Monday, 22 October 2012

Energy from air - more energy storage options

A small company near Stockton and Tees has demonstrated how to make petrol from air (or rather carbon dioxide extracted from the air) and water [1].   What they aren't saying is how much energy it takes to drive the process. It takes a lot of energy to make petrol because petrol is very concentrated energy. There was another sort of energy storage in the news recently [2] which also involves air - by cooling the air down to cryo temperatures and keeping the liquid nitrogen. To get the energy back you let it warm up again and force the rapidly expanding gas through turbines which produce electricity. This process is reasonably efficient but not suitable for cars and lorries. Compressed air storage is another technique already in commercial use for smoothing out the power available from wind turbines and there are exciting new developments coming along to make this more efficient [3] but this isn't suitable for vehicles either. What you need for transport is high energy density, so maybe the energy efficiency isn't always important.

Fossil fuels are a form of chemical energy but renewable sources such as solar PV, wind, wave and tidal power generate electricity directly. Electricity is the most versatile form of energy. You can use it for heating or cooking or run TVs and computers or generate very high temperatures - plasma arc welding generates temperatures up to 25,000 C. However, electricity cannot be stored directly. You have to convert it to something else - chemical energy in a battery, gravitational energy in a hydro-electric storage plant, mechanical energy in a flywheel and so on. The ideal storage technology, which unfortunately does not exist, would be
  • efficient - so we don't lose too much in the cycle to storage and back again
  • high energy density - so we can find enough space to store it
  • cheap   
Which of these factors is most important depends on how you are going to use it. For large scale grid storage, cost is the primary concern but for transport, energy density is critical because if your energy store is bulky or heavy it limits the range of your vehicle.

Grid storage 

When I looked before at options for  Large Scale Energy Storage Options I was mainly concerned with energy density but for grid scale energy storage it is cost that is the key concern. Energy density is only important in so far as it affects cost. At the moment most of our power stations run on methane gas which at ordinary gas mains pressure has quite a low energy density: 10 kWh/m3. Gas to produce 900 GWh - one day's worth of current UK electricity - would be 90 million cubic metres - 36 times the volume of the Great Pyramids. In practice we need to store much more than one day's electricity supply and for heating too. The current UK storage capacity for methane is more than 4500 million cubic metres, mostly in depleted oil fields [4]. Storing methane is not a problem despite its low density because containment is cheap.

There is however a strong link between cost and energy efficiency. The aim is to be able to store intermittent renewable energy when there is excess, from wind or PV panels or wherever, so that we can use it later when there is a dearth. If round trip storage efficiency were 100% we only need to build just enough wind turbines and PV panels so that we have enough capacity on average. If it the efficiency is 50% we need to build up to twice as many. The cost is not in the storage itself but in the extra generation capacity needed. Methane is a very poor storage medium for this application because although we can make it from excess electricity the round trip efficiency is only about 30%[5]

There are new developments both in compressed air energy storage (CAES) and in liquefied air storage which should be able to achieve 75% round trip efficiency.  Normally CAES  has only moderate energy efficiency because when you compress air it gets hot which makes it difficult to contain - cooling it down to make this easier reduces the efficiency. Nottingham Professor Seamus Garvey is running trials using a multi-layer bag which retains most of the heat in the air, up to 450 C. The bag has concentric layers filled with sea water, oil and finally molten salt and the bag is kept in the sea, 600m below the surface, to maintain the pressure.

The new cryogenic liquid nitrogen technology has the opposite problem storing cold instead of hot.  Nitrogen is liquid at -190C. To get the high efficiency you need to have a heat source to warm up the gas and speed up its expansion which is why the prototype is associated with power station which has plenty of spare heat.

The energy density in both these cases is higher than with methane but the containment is more expensive. The cryo nitrogen technique has the greater energy density but also high capital costs: Highview Energy Storage predicts at least 250$/kWh [6]. Professor Garvey thinks the gas bags will be less than 10 times that price. Time will tell.

Energy for transport

Apart from electric trains and trams all our transport options carry their fuel with them which is why energy density is so important and petrol is the ideal fuel. During the war, when petrol was not available, some buses and cars were modified to run on gas. The fuel was carried in a big bag attached to the roof of the bus. Here are some examples. However, vehicle range was very limited.

Petrol has high energy density both by volume -  900 times that of natural gas - and also by weight. We pay three times as much for our petrol energy at the petrol station as we do for natural gas through the mains and electricity for electric vehicles is cheaper than petrol in terms of pence per mile, but we still prefer petrol or diesel for our cars because it is useful to be able to drive 100s of miles between fuel stops.

The company Air Fuel Synthesis has put together a demonstration system for capturing carbon dioxide from the air and turning it into petrol. This system uses a lot of energy but as long as that energy comes from renewable sources the synthetic petrol is overall carbon neutral. The system uses well known technology - the only innovation is in putting it all together. The system runs in several stages all of which require energy:
  • Capture carbon dioxide from the air 
  • Split water into hydrogen and oxygen
  • React hydrogen with carbon dioxide to make, after a number of reaction steps, petrol
Overall, the process  may only have an efficiency of 10% or less - Air Fuel Synthesis are carefully avoiding answering that question - but if we require liquid fuel for our cars it still could be economic, in time. It only has to be cheaper than other renewable sources of fuels suitable for cars. Transport is responsible for more than a quarter of the world's energy use [7] so we can't just carry on relying on fossil fuels for it. We can make transport fuels from energy crops, if we can spare the land to do so, but it seems we need all the land we can get for food.

However, we don't need high density fuel for all transport applications - only the long range ones. Only 2% of the trips we make are more than 50 miles so 98% of all our journeys are well within the typical electric car range [8]. We could use electric cars most of the time and borrow a petrol car from a car club or car hire firm when we need it.

[1] UK Engineers make petrol from air Institute of Mechanical Engineers

[2] Liquid air offers energy storage hope (BBC)

[3] Compressed air storage has bags of potential The Engineer 2011

[4] Digest UK Energy Statistics DECC (2011)

[5] Got Powered  (60% efficiency to methane but then you have to convert that back to electricity so 30% overall)

[6] Storing Electricity Using Cryogenic Technology in a UK Policy Context Tim Fox Institution of Mechanical Engineers

[7] How much energy is consumed in the world by sector IEA (for 2008)

[8] National Travel Survey table 0307 Dept. for Transport

1 comment:

  1. Highview has now received funding to build a commercial scale plant: able to supply 5 MW for 3 hours