The Problem With Solar Thermal Power

Posted on Sat 07/25/2009 by


Concentrating Solar Power Plant utilising the Tower method. Click on the image to open in a new and larger window.

Concentrating Solar Power Plant utilising the Tower method. Click on the image to open in a new and larger window.

In this mad rush towards the use of so called renewable power to replace existing coal fired power generation, many wild claims are being made. Because of that, those members of the public who have little knowledge of exactly how power is generated can be lured into believing that some of these methods actually can be used to generate the electrical power we take so much for granted.

The public believe that electricity is just something that is always there, and because of that, then the question of how it is actually generated rarely enters the equation, because the belief that electricity is, well, just electricity. That being the case, then as long as it still comes out of the ‘hole in wall’ or when the switch is turned on, then the perception of how it is generated is probably just a matter of moving from one source to another.

There are important questions that need to be addressed about the variability of these renewable methods of generation, and how they cannot supply power that is constantly required for 24/7/365, and not just on average 8 hours out of every 24.

In numerous other posts I have gone into intricate detail how the three favoured methods of using renewable sources to generate electrical power are problematic. The cost factor alone of using these three methods should be the most worrying factor of all, but in this day and age when the word Billion just rolls so readily off the tongue, we have all become inured to this monetary cost, and these three methods are incredibly costly, which sounds sort of incongruous when the original thought processes told us that these forms of generating power provided a free source of generating power. After all, the light from the Sun, and the blowing of the wind are free to all. Now we know that while that light and wind are free, the ability to actually harness it means an intricate construction that is incredibly costly.

I have also dealt at length with that exact variability of these methods, and this would have to be the most worrying aspect of these forms of generation, and these have also been glossed over by those with an agenda to replace those coal fired power plants.

To that end, we are being told that of the three forms of power, Wind, Solar Photovoltaic (Solar PV), and Solar Thermal, one of them, Solar Thermal CAN actually be used to replace those coal fired power plants.

So, let’s look in detail at that Solar Thermal form of generating electrical power.

More correctly, it is called Concentrating Solar Power. Whereas Solar PV uses the light of the Sun to generate electricity in solar cells, Concentrating Solar Power is completely different. It uses specially manufactured mirrors to focus the light to a focal point, creating an intensely hot area.

Concentrating Solar Power Plant utilising the solar trough method. Image by Alan Radecki. Click on the image to open in a new and larger window.

Concentrating Solar Power Plant utilising the solar trough method. Image by Alan Radecki. Click on the image to open in a new and larger window.

Some plants use a large tower with all the mirrors focused onto a single point, at the top of a tower, as shown in the image at the top of the page. Other plants use vast arrays of mirrors in rows with a central pipe passing through the focal point, as shown in the image at the left. Both of these methods use mirrors that are mounted on tracks so that they can follow the movement of the Sun across the sky, and these are called Heliostats. Both use compounds that are either stored in that tower or are passed through the pipes. These compounds can be salts, graphite or other compounds. The immense heat generated by the focused Sunlight heats these compounds to a molten state. Through these compounds are passed pipes carrying water, and the molten compounds boil that water to steam, and that steam then drives a turbine which then drives a generator, a similar process to that used in coal fired power plants, and also in nuclear power plants.

So, this similarity in using steam to drive a turbine, which then drives the generator gives people the belief that because they are similar, then this form of Concentrating Solar power CAN in actual fact be used to replace coal fired power plants.

This is a patently false belief, and I will explain exactly why.

People have no concept about the actual process of conventional steam turbine power generation, and as an example, ask anyone how coal is used to produce electrical power. If one person in ten answers anywhere even close to the mark, I would be surprised, in fact dumbfounded.

The problem with Concentrating Solar Power lies not with the process of providing the steam to drive the turbine, but with the actual generator itself.

To produce large amounts of electricity, as those coal fired plants provide, the generator has to be large, and by large I mean huge.

A huge magnetic field (the Rotor) rotates inside of a huge system of electrical windings (the Stator) wrapped closely around that magnetic field. An electrical current is induced in that Stator, and this is the generated electrical power, and believe me, I have simplified it to the most extreme so it can hopefully be understood.

The greater the size of the Stator, then the more the electrical power that is produced. The greater the Stator, then the greater the Rotor.

The Rotor in one of those large coal fired power plants can be anything up to thirty feet across the diameter, and up to 50 feet in length. This huge size encompasses huge metallic cores wrapped in vast lengths of electrical wiring. The current passing through these windings generates a magnetic field in those metallic cores, and special materials are used for these cores, and these are the super conductors you hear about. Superconductors are erroneously thought to be the wiring that carries the electrical current, and in some cases they are called that, hence the misconceptions, but superconductor technology mainly deals with the generation of huge magnetic fields in certain metals.

This huge complex of metal and wires to create those huge magnetic fields can weigh anything up to 250 to 400 tons, and read that again …. TONS.

That huge weight has to rotate at the desired speed, around 3600 RPM, and that is 3600 revolutions per minute, or 60 revolutions each second, 60 per second, got that?

To do that, you need a huge turbine, and in most cases with those coal fired power plants, that turbine is a multi stage turbine, three stages, the first driven by high pressure steam, the second medium pressure, and the third low pressure, as the steam cools slightly. Each stage is similar to a jet aircraft engine.

This turbine is mounted on the same shaft (naturally) as the generator itself, further increasing the weight of the complex that needs to be driven at that speed.

I can understand it can be a little difficult to try and comprehend that, but walk out onto the road in front of your house and look back at the house. Then imagine what it might take to make your house rotate at 3600 RPM. That is the scale of this.

To drive all that weight then an immense amount of high temperature and high pressure steam is required, and required all the time, 24/7/365, to keep all of that revolving.

So then that is the scale of conventional steam driven power generation.

Let’s then look at Concentrating Solar power, where the light of the Sun is focused to a central point to make a compound molten, to boil water to steam, to drive a turbine, which in turn drives a generator.

Never, and let me repeat that, NEVER. will you see a concentrating solar plant of even a fraction the size of a large coal fired power plant.

They will be small boutique plants producing an absolute maximum power less than 10% of a single coal fired power plant.

Why is that?

No matter how many mirrors you have and no matter what the compound, it can only boil water to steam at the required temperature and pressure to drive a much smaller turbine, hence a much smaller generator, hence considerably less power.

Build a huge generator, with a huge turbine, similar to a coal fired power plant, and it will NEVER run up to speed, because that amount of steam can never be generated. So, it is carefully calculated how much steam CAN actually be generated and the turbine/generator complex is designed accordingly from that.

Okay, so now we have carefully designed a complex that can actually generate electrical power, let’s just build a whole bunch of them to replace that one coal fired plant, and hang the expense.

Second problem, and one completely ignored by those who advocate this form of power generation.

That steam is only there to drive the turbine/generator while that compound remains in the molten state. As soon as the Sun sets, that compound starts to cool, and cool rapidly. As it cools, then the steam, while still remaining as steam, will lose temperature, and pressure, and in so doing, the fraction of a second it reaches the level where it can no longer drive that turbine/generator’s huge weight, then the whole complex just stops completely, and no electrical power is generated at all. Nothing.

It will then not start to rotate again until that compound again reaches the molten state when it can produce that level of steam. This will be a couple of hours after the Sun rises in the morning.

So, from this, you can now see with remarkable clarity that even though we have been told that Concentrating Solar is the plan for the future, and can actually replace those large coal fired power plants, the exact opposite is the case.

These plants at best can provide power for maybe ten hours in a 24 hour day, on average, extrapolated across a whole year. In the deepest of Winters, there is every chance it will never be able to generate the steam needed, and in the North, that problem will be even further exacerbated.

The problem is a physical problem that no amount of wishing and hoping can overcome.

To drive that huge weight to produce those large amounts of electrical power, you need huge amounts of high temperature high pressure steam.

So, these Concentrating Solar plants will always be of a much smaller nature, and until someone can tell me how they can make the Sun shine over one of these plants for 24 hours of every day to keep that molten compound actually molten enough to provide the required amount of steam to drive all that, then these plants will only ever be of a boutique nature, and will never come into widespread use.

Because of these inherent problems, entrepreneurs are not flocking to make these plants, and those who are have been subsidised by huge amounts of Government incentives, and monetary backing both at the construction phase and at the sale of electricity to the grid phase, because they will NEVER return a profit to anybody, and the cost of the power actually provided to consumers will be prohibitive, if any of those costs are ever to be recovered.

The problem is not one of will, be it environmental will, or even political will.

The problem is a physical problem, that can never be overcome to the point of replacing those coal fired power plants.

This has been a technical post, but what I have specifically attempted to do is to try and explain the problem so it can be easily understood.

It is just so easy for people to say that this is the way of the future, and because of that, then lay people will accept that, and puzzle as to why people actually try and speak against it.

I caution against these renewable plants not because of a political agenda, or a perceived anti environmentalist vandalism approach, but from a technical viewpoint, which as you can see from reading this, is not readily understood.