The EROEI of energy balancing

See also The EROEI of electricity generation.

In a future with the huge increase in variable renewables predicted in such places as zerocarbonbritain, the Offshore Valuation, and Sustainable Energy Without the Hot Air, it will be necessary to find ways of matching up supply and demand. Currently this is done by trimming the output of fossil fuelled power stations, with some unintended consequences for carbon emissions, at least in some places.

In part, balancing can be managed by ensuring as large a network as possible to increase the diversity of renewables and weather conditions as has been shown by Sinden, amongst others. In addition to this, demand side management can play a part (edit: potentially a very large part as pointed out by Dave in the comments, Tom Konrad, and Mark Barret in the work linked to below) in reducing the peaks of demand when power is not available. However, the most important way of matching supply and demand, both now and in the future, is through storing energy.

There are two types of storage – fuel stores and rechargeable stores. Fuel stores in the form of stockpiles of fossil fuels and dispatchable power stations have been the traditional way of matching supply to demand. This is supplemented to some extent by pumped storage and by interconnectors to France and Northern Ireland. These interconnectors are being added to as I write this with a North Sea Grid planned.

A way of looking at the effect of this requirement for storage on the efficiency of renewable energy is by calculating the EROEI of the storage and transmission technologies. This EROEI of balancing is also known as the lifecycle efficiency of the system. The EROEI in the case of rechargeable storage is a combination of the round-trip efficiency of the battery and the embodied energy of manufacturing, transporting and maintaining it.

The EROEI of balancing through transmission is a combination of the embodied energy of the Grid itself and the transmission efficiency. It is fair to count the whole of the embodied energy of electricity grids as the footprint of balancing. This is after all what grids are for. However it is less fair to lay it all at the feet of balancing variable renewables, as it has always been used to balance geographically dispersed and variable (or intermittent) fossil-fuelled power stations with equally dispersed and variable demand.

I’ve trawled around and found a few analyses of balancing technologies which I’ve shown on the chart below. The two grid systems have significantly higher EROEIs than battery technologies. It should be noted that only a proportion of the electricity produced will need to be stored, even with high renewables penetration, whereas all electricity will need to be transmitted over the Grid.

EROEI of balancing

Storage table

Edit: references are here.

In the graph, the top of each column represents the round trip efficiency. The red section at the top of each represents the loss of life-cycle efficiency from the embodied energy of manufacture and maintenance. The blue column represents the EROEI. Dark grey columns are technologies for which I couldn’t find any embodied energy figures and so just represent the round trip efficiency.

All of the figures required a few calculations to make them into EROEIs – in the case of the HVDC figures I’m not 100% convinced they’re right but you can check them for yourself at the bottom of the page). It would appear that grids, whether National Grid or a 4,000km long HVDC connection, are the most energy efficient way of balancing. Of course this depends on there being a match between supply and demand.

By the way, one obvious omission in all this is the other type of storage – fuel storage. Even though gas and coal have among the lowest EROEIs going for energy generation technologies, they are easily stored and that storage has a low energy cost. It would also be interesting to try and put a figure on what the lowest capacity factor for say CCGT would be before the EROEI fell to lower than one. That also raises the question of carbon return on carbon investment (CROCI) which I’ve not written about so far. The argument for balancing with fossil fuels may well not stack up so well in the carbon accounts as in the energy accounts, although that depends on how much we need to use it. At least one academic, Mark Barret, has created a detailed model of the UK’s electricity supply with only 5% fossil-fuelled balancing, which also incorporates additional HVDC inter-connectors and demand side management.

What would be interesting, now that we have figures for transmission, storage, and generation, is to try and model the impact of different mismatches between supply and demand, given different capacities of the storage and transmission methods available. A project for the future I think.

Sources

Harrison et al (2010) “Life Cycle Assessment of the Transmission Network in Great Britain”, Energy Policy

May, (2005) “Eco-balance of a Solar Electricity Transmission from North Africa to Europe”, Diploma Thesis for Technical University of Braunschweig

Denholm & Kulcinski (2004) Net energy balance and greenhouse gas emissions from renewable energy storage systems” Energy Conversion and Management

HVDC calculation

At 1.7% losses per 1,000 km (p. 35) that is 93.4% efficient over 4,000 km. That means 1.071kWh generated per kWh delivered.

The embodied energy of a concentrating solar plant plus an HVDC line is calculated as 0.21MJ/kWh (p. 119) and of that the embodied energy is 3.5% (from the graph on p. 114). That makes 0.002 kWh embodied per kWh delivered.

Add the two together and you have 1.073 kWh embodied per kWh delivered and 1 / 1.073 = 0.93.

11 Responses to The EROEI of energy balancing

  1. Dave says:

    Interesting. We’re looking at this topic this year and possibly next too. I’m not entirely sure that I agree that energy storage will be the most important way of matching supply and demand – there will be a kaleidoscope of alternatives including demand-side management (of course load-shifting might be seen as another form of energy storage in some respects!), but the broad thrust is spot on. Some Danish Universities are also looking at this sort of challenge. DTU Risoe and Aalborg from memory…I may have some useful contacts for you if you’re involved in the UK consultancy arena. Send me an email if you’d like to correspond.

  2. Jamie Bull says:

    I totally agree on the demand shifting point – and have slightly edited the post to reflect that. It is a point well made in Mark Barrett’s work that I linked to. He particularly looks at the shifting of heat demand.

    However it’s not a strategy been studied with respect to EROEI that I know of, although there’s no reason why it shouldn’t be – I’m thinking of PCM-enhanced hot water tanks for example, which are certainly a form of storage.

  3. Tom says:

    A couple major energy storage technologies you omitted are some of the most effective: thermal storage. Combined with CSP, this is, in effect, a form of fuel storage. In combination with a parabolic trough CSP plant, the RTE is around 85%, due to losses from heat exchange between the mineral oil working fluid and the molten salt storage medium. In contrast, Power tower CSP plants use molten salt as the working fluid, and so do not suffer from heat exchange losses, and hence have a RTE of 95%+

    The other thermal storage technology (which can also be considered a Demand Response Technology) that’s worth considering often has no embodies energy at all: Ice based thermal storage in conjunction with A/C. The reason for the no embodied energy is that the A/C operates at night when it is more efficiency, and can often be downsized in conjunction with adding thermal storage. During the day, the system only uses fans to melt the ice formed at night. The effective RTE can reach 100% or higher depending on the day/night temperature differential, because the ice is created at night when the A/C heat pump operates more efficiently.

    These technologies are probably a lot more important in the United States than the UK, because both work best in hot or sunny regions.

    • Jamie Bull says:

      Some good options there. But as you say, not really much use in the UK or much of Europe (with Spain a notable exception on the CSP front – I got some nice pics of one near Seville recently). Do you have refs for the RTEs?

      Interesting point on the heat pump working better when charging up the “coolth” store at night. I wonder whether that applies to charging up a heat store during the day for heat in the evening.

      • Tom says:

        The calcs for RTE were based on interviews for an article on CSP I did last year:

        http://www.altenergystocks.com/archives/2009/04/the_future_shape_of_csp.html

        The thermal tanks lose about 1%/day, and here’s a quote from the article: “According to Gould [of Solar Reserve] and Glatzmaier [of Sandia National Labs], the thermal storage systems systems at the Andesol [parabolic trough] plants suffer 7%-10% round-trip energy losses in heat exchange.” I’m afraid I had a computer crash a couple months ago and lost the original notes.

        Storing thermal heat during the day would produce higher efficiency electricity generation IF the plant is air-cooled. If it is water cooled, there would probably not be much difference. However, this would probably not make much difference in practice, even for the (rare) air-cooled plant, since the main current use of thermal storage on CSP plants is to shift production just a couple hours into the evening when demand is still high (and it’s still hot.)

        Ice based thermal storage would probably be applicable to most of Southern Europe: anywhere they use air conditioning in commercial buildings.

  4. Richard says:

    I’m interested to hear your thoughts on the EROEI as applied to the use of hydrogen for energy storage and energy balancing. Similarly there is an increased interest in the use of the various advanced batteries designed for Electric Vehicle use as plug-in storage and local distribution system support services. I agree with your overall direction of getting the big picture parameters for all the options on the table – perhaps balanced with other more operational matters such as, how can these storage / balancing facilities be most easily evolved / introduced into the existing energy system. Sometimes the solution with the best ultimate macro-credentials don’t make it through these market evolutionary stages / barriers.

    • Tom says:

      The Hydrogen electrolysis/fuel cell cycle has horrible round trip efficiency of about 40-50%… it should not be seriously considered for grid based storage.

      Plug in vehicles can make more sense because the embodied energy of the batteries can be attributed mostly to the vehicle, which makes the overall EROEI picture look better. However, this assumes that the EREOI of plug-in vehicles makes sense. I have not seen an analysis of electric vehicles or PHEVs that takes into account the embodied energy of the batteries, so there is some doubt in my mind that EVs are actually better on an EROEI basis than petroleum fueled HEVs.

    • Jamie Bull says:

      I haven’t seen any study of the EROEI of hydrogen storage. The RTEs don’t look great though and it doesn’t gel too well with existing distribution methods. On the other hand I like the idea of hydrogen stores at the foot of offshore turbines, or alternatively at landfall, with fuel cells to allow power onto the grid when required.

      I note that German researchers have begun making natural gas using excess wind energy. Now if that’s not perfectly placed to tap into existing infrastructure (at least in the UK), I don’t know what is. There, though it will be important to avoid any methane leakage given its high CO2e.

      On the question of EV batteries and V2G storage/balancing services, that is definitely a promising area of the research for the future. I remember seeing some figures claiming the resource is nothing like the size it is sometimes made out to be but I can’t seem to find them. My back of an envelope calculations just now seem to suggest otherwise so I’ll take a closer look. If you’re in the UK there’s a conference on V2G coming up at the end of June in London.
      http://awbriefing.com/events/10-06-24.php

  5. […] talk of self-sufficient buildings makes me think that we are talking about the embodied energy and energy storage losses of an off-grid home. A (net) self-sufficient home which is still connected to the grid shouldn’t […]

  6. […] an analogy to be made here with electricity, as there so often is when talking about water. It is much more efficient to transmit electricity around the grid when your rooftop PV installation is producing more than […]

  7. […] EROEI references These is a list references for the two linked posts on the EROEI of electricity supply and the EROEI of electricity balancing. […]

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code class="" title="" data-url=""> <del datetime=""> <em> <i> <q cite=""> <strike> <strong> <pre class="" title="" data-url=""> <span class="" title="" data-url="">