An impressive idea is out in the International Journal of Energy Research from the University of Leeds and the Chinese Academy of Sciences. The research group has invented a new way to answer quick peak electricity demands.
Peak demand and particularly quick and short-lived peaks are when demand for electricity soars, causing a problem for electric grid operators. The amount of electricity drawn from national grids varies enormously at different times of day. It usually peaks in the early evening for a couple of hours after homes are occupied from people leaving school and work. But it’s the short duration peaks that cause such concern. Those common spikes turn up after major televised sporting events, during commercial breaks and in the morning hours. It’s ‘the everyone hits the microwave and refrigerator’ and those industrial startups with homemakers staring the clothes dryer moments that pull down the available volts and amps.
Grid operators matching the highs and lows in demand with a steady supply is a major challenge. The companies typically top up a ‘base’ supply of energy with electricity from power plants that are just switched on to cope with the peaks. But those natural gas-fired generators often used to feed these peaks are notoriously inefficient, expensive to run and sit idle for long periods of time. The system as it works now is both energy consumption dense and financially consumes lots of money for very little operating time. Answering peaks is a huge chunk of your power bill.
University of Leeds Professor of Engineering, Yulong Ding, and colleagues are proposing a more environmentally friendly system that could also be much cheaper to run.
Of crucial significance, the system would store excess energy made by a plant supplying the ‘base’ demand and use this to supply the ‘peaks’ in demand – as and when they happen. The clever boffins of the UK and China have a fascinating take on forming a fuel to store energy.
The practice is to use excess electricity to run a unit producing liquid nitrogen and oxygen – or ‘cryogen’ from right out of the atmosphere. At times of peak demand, the nitrogen would be reheated to a boil – using waste heat from the power plant heat and as needed from the environment. Step one: the hot nitrogen gas would then be used to drive a turbine or engine, generating the peak demand’s ‘top up’ electricity.
Step two: the oxygen would be fed to a combustor to mix with the natural gas before it is burned. Burning natural gas in pure oxygen, rather than air, makes the combustion process more efficient and produces almost no nitrogen oxide. Instead, the ‘oxygen + fuel’ combustion method produces a concentrated stream of carbon dioxide that can be removed easily in solid form as dry ice. Clean, neat and the only effluent would be what’re produced when making the cryogen. Smartly managed with adequate storage, the efficiency could be quite high.
Operating an integrated system with cryogen and the down process methods the amount of fuel needed to answer peak demand could be cut by as much as 50%. Greenhouse gas emissions would be lower too, thanks to the greatly reduced nitrogen oxide emissions and the capture of carbon dioxide gas in solid form for sale. The base production efficiency if effluent free would make peak demand effluent free as well. It’s an elegant, innovative and simple design that begs the question how could this not have been thought of before?
Professor Ding said, “This is a much better way of dealing with these peaks in demand for electricity. Greenhouse gas emissions would also be cut considerably because the carbon dioxide generated in the gas-fired turbine would be captured in solid form. On paper, the efficiency savings are considerable. We now need to test the system in practice.”
Technically speaking the new system combines a direct open nitrogen (cryogen) expansion cycle with a natural gas-fuelled closed Brayton cycle and the CO2 produced in the system is captured in the form of dry ice. Thermodynamic analyses were carried out on the system under the baseline conditions of 1 kg s−1 natural gas, a combustor operating pressure of 8 bars and a cryogen topping pressure of 100 bars. The results show that the energy efficiency of the proposed system is as high as 64% under the baseline conditions, whereas the corresponding electricity storage efficiency is about 54%, an 10% gain or nearly a 20% improvement.
A sensitivity analysis has also been carried out on the main operating conditions. The results indicate that the baseline performance can be enhanced by increasing the gas turbine inlet temperature, decreasing the approach temperature of the heat exchange processes, operating the combustor at an optimal pressure of ~7 bars and operating the cryogen topping pressure at ~90 bars. Further enhancement can be achieved by increasing the isentropic efficiency of the gas turbine and the liquefaction process. The results of this work also suggest that the power capacity installation of peak-load units and fuel consumption could be reduced by as much as 50% by using the newly proposed system. Further work is suggested for an economic analysis of the system.
The engineering choices for a working design are a huge list with lots of variables to work through for different situations. The outstanding point is the existing generating capacity could fuel up for the peaks leaving the whole investment for fresh fuel sourced peak demand generation out of the cost equation. It’s a superb idea with lots of potential, not just for power plants either.