Wednesday, May 30, 2012

Base Load and Clean Energy

A commonly stated claim by advocates of business as usual is that renewables cannot provide base-load power generation. In the industry, base load is the minimum that has to be continuously available. With large coal-fired plants, keeping them running at a constant rate of output is a lot cheaper than reducing or increasing output – provided you want all the power generated:it isn’t cheap to produce something if it’s wasted.

Power utilities attempt to balance demand around the clock to keep it reasonably constant. That’s why for example your electric hot water system has a ripple relay, a switch that can be turned on or off remotely by varying the usually regular AC sine wave. During times of low load, all the hot water systems on the system are turned on; when load starts to get high, they get turned off. This works because you are storing energy in the form of hot water.

For renewable energy systems to work so that you can flick a switch whenever you want power, they need to work even when the underlying energy source is not there, e.g. at night if you are using solar power, or on a windless day if you use wind turbines. The argument is that these systems cannot provide base-load power because they only supply power when their energy source is available.

We’ve already seen that a coal-powered system relies on energy storage of a sort to smooth out demand in the form of storing hot water. This is a remarkably inefficient way of storing energy. To start with, a coal-fired power station loses up to 70% of the energy it consumes as waste heat. Of the remaining 30%, about 10% is lost in transmission. Then when you heat hot water using electric resistance, that’s also a remarkably inefficient use of electricity – and you are turning it back into heat, which is what you started with. Finally, since the water tank has to store energy across periods when the ripple relay is turned off and even when that’s not the case, it has to store enough energy that taking a bath doesn’t make the water too cold for the next use, you need to heat – and keep hot – a lot more water than you actually use for any one purpose.

The massive losses inherent in a fossil-fueled power grid are to some extent masked by the fact that coal is relatively cheap. However, coal is a depleting resource and will become more expensive over time – even if we don’t count the environmental costs of mining and burning coal.

To work this all out properly, you need to work out the available energy and costing. What I offer here is an idea of how a clean energy system could work.

First, a solar thermal (or concentrating solar) system an provide base load, if a sufficiently large store of heat is included. The way these systems work is by heating a salt mix past melting point, and using that stored heat to produce steam for a turbine. Heat storage on these systems can potentially extend their use over more than a day of low sun. What if you need to extend use even longer, if there’s an extended period of low sun? A solar thermal plant has to have a way of heating the salt mix initially, usually using gas. An option is to use excess power generating at off-peak times to create hydrogen by electrolysis, then burn the hydrogen when the sun is not hot enough to maintain the temperature. A system like this combined with wind could work well. When there was sufficient wind available, the wind power could provide base load and top up the stored hydrogen if there was any excess. On less windy days, the solar thermal plant would provide more of the power.

Is it not wasteful having all these alternative power sources? A conventional grid does this anyway. The large base-load power stations are backed up by smaller generators that can be turned on and off quickly, but at a higher cost per unit of electricity. Making all this work will require some hard engineering, but describing clean base-load as impossible is wrong. Hard, not impossible. So why should we bother? Because coal will not last forever, and because coal is dirty. Coal mines are massively polluting, coal transport is energy-inefficient and coal emits many harmful substances including heavy metals when burnt.

All of these would be good reasons to look for alternatives even without climate change. Add that in, and we need to start working seriously on these options. In Australia, there’s a 100% renewables campaign and the Beyond Zero Emissions group has researched clean base load power extensively. If South Africa could beat Australia into the lead position on the Square Kilometre Array telescope project, what’s our excuse for being so backward on clean energy?

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