Efficient Conversion of Solar Energy into Electricity using Convection Currents and Wells Turbines in a Constant Volume Closed Cycle with Thermal Bricks for Energy Storage and Overnight Generation.

The author proposes a new method for the conversion of solar energy into electricity which is illustrated. The principles should apply from small-scale domestic generation to commercial solar farms.

The solar collector and energy store are securely sealed and contain air at atmospheric pressure. The solar absorber is placed midway between ground level and the glass; it is made of metal with selective black coating and is abundantly perforated to allow the vertical passage of air. Wells turbines are suggested for conversion of air flow into electricity as they have the remarkable feature of rotation in the same direction when air flow is reversed. The energy store could be of thermal bricks as in electricity storage heaters though cheaper materials such as concrete, rock pile, stone or pebble beds would be adequate. There must however be good air flow through the storage material. Structural support on the turbine side should be perforated to allow lateral access of air. The entire structure should be exceptionally well insulated to minimise any loss of solar energy absorbed.

During the day solar energy will be transmitted through the glass with 90% efficiency. The air immediately above the absorber is warmed and will rise. This in turn will draw air from beneath the absorber. A cycle of convection currents will be set up. Warm air will rise from the absorber to the glass and the additional pressure will drive the upper turbine. As it meets the thermal bricks which are cooler, it will surrender its heat energy and descend. The cool air at the bottom of the energy store will be drawn through the lower turbine into the solar collector at ground level to repeat the cycle. There will be a continuous transport of energy from the solar collector via the turbines into the energy store.

All the above changes take place at constant volume. Solar energy that passes through the glass warms the air above the absorber. The air cannot expand so the additional energy raises temperature and pressure in equal proportion. Turbines convert increased pressure into electricity with high efficiency as in the steam turbine, cased wind turbines, compressed air energy storage or the Wells turbine in wave energy applications. An efficiency of over 60%, perhaps over 90% should be achieved. Beyond the turbine residual heat in the warm air is absorbed by the thermal bricks which act as a daytime energy sink.

At night, cold outside air sets up a reverse cycle of convection currents. Air underneath the glass is cooled and falls inside the solar collector. This draws warm air from above the energy store through the upper turbine. The cold air underneath the solar absorber passes through the lower turbine and is drawn upwards through the bricks. Thus during the night the temperature of the bricks will gradually fall to external ambient temperature.

Again, however, all changes take place at constant volume. Every unit of energy that exits through the glass lowers the temperature and pressure of air in the solar collector in equal proportion. Every unit of energy released by the bricks will raise the temperature and pressure of air in the energy store in equal proportion. The turbines convert that pressure difference into electricity with high efficiency.

The energy store should have sufficient heat capacity to retain up to one half of the entire daily insolation. This is surrendered at night to produce electricity. It is important that the bricks lose all heat stored and cool throughout to the minimum night temperature. During the day warm air from the collector will raise the temperature of the upper layers of bricks but the lowest levels will remain at the previous night’s minimum temperature. This ensures a relentless supply of cold air to the solar collector throughout the day.

Solar energy will be transmitted through the glass with over 90% efficiency. Heat losses from the collector can be minimised by supra insulation and double glazing. It is important to note that the absorber plate operates at around ambient temperature, cooled continuously from below. This will virtually eliminate heat loss through the glass. The solar collector should have over 80% efficiency.

It is my assertion that with a solar collector efficiency of over 80% and a turbine efficiency of 60-90%, the proposal outlined allows conversion of solar energy into electricity with an overall efficiency of over 50%.

The principles above should apply from small-scale domestic generation to a rural village scheme or to a commercial solar farm. Calculations indicate that a solar farm one kilometre square, operating with a 20ºC difference between daytime maximum and night minimum temperatures, with tropical insolation of 6KWH per square metre per day would need an energy store one tenth the area of the solar collector and of 5 metres height. The economics should be outstanding – the materials involved are relatively cheap; there are no cumbersome structures; there are no advanced technologies. Every component is tried and tested and well developed. The solar farm described would cost about £100 million and if an overall efficiency of 50% can be achieved, should give an average output of over 100 MW concentrated into daytime and evening in line with demand. It would mean electricity cheaper than from any other renewable technology and cheaper than from fossil fuels.

No experimental work has been carried out on the above proposal – it is merely a set of theoretical ideas. There are many variations of the overall configuration, the design of the turbines, the nature of the glazing and the energy store and the routes for the warm air and cold air. But there are two key features:

the use of thermal bricks as a daytime energy sink and for night energy supply

a closed system that allows all changes to take place at constant volume. There are no losses due to the expansion of air. Energy absorbed raises temperature and pressure in equal proportion. Turbines convert pressure differences into electricity with high efficiency.

It may be that all the above system achieves is the efficient absorption of solar energy into thermal bricks during the day and its efficient surrender through the glass at night with the turbines absorbing little of this energy!! Indeed some scholars will quote the second law of thermodynamics and if the temperature range is 10ºC/30ºC insist on a maximum efficiency of ²⁰/₃₀₃ or 6%. If so there are variations that could produce a higher temperature range and higher efficiency.

It is my assertion however that the second law does not apply. There is no Carnot cycle. These are changes at constant volume that will allow the conversion of solar energy into electricity with an overall efficiency of over 50%.

                                                                                                                                                                    March 2002

E-mail address:  williams.a(AT)globalwarmingsolutions.co.uk