Clean Energy Intro: Solar Thermal

March 10th, 2008 by Michelle Bennett

What if you could produce clean solar energy night and day, rain or shine, and never hit the bank for a single P.V. solar panel? Photo voltaic panels can be pricey, so the solar industry is always trying to lower costs and boost efficiency in the quest to compete with coal. One fast-growing, cost-effective solar technology uses heat to generate energy 24 hours a day, and it can store energy without batteries. In this post I’m going to investigate solar-thermal technologies.

It’s called “Concentrated Solar Power” or C.S.P. The idea is simple; no complex chemistry or fancy silicon wafers required. Glorified mirrors shaped like satellite dishes (or parabolic troughs) direct the sun’s rays towards a reservoir. The concentrated solar heat boils water into steam, and steam powers a turbine. When the water cools off it’s collected and cycled back through the system. The mirrors can even track the sun across the sky to maximize efficiency. Water is not the only fluid that can be used, but its unique properties have made it popular. More on that below.

If you remember your history, the industrial revolution was powered by steam – factories, trains and boats boiled water by burning coal. Eventually we moved on to the internal combustion engine because you got more bang for you buck – literally. These ubiquitous engines convert controlled explosions into propulsion or electricity. The problem with combustion is the waste products and their effects on our health and environment. With solar thermal, the only waste would come from building the power plant.

So how does C.S.P. produce electricity at night? It takes advantage of a unique property of water: very high specific heat capacity. That means it takes a lot of energy to increase the temperature of water. It also means that water stores heat energy for a long time. Think of a pool, lake, or the ocean. It stays warm long after the sun goes down and even through cooler days in autumn. Water has the second-highest specific heat capacity of any known substance, which makes it perfect for storing large amounts of heat energy, just as a battery stores electricity. In the form of steam, water can continue to generate electricity when the sun is down and recharge the heat “battery” when the sun is up.

Another way to store that heat is with salt – molten salt. Sandia National Laboratories is experimenting with molten salt as a storage medium because of its heat-transfer and economical properties. It also removes the heat storage process from your dynamo so you can recharge your heat batteries and generate electricity separately. The benefit to this system is that if the sun doesn’t shine for days at a time you have a strong back-up “battery”. Their system can store 2-12 hours of energy for a week. We’ve heard from Sandia before, read about their “energy from air” scheme or their solar efficiency world record – both use C.S.P.

A lot of exciting news has been emerging about solar-thermal. It’s been demonstrated as reliable for over twenty years, and new plants are coming online, or being planned, across the world. Energy is not the only benefit of Concentrated Solar Power; it also creates jobs and can use otherwise unproductive land. Empty desert is ideal for this technology – abundant clear, bright days. Some estimates even claim that we could power the entire USA with C.S.P. or even the world (1% of the world’s deserts to be exact). Regardless of such high hopes, the future looks bright for C.S.P.

For …

An animation explaining how C.S.P. works, check out Ausra.

A useful fact sheet, surf Solar Developments.

Hard data, try the Department of Energy.

DIY action, swing by Treehugger.

Solar ovens and more, see this post.

(Image courtesy of Treehugger)









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