Solar Energy

 

 

 

Here in Britain, we are becoming anxious about energy supplies. Our offshore oil boom is over, gasoline prices are at a record high, and the past couple of years we have seen double-digit increases in the cost of home heating. To some degree it's part of an international energy crisis - sometimes called the 2000's crisis. I am a bit young to remember the last one, the oil crisis of the 1970's - when the OPEC cartel cut oil supplies to the West. Then there were absolute shortages of gasoline - the gas stations ran dry. The modern energy crisis is different. The pressure of inexorably rising demand and peaking production has led to sustained high prices for oil and for many other commodities. There is gasoline and natural gas for sale but many people are struggling to pay for it. This problem isn't going away - it can't be resolved just by diplomacy or concessions from OPEC.

The implications are far-reaching for high-consumption and resource-poor countries like Britain and the rest of the European Union. A lot of people seem puzzled by the sluggish economy, but it's been predicted for a long time that peak oil would bring an era of zero growth. With China claiming a rising share of the world's resources, other countries may have to cope with sustained economic decline. My impression, living in Britain, is that we are not well prepared for it. Politicians here debate how much stimulus - government borrowing and money printing - is needed to restore sustained, vigorous growth. We don't hear much discussion - in public anyway - about economic constraints. Some people may be confused because the financial crisis and the energy crisis hit at the same time, leading them to miss the significance of the latter completely. 

So lately, I have been looking into the subject of renewable energy, and trying to get it into perspective.  The graph below is derived from US Department of Energy data, and shows past and future energy consumption by source. Their projection of energy from liquids ( oil ) shows a steady rise in the future - no sign of peak oil in their figures. Time will tell. The US government seems to be making an optimistic assumption about the yields from shale oil and other hard-to-get-at deposits.

 

global energy sources - from US dept of energy figures

This graph measures energy sources in Terawatts (TW), which are equal to a million Megawatts (MW). The total energy we get from oil is equivalent to the continuous, round the clock generation of about 5 TW. We normally measure oil in barrels and with some easy maths we can convert. One barrel of oil has a chemical energy of 1.7MWh. To generate 5TW we need to burn 70 million barrels of oil per day (bpd).

So what potential does renewable energy have to replace this ? If we add together oil, coal, gas and nuclear energy the total is around 15TW. That's the target for proponents of renewable energy.

bio-fuel

This is a controversial energy source, which according to its critics involves converting perfectly good food into a relatively tiny amount of gasoline. How tiny exactly ?

Soybean yields on average are about 1 tonne per acre, and 1 tonne of beans produces about two barrels of oil, which is 84 US gallons.

We normally measure the energy of food using calories (kcal). This is the energy required to heat one kilo of water by one degree C, and is equivalent to 4.2KJ. One tonne of soybeans contains 4.1 million kcal as a food source. In energy terms that is a bit more than the energy yield from two barrels of oil - when we convert soybean to gasoline we are discarding some of its energy value.

So that tonne of soybeans, grown on an acre of land, would feed an average person for six months - or provide about six tanks of gasoline. Whether that is a reasonable yield depends on your point of view. However to meet the total fuel demand of 70 million bpd, you would need 20 million square miles of land under cultivation for biofuel. That is almost half the land area of the earth and more than the total amount of arable land.  So it's not practical to replace fossils fuels with biofuel given the sort of crops like soy that we are currently using.

Could there there some other crop, perhaps genetically engineered, that might yield much more ? Biofuel ultimately depends on the energy of sunlight trapped through photosynthesis. Plants like soy are being used like organic solar panels. How efficient is this compared to man-made panels ? That's the next question I want to answer.

solar power

The graphic below shows how much solar power reaches the earth's surface, measured in Watts per square metre, averaged over a year - day and night.

solar insolation - from work by Matthias Loster

An acre of land is 4000 sqm, and the central USA is getting about 200W / sqm, so the acre gets an average power of 800KW from the sun. That's quite a lot. That's 19MWh in a day, equivalent to about 10bpd of oil. Compare that with the biofuel acre, which gives us a meagre 2 barrels per year. Put another way, the soybean crop yields less than one thousandth of the energy potential of the sun. For me the issue with biofuel is not that it pushes up the price of food, but that it's such a poor technology for generating power.

We can do much better with solar power. The generators operate in one of two ways. Solar concentrators work by focusing sunlight onto a boiler. The principle is simple enough, but they are useless on a cloudy day. The alternative is photovoltaic cells - commonly known as solar panels. These work with light from any direction - including light scattered by clouds. Both technologies can convert a reasonable percentage of the energy of sunlight into electricity - the best installations reaching about 30% efficiency. So could this generate the 15TW we currently get from fossil fuels ? The answer, surprisingly, is yes. In a sunny location like the southwestern USA, assuming sunlight of 250 W/m2 and 20% conversion efficiency, you need to cover 300,000 km2 with panels to generate an average 15TW. That is the land area of the state of Arizona - to generate enough power for the whole world.

Engineering on that scale might seem like science fiction, but then consider the Herculean efforts already being made by the oil industry to find new deposits. From this point on, it only gets harder for them. We are getting to the point where a Watt of solar power costs about the same as energy from oil and other non-renewable sources. If we spent as much money on renewables as we spend on fossil fuels then we could conceivably build a sustainable energy infrastructure. There is a further challenge, beyond just installing solar panels : to store and transport the energy, and to convert it to the required form. But it's basically the same problem faced by any source of energy. Natural gas, for instance, might be sourced in the middle east, liquefied, shipped to a terminal in Europe, piped to a power station, and burnt to generate electricity, which is then transmitted at high voltage and converted to low voltage for a home user. The cycle for solar power might involve using electricity to generate hydrogen from water, then shipping the hydrogen around as a fuel. Yes it's a lot of effort, but once the infrastructure is in place your energy problems are solved permanently. Contrast that with the oil industry, which is constantly on the move to get to new deposits, and pollutes the environment wherever it goes.

The potential is there for solar power - and the revolution is just beginning.

 

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