Beyond Zero talks to Rainer Aringhoff of Solar Millennium about baseload 24 hour solar with salt storage

Rainer Aringhoff of Solar Millennium discusses Thermal Solar technology with Matthew Wright and Scott Bilby of Radio 3CR.
Thermal Solar plants with storage can now produce steady energy 24 hours a day in summer, and the addition of salt storage to solar thermal plants actually reduces their overall cost. Larger plants, to around 250 Megawatts, are now being planned.

Rainer began working in solar energy in the 1980s. He worked in California after the first ‘oil crisis’ when solar plants were first established there. These are still in operation today with outputs higher than they had originally due to new technology.

Spain has become Solar Millenium’s most important market for solar technology with their Andasol plants. Andasol 1 for example, will supply up to 200.000 people with climate-friendly electricity and save about 149,000 tons of carbon dioxide per year compared with a modern coal power plant.

The technology uses a ‘trough plant’.
How does this work?

A steel structure supports a parabolic trough 200m long. Incoming solar radiation is concentrated on the focal line of the absorber tube where radiation is converted into heat. Heated synthetic oil is pumped through the field, heated up in collectors and then pumped to a central power block. A series of heat exchanges produces steam which runs the steam turbine generators and produces electricity.

Thermal storage was introduced in Spain due to an artificial hurdle faced by Solar Millenium when engaged to establish solar energy plants. ie. to be eligibile for an attractive tariff, they had to meet a capacity restriction. (50MW net – whereas 150-200MW is more the optimal size).

Solar Millenium’s response to this restriction was to extend operation after sunset through the deployment of thermal storage. They effectively oversized the solar field. (Rather than a certain ratio of concentrated square metres of collectors to produce heat).

Consequently they are producing more heat and storing it and producing steam after sunset and are able to operate around the clock in summer ie. baseload around the clock!

The tanks?

Each tank has a diameter of 36m and height of 14m. They are well insulated and filled with molten salt. There are two tanks containing 31,000 tonnes of molten salt heated to about 390C.

Thermal efficiency?

Overall power cycle efficiency is about 39%.
Overall system efficency is about 16%.
Storage efficiency is about 97%.

California?

Utilities there are interested in covering the extremes in demand in summer and have been paying three times the normal cost for electricity for the peaking hours.

There is the perfect match of electricity demand being at the same time thermal plants are most productive. So in California Solar Millenium are designing systems where they increase the capacity of steam turbines and the ‘flushing out’ in the afternoon and evenings. These are ‘peaking plants’. ie. there is flexibility in these systems as they can shift power from morning to afternoon or evening, to whenever the power is required.

California is one of the three highest potential areas in the world for creating solar energy, due to their ideal conditions. The other two are Chile and Australia. In California the load centre is only 80 to 100m south of the highest radiation. Although Solar Millenium is currently struggling with regulatory problems, there are many projects underway. One is a 250MW unit which brings costs down by about 20% compared to the Spanish plants and also the radiation is about 20% better.

Cost of storage cheaper than building another plant and turbine!

This was the motivation in building the plants in Spain due to the government tariff restricting capacity. It was cheaper to increase the solar field and add storage than build a new plant with another field and turbine.
NB: In Australia the opposite point of view is being argued seemingly in ignorance of Solar Millenium’s experience).