cost just $1000 per kilowatt because it requires fewer expensive materials, claims Brett.
Colin Bennett on 31 Mar 09By Jorge Moreno, Environmental and Building Technologies, Frost & Sullivan With more end users focusing on reducing energy costs, energy-saving water-to-water heat pump (WTWHP) chillers are being deployed to reduce a facility's utility bills. A WTWHP chiller is a water-cooled chiller that is designed to produce hot water at a specified temperature. The use of a WTWHP chiller is very similar to a conventional centrifugal chiller except for the fact that it uses two compressors, slightly different piping configurations, and more advanced controls in order to balance cooling and heating loads. In a conventional chiller, cold water is produced for comfort cooling, and the hot water that is extracted from the refrigeration process goes into a cooling tower and is released into the atmosphere. In a WTWHP chiller, this hot water is captured and relocated to a second heating stage, where the temperature is raised and the water is used as a heating source for a building's heating requirements. The key strength of WTWHP chillers is the high coefficient of performance (COP) that translates into significant energy savings and a shorter payback period. On the other hand, the key weakness is that it can only provide such benefits in a narrow range of applications primarily due to its coincident need for cooling and heating requirements throughout the year to ensure efficiency. A coincident need means that the application demands sizable water heating load along with the typical high cooling requirements in summer, and a sizable chilled water load along with the typical heating requirements during winter. Cooling output is directly dependent on the demand for heating, and vice versa. Consequently, in the absence of sufficient heating requirements, there is only a limited amount of cooling that can be produced. Any excess heating or cooling cannot be stored and hence, it is critical to align the cooling with the expected heating requirements. Coincidentally, in the absence of suf
