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Friday, June 5, 2009

About the efficiency in renewable energy power plants

This issue comes because some comments in the spanish blog. The issue of renewables efficiency I think is key and open to many misinterpretations so I wanto to add my comments on that in this post.

I will begin by the end, efficiency in a renewable energy system has no value, an efficient, renewable, is, in principle, not better or cheaper.

We are used to think about the efficiency in renewables in the same way we used to think about the standard generation systems and it is really a mistake to judge renewables in the same way. A liter of oil generate about 10 kWh of heat. Let's say you have a hypothetical cost of € 1 and that when burned produces 10 kg of CO2. If I have an electrical generation system based on oil with an efficiency of 20%, the burning of a 1 liter of oil (10 kWh) will generate 2 kWh of electricity. Therefore I need to generate 1 kWh, 0.5 l of oil, I'll have a cost of 0.5 €/kWh and will generate an emissions of 5 kg of CO2/kWh. These are the economic and environmental costs of this generation.
If I have an electrical generation system based on oil but has a 40% efficiency, burning 1 liter of oil (10 kWh) I'll genetate 4 kWh of electricity. Therefore I need to generate 1 kWh 0.25 liter of oil, at a cost of 0.25 €/kWh and will generate 2.5 kg of CO2/kWh emissions.

So, in the standard thermal generation systems the energy efficiency is really important. The more efficiency you have, the less fuel is used to make 1 kWh of electricity and electricity is a cheaper and has less emissions. Efficiency is a core issue in conventional generation plants. By the way, I make a point, the efficiency of conventional plants is not really phantastic. In fact if you look CSN web site (it is the governamental intitute to assure the nuclear safety in Spain) you will see that the Trillo nuclear power plant has a thermal power of 3010 MWt and an electrical power 1064 MWe. This means that the reactor at full power generate 3010 MWt of heat, of which the plant can only turn into electricity 1060 MWe, which gives an efficiency of 35%. Nothing impressive. We have experimental PV cells with an efficiency little bit higher than 40 %.

Let's see what happens now if we replace 1 liter of oil, per 10 kWh of wind at a cost of € 0 ¿idoes t matters the efficiency? Whether it is 20% or is 40%, the fuel to generate 1 kWh of electricity will continue to cost 0 € meanwhile, the CO2 emissions will remain at 0 kg CO2/kWh.

So we have to as the 'fuel' used by nearly all renewable plants is free, then we really don't care about the efficiency. What matters is that the cost of construction, divided by the kWh generated along the life cycle is as low as possible. And this sometimes can be achieved by increasing the efficiency, but it isn't always the case. Es decir si un Wp de panel fotovoltaico con una eficiencia del 15% cuesta 2,5 € y otro con una eficiencia del 25% cuesta 3 €, siempre me interesará más instalar el panel con un 15% de eficiencia ya que el coste de mi electricidad será más económico. I.e. if a Wp of PV module with an efficiency of 15% costs € 2.5 and another with an efficiency of 25% costs is 3 €, I am always more interested in installing the panel with a 15% efficiency as the cost of my electricity will be cheaper.

And there is a general exception to everything I said. In the case of biomass, from the standpoint of efficiency, we must consider it as a conventional power plant. This is the only exception that applies.

And one final point is that is always a maximum efficiency that can never be exceeded in all technologies. For example in the wind energy Betz theorem says that the theoretical maximum efficiency of a wind turbine is 16/27. A perfect wind tutbine can only generate a 5,9kWh from 10kWh of wind.

(This article is adaptated from the original in spanish: ("Sobre la eficiencia de las plantas de energía renovable").

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