Two new roles for heat

It’s typical isn’t it, nothing for ages and then two new technologies come along together! In the last few months two new(ish) technologies have been floated which could benefit any future low-carbon economy.

Figure 1, Compressed energy storage [Credit: Phys.org]

 The first of these technologies is the concept of liquid air energy storage. The idea of compressing air and using it to store energy isn’t anything new. For several decades now it has been used, often in the form of large underground caverns being used as the repository connected to a surface turbine/compressor. On a smaller scale, such systems have even been proposed for use as in cars, where the vehicle runs off the compressed air stored in tanks.

Figure 2, Compressed air cars [Credit: BBC]

However, such systems have a reputation for not being terribly energy efficient, typically at around 40% or less. Furthermore there is the small matter of storage of the air itself. All well and good, if you’re sitting on top of a large salt cavern, but not much good otherwise! A proposal from Germany involves using large air bags tethered to the sea bed as the energy storage medium (the pressure at depth naturally keeping the bags pressurized).

However, it’s important to remember that with compressed air storage a substantial portion of the air loss is down to the heat generated (when you compress any gas, according to the gas laws it will increase in temperature, thus consuming energy that will be dissipated as the gas natural cools down again).

The trick with this latest technology is to liquefy the air, making it much more compact and easily stored inside an insulated tank (something like a giant Dewar flask). When you need to release the energy, the liquefied air is simply released, heated back up to ambient temperature and this drives a turbine, returning the power to the grid.

Figure 3, An overview at the liquid air energy storage proposal [Credit: Sheffield University]

Ordinarily such a system would only be about 25% efficient. However, by locating the facility next to a thermal power station one can use the waste heat from the power station to provide the heat input load. This raises the cycle efficiency up to 70% and serves to provide another role for waste heat from power stations or other major heat users, beyond what I discussed in an earlier post. It also serves to reinforce the point that it is the production and consumption of heat energy (37% of UK energy consumption), not electricity (20%) or transportation fuels, which is crucial to maintaining an industrial society.

I would note that liquid air storage would probably only represent a mechanism for short term load balancing on the grid. Storing the energy generated by wind and wave or solar systems overnight to even out the peaks and troughs in the daily usage cycle. Typical the boil off rate of a liquefied gas runs at about 0.5-1% per day. So such a storage system would only be effective for a matter of days or a few weeks. Beyond this serious energy loss penalties are incurred.

However, the major stumbling block to a future fossil fuel free grid, isn’t short term energy storage, but long term energy storage. The bulk of UK’s energy consumption occurs over the winter months, when fortunately wind energy tends to be more reliable, but there are periods of completely windless days when it is also very cold. It is the ability of fuels such as oil, coal and natural gas to be easily bunkered and release as and when the demand rises that is essentially the “killer app” that has allowed fossil fuels to dominate energy production for the last century.

So there are limitations to what benefits liquefied air storage will bring. Converting energy from renewable sources into hydrogen, seems the more obvious solution for certain energy loads. As hydrogen can be stored in a non-cryogenic form, transported long distance and can be directly used for the generation of heat (which again is 37% of UK energy consumption, not including this new role of liquid air manufacturing).

Methanol Economy?

The other idea that has recently emerged, is that of producing Methanol as a liquid fuel replacement for oil. The idea of a so-called “methanol economy” is nothing new. But previous proposals have generally focused on producing the methanol via the synthetic gas route or manufacturing it from biomass.

However, scientists have now proposed producing Methanol by using heat energy to crack carbon dioxide and water into Methanol. Such a system is currently being tested on an industrial scale in Iceland, using Geothermal heat as the heat energy source.

Figure 3, Methanol is frequently used in Drag-race vehicles [Credit: Machine Design Mag.]

Using methanol as a substitute for petrol, in place of hydrogen, would have a number of advantages. It has a high level of energy density (although its energy density by volume is higher than compressed hydrogen tanks, its only about half the density of petroleum), is a liquid (unlike hydrogen, a compressed gas), is easily storable and comes with a high octane number. Methanol is already in use by a number of high performance cars, notably Drag racers, and it is also fully compatible with certain types of PEM fuel cells (it does not need to be converted into hydrogen first like other fuels).

Figure 4, The Direct Methanol Fuel Cell (DMFC) [Credit: DTI Energy]

However, Methanol has the disadvantages of high flammability (compared to hydrogen, as being a gas hydrogen will simply burn straight up), corrosive (meaning you’d need to refurbish existing liquid fuel delivery systems before using it) and a potential issue with toxicity, as Methanol and its vapors are potentially toxic as well as carcinogenic. While this is an issue with petroleum products too, it isn’t a problem with hydrogen or certain other types of biofuel alternatives (such as Ethanol).

Incidentally, bringing up the topic of Ethanol, it is often stated as “fact” that Ethanol fuels consume more energy in their production than is returned when consumed. But several studies suggest, that while the situation for US corn ethanol is poor (but still a positive) it is better for other forms of ethanol production, and Ethanol still represents an effective means of fuel production.

A new role for Nukes?

Least I be accused of anti-nuclear bias (as some of my critics would have you believe) as opposed to what I would label “nuclear realism“, I would note that both of these technologies could provide a new role for nuclear. Previously, the idea of nuclear CHP (combined heat and power) has suffered (see here, section 10.4.3) due to the obvious fact, that in order for the waste heat from the station to be utilised in some way, the nuclear power plant needs to be located close to the users of such heat (homes, businesses, factories, etc).

Of course while it might make perfect technical sense to plonk a nuclear plant right down in the middle of central London, not even the most fanatical of the UK’s nuclear lobby’s supporters would actually advocate such a policy. Without a massive change in public perception, it is doubtful the UK public (or the public of any other country with a free press and a democratic government) would ever be willing to accept the location of nuclear power stations near to heavily populated areas.

However, it really doesn’t matter where on an electricity grid a liquid air plant or Methanol production facility is located, so long as the local transmission lines can tolerate the applied loads (which shouldn’t be a problem for a power station). It would make little use to actually use the electricity from a nuclear power station directly for this role (if you’re going to the expense and difficulty of building a nuclear plant, it makes far more economic sense to simply export that power to the grid as it is generated). Far better to use a cheaper (but more variable) source of electricity such as wind energy to provide that input. The heat from the nuclear power station then being used to complete the process and square the circle.

Such a role might make nuclear a bit of an easier pill to swallow for many. Indeed, given the IMechE longstanding pro-nuclear stance, I suspect this may have something to do with they’re promotion of liquid air storage.

Although, as I’ve previously pointed out on numerous occasions there are serious obstacles to the further roll out of nuclear power, notably economic barriers, the slow rate of construction, nuclear power’s own problems with “intermittency” and long term limits on the supply of nuclear fuels. While the idea of harvesting waste heat from reactors would certainly make the case for new reactors a little more feasible, it certainly does not “close the deal”.

Renewable Heat

Fortunately there are plenty of other sources of waste heat that can also be used. Biomass or geothermal power stations are obvious sources (indeed as noted, they are already being used for these roles). Also there are numerous heat producing industrial plant, which are currently incompatible with CHP (see my previous article on CHP and heat pumps from more on that). Co-generation generally requires a fairly constant level of heat demand for a large portion of the year to remain economically viable. But often at certain times of the year, most notably the summer months for building heating systems, there is little demand for heat, making CHP less economically viable.

Figure 5, Combined Heat and Power [Credit ESRU Strathcylde University]

One could see such liquid air torage being used instead to help back up wind energy to cope with swings in summer power output, while at the same time making CHP projects more cost effective.

Of course, as noted earlier, the bulk of the UK’s energy demand peaks in winter (although of course it should be stressed in other climates, such as southern Europe or the US the peak demand is in summer) so presumably liquid air will merely be one of a number of energy storage technologies in play. It will likely work alongside hydroelectric pumped storage, tidal lagoons, hydrogen production and electric car battery storage to help square the circle for a future renewable energy grid.

But clearly the dwindling number of fossil fuel power stations (or the hydrogen fueled peaking power plants which will ultimately replace them) would not only provide a ready source of waste heat, but carbon also. Furthermore, in the case of Methanol production, this would turn the carbon output from such plants into a valuable commodity, making the case for CCS (Carbon Capture and Storage) much more economically viable. Even if the vast bulk of the carbon is simply injected underground, it would still make the extra cost involved with making the plant carbon capture capable, much more worthwhile.

Figure 6, Carbon Capture and storage at a modern power station, could liquid air and/or Methanol production add to this picture? [Credit CCSa.org]

This last option is much more applicable when you consider the other major roles that fossil fuels provide. Notably, that they are often used to make both Ammonia and nitrogen based fertilizers (approximately 80% of Ammonia production going into the manufacture of fertilizers). Maintaining production of these products is vital in a post-fossil fuel world. I’ve heard proposals to utilize concentrating solar power stations or geothermal sources to provide the heat to crack carbon and nitrogen into Ammonia, but the crucial source of carbon still appears elusive. Naturally, CCS seems to be an obvious way to provide the necessary carbon input (even if, again, the bulk of the carbon is simply injected back underground). Liquefying carbon dioxide (presumably using the same kit used to liquefy air) would also makes it easier to transport.

Balancing act

All in all, neither liquefied air nor Methanol production (via renewable heat) represent a silver bullet solution to our future energy problems.

Liquefied air will be incapable of providing a complete solution and will merely make existing heat energy generating and “variable” renewable energy projects that bit more economically viable.

Methanol will probably fill a wedge between existing petroleum and biofuels on the one side, and natural gas and hydrogen on the other.

But both of these technologies do add a pair of potentially useful tools to our ever growing post-carbon tool box.

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About daryan12

Engineer, expertise: Energy, Sustainablity, Computer Aided Engineering, Renewables technology
This entry was posted in clean energy, climate change, economics, efficiency, energy, fossil fuels, nuclear, peak oil, power, renewables, sustainability, sustainable. Bookmark the permalink.

6 Responses to Two new roles for heat

  1. mollymay5 says:

    Very insightful read. Thank you I have learned a lot about this form of sustainable energy. If Australia could adopt a similar program, especially considering our abundance of agricultural resources, I think that this would be quite a success.

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