Renewable energy sources more imagined than real
We are often attracted to renewable sources as there seems an almost unlimited amount of potential energy available if only we can get our hands on it. We often we hear of proponents of renewable sources coming up with fantastic pools of resource at high efficiencies that we can access at no (apparent) cost. Take for example tidal energy. The amount of energy in tidal power generated by the gravitational energy of (mainly) the moon comes to a total of about 3.5 terawatt hours (1012) which seems at first sight a great deal of energy but is in fact only 20% of the global demand in total. Furthermore there are twenty places in the world where tidal flow exceeds the around 1.5 meters per sec at peak flow which would be sufficient to drive a turbine farm which means the actual accessible power is very small (and periodic) at around 200 gigawatt’s (109) of available energy – or around one thousandth of the available energy supply. So there is in principle a large amount of energy there but unfortunately we are unable to access it in any reasonable way.
I was drawn to these interesting facts in the New Scientist after a meeting where it was proposed to me that if we could all have Sterling engines installed in our homes and drive them with natural gas instead of the big power generators doing it for us we could enjoy 95% efficiencies in conversion as against the around 30% the best generators achieve. However this was a mistake and would breach the laws of thermo dynamics if it were true as a typical sterling engine has a thermal efficiency of between 20 to 30% which is nicely comparable to a car engine but not I am afraid in the 90% range. Given that most Sterling engines require exotic materials and tend to be quite expensive I doubt this is a starter in cost terms alone – although up to 150 or so KW that might be in the running for a home generator utilising waste head from the house say in summer (acting as a air conditioner).
As so often is the case spokespeople appear every so often extolling the virtues of a green half baked idea when the engineering aspects almost always rules out getting anywhere near such stratospheric benefits. We have only to look at wind power, many of the turbines operate as miserable efficiencies and blight the landscape for miles around. What is needed is a bit more evidence based evaluation and a little less hype and hope.