The latest demand for renewable energy sources in general, solar & wind in particular, raised again the need for feasible energy storage solutions. Solar power is currently a daytime-only energy source because storing extra solar energy for later use is prohibitively expensive and grossly inefficient. Wind energy is also dependent of sufficient wind power, which exists in many areas mainly at night time only.
Various storage solutions for solar power plants were and are being developed; most of them are based on thermal storage. Much effort is invested for developing storage for trough technology (since it requires lower temperatures, up to 400ºC). Usually phaze changes materials (PCM) are being used, since it enables higher density in the storage and minimal temperature losses between charge and discharge. The main problem is the low heat transfer (due to low thermal conductivity of the salts), and this affects directly the amount of power that could be extracted from the storage.
Several research is being executed for developing enhanced solutions, mainly by enhancing the heat transfer between the salt and the heat transfer fluid (in the molten salt receiver/hot storage tank), reducing transient effects, optimization of the storage materials.
Various solar tower electricity generation systems were developed and the most advanced of them was installed and tested in California. This system, Solar Two, generated 10MW electricity using an eutectic molten nitrate salts mixture pumped and piped from a ground-based cold tank to a receiver mounted on the top of a tower. The hot salt from the receiver is then piped to a second, hot tank on the ground. In a secondary loop, the hot salt flows through a heat exchanger to generate steam and returns to the cold tank. The third loop includes the steam generator, which supplies steam to a steam turbine electricity generator. This plant was closed on 1999. Now Sener is trying to do something similar in Spain.
For the trough, – in Acciona’s Nevada plant, for example, there is no storage (only for about 30 minutes, that is achieved by the fluid that is in the pipes). On the other hand, at Andasol, – Flagsol (Solar Millenium’s subsidiary) together with ACS Cobra are developing thermal storage based on molten salt.
A simple, inexpensive, highly efficient process for storing energy is the bottle neck for deploying renewable energy for electricity base and peak load demands.
Later on we’ll discuss possible other energy storage options.
