IAN LANG ELECTRONICS

If you look at your appliances you'll see they've all got a wattage rating. The kettle, for instance is 2500W. The toaster is 850W. The Microwave wants 750W and the oven wants in excess of 3000W. The dishwasher needs 2000W, and the washing machine wants about the same. The computer wants 600W or so whereas the radio requires a widgy little 11W at it's loudest.

 

Let's run them all at the top of the tree together. I'm using 11, 711W or 11kW and if I ran all of them at that rate for an hour I'd get charged for 11.7 kWh on the bill. At 15p that'd come to about £1.75 and in the case of the microwave I'd have something very overcooked; and if it was the same bread in the toaster for that long I'd probably be too busy putting the fire out to worry about the cost. I'd be able to have a nice hot cup of tea afterwards though. But no toast.

 

All of which serves to illustrate a point that you should choose the output of your invertor carefully. If the maximum power you are going to use at any one time is 580W, get a 600W invertor. If it's 5800, Get a 6000W invertor. It's important because the bigger your invertor output is the faster the drain on your battery. Remember there's a transformer in there. The power on one side of the transformer equals that on the other.

Coming out, there's 6000W or 6KW at 220 V and so 27A available.

 

Going in there's 12V, but still 6 kW and that's a staggering 500 Amperes. If you're using this much electricity in one go, you'd have to go to a much bigger voltage input at the battery end. Possibly a 72V system with a large Ah capacity, and that would be very expensive.

 

And this point illustrates that you can't run a 12V domestic system fully and without reservation from solar alone, particularly not in the Winter.

What you can do is reduce your bills by having things like lighting fitted into a ring feeding off the invertor. With the advent of low-power lighting you could in fact light up your house all night on a 600 or even 300 watt invertor. A 600W one would take 50A and this would drain a 500Ah battery in ten hours. Using a 24V system, you could take 25A, and a 48V only 12.5A, and a 96V system 6.25A

 

A recent development is to feed the panel output directly in daylight hours to the invertor and the batteries, switching between needs and prioritising the invertor, but this may not be a desirable option and time will tell how succesful this approach is. More promising is an invertor which varies its current input as needed. This approach can mean batteries last much, much longer and consequently do not need such a long recharge time and is the way forward for solar systems. At the current rate of research, it may be many years, if ever, before a viable solar system is available for unreserved use in the United Kingdom.

 

In other parts of the World though, it is possible to have a commercial power plant that generates enough to feed a grid system. These are CSP stations, which stands for concentrated solar power.

 

Nevada Solar One in the USA can generate 64 MW, whilst the SEGS arrays in California can do 354MW. Cuidad Real and Andasol in Spain can generated 50 MW each.

 

Nevada. California. Spain. Can you follow the breadcrumbs and make assumptions about climate patterns here? Suffice it to say it's not going to be worth building a similar enterprise in Aberdeen any time soon unless the Global Warming Lobby is completely right.

 

 

There are four main technologies involved in this. The first is:

 

Concentrated Photo-Voltaic.

 

In this technology, Fresnel lenses or mirrors can be used to focus a beam of sunlight on to collectors. Although these could be solar panels, they aren't; they are actually smaller PV chips which can be twice as effective at converting solar energy into useful electricity. Providing there aren't any clouds that is, because in diffused conditions the light can't be focussed , the efficiency drops dramatically and is actually worse than a solar panel. A further complication of this technology is that as it is a focussed beam, the light puts heat on to the chip, and the hotter it gets the less efficient. So, some of that electricity you've just generated you now have to use for a cooling system.  There are three broad classiifications:

 

Low concentration:

 

Up to about a hundred suns. The collector can have quite a high acceptance angle (where the beam hits the collector) before efficiency is unacceptable. Moreover, the beam isn't  too hot, and so no cooler is required. Because of the high acceptance angle, often no solar tracking system is required.

 

Medium concentration:

 

Which carries on from low concentration and goes up to about 300 suns. The beams here do get hot, and because of the density of the collectors in a given area, an accurate tracking system in two axes is required.

This make them more complex and consequently more expensive to construct.

 

High Concentration:

 

Which is always over 100 suns and has a large density of collectors in a given area. They are multi-junction cells, which mean that they contain several pn junctions which are respondent to different wavelengths of light and, theoretically, should give an 87% conversion eficiency in highly concentrated sunlight. They don't though. 40% is very good, and 30% is the industry standard. But the complexity leads to a high price to perormance ratio which means that they are hardly ever used on the ground except where good conditions are predominant. They do find a market in aerospace though as they can kick out a lot of power for how small and light they are.

 

The second technology is :

 

Parabolic Trough

 

This technology is in fact the oldest and has been working in California and Nevada since the 1980s. California's plants are known as SEGS 1 to IX  (Solar Energy Generating System) and are placed in the Mojave Desert  where insolation is the best in the United States. You're now thinking along these lines:

 

"Insolation the best in the United States is it? Oh good. "

 

Insolation is a word very few of us ever come across as it's techno-babble for solar irradiation and is one of those words made up by people who work in this industry and don't want the rest of us to know what they are talking about. Put simply, it's the total of radiation put down by the Sun in a given area, over a given time. The scientific unit is the Megajoule per meter squared per period (as the time can be measured in hours, days or months and then you get an average) but most technicians use the Watt-hour per metre squared.

 

This technology has nothing to do with panels, cells or PN junctions but it does have a lot to do with mirrors. The mirrors are shaped parabolically and in a long line. They collect and focus the sunlight to the central area of the parabola, down which runs a pipe, through which runs a liquid, in the case of SEGS this is a synthetic oil but it could just be seawater. The liquid heats up, the heat from the liquid is transferred to water and this turns to steam which then drives a turbine. Consequently this method is known as a Solar-Thermal System.

 

The problems here are immediately obvious. If you want to achieve maximum efficiency, you have to keep moving the mirrors. They're mirrors, they break, and they replace about 3000 a year at SEGS. They're parabolic and reflect 94% of the light back, whereas a domestic mirror reflects about 70% at best. Consequently they are expensive. The biggest cause of mirror breakage is wind and during intense storms the mirrors are turned out of the wind- rendering them almost useless. At the time of writing research is going on to replace them with steel parabolas coated with polymer sheets that will perform just as well but won't break and will be much lighter. 30% cost efficiencies are predicted at SEGS.

 

The following is a graphic produced by Nextera Energy who own SEGS and shows clearly how it works:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

You'll notice there's a supplementary boiler there. The Solar-Thermal System can't work efficiently by night.

This is a very good graphic and clearly shows the parabolic shape of the mirrors. The focal point is at the the centre of the half circle. Let's have a deeper look at it:

Principles of Solar Power

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The diagram on the left is from trueimagebank.com and shows perfectly how the mirror works. The curve of the parabola is symmetrical and so wherever the light hits it it will reflect to one and only one point. This has the effect of amplifying the energy received in the radiation from the Sun at that one point, and consequently putting your pipe here will have a heating effect on it and the liquid inside. In fact SEGS use therminol as the liquid, and get it up to 400 degrees centigrade; more than enough to boil water and provide steam for the turbines. A curve of lesser radius will move the focal point down the diagram, greater will move it up.

A variant on this theme is CLFR (Concentrating Linear Fresnel Reflectors) which use thin mirror strips to concentrate on one spot. The principle is exactly the same, but it is much cheaper to make and install, though not as efficient.

 

The turbine used at the end of the process is most likely to be a Rankine Cycle Engine, and the diagram above bears out this is the method used at SEGS. Over the page we look at this device and see how the heat generated by the parabolic trough turns into useful electricity.

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