# New paper in Applied Energy

I am pleased that our paper “Projecting battery adoption in the prosumer era” has been published in Applied Energy!
The website Science Trends also featured an article highlighting our work.

The paper investigates two aspects of battery adoption for residential consumers with PV generation. We use a data driven methodology based on 15-minute electricity consumption and PV generation data from the Pecan Street Project, from which electricity consumption data for several hundred households is available. The households are primarily located in Austin, TX, however other locations in the US are also included.

Firstly, the paper investigates what size of batteries are required for consumers to reach certain thresholds of self-sufficiency, and secondly, it investigates how the economic case for residential batteries looks under a range of electricity tariffs, including both current tariffs and future projections.

Figure 1: Illustrating self-sufficiency with PV and the effect of a battery. (A) The demand and generation from an example household – we see a significant amount must be exported. (B) The mean increase in household self-sufficiency for a given battery capacity. (C) The mean marginal increase in self-sufficiency as a function of battery size.

In the first part, we find that for the studied 369 consumers, the mean self-sufficiency from PV-only (without storage) is 36%, meaning that when a household installs a PV system that produces the equivalent of their yearly consumption, we expect them to export 64% of the total PV generation to the grid as it does not align with their consumption (see Fig 1A). Using batteries, we find that modest sizes can increase consumer self-sufficiency significantly, for example, a 10 kWh battery leads to a mean self-sufficiency increase of 25% across all households (see Fig. 1B). However, we find that (in terms of self-sufficiency), there is very little benefit of installing a battery with more than 3 kWh of storage per MWh of yearly PV generation (Fig. 1C).

Many have watched the company with interest given its hype, larger than life founder and large amounts of investor funding – sources indicate this exceeded $70 million. Their various claims of high thermodynamic efficiencies which had already been achieved were also interesting – although it is hard to tell from the little available information whether this was simply a result of including the compression heat in their calculations (if you are interested in compressor performance you typically only consider the difference in air enthalpy between compressor inlet and outlet). As such, a little skepticism regarding this is probably healthy, which is emphasized by the company’s switch from CAES system to high pressure storage container manufacturer. Their water-spray injected compressor was an interesting idea, but was probably difficult to manufacture in practice. Moisture in compressed air can cause many problems including corrosion, aiding the build up of sludge deposits by mixing with oil/dust and as a result increasing pipe pressure losses, and causing faults in sensor and control equipment to name just a few issues off the top of my head. Crucially, it must also have been very difficult to avoid freezing during their expansion process, and water droplets/icicles in fast moving machinery can also be incredibly damaging. I should also note that their patent does also refer to oil spray injection. Lightsail’s move to high pressure container storage manufacturer smacks of desperation and suggests to me that the management realised that they had completely underestimated the scale of the ACAES challenge – indeed a review from a former employee on glassdoor quotes the Cons of working at Lightsail as “Large scale energy storage is NOT an easy problem to solve”! The high pressure storage tank is probably the only part of the ACAES system that actually exists off-the-shelf, which throws up questions of why this would be the component of choice to manufacture. I have discussed in previous posts my opinion that it is most certainly a misconception to think that a ACAES system can be constructed with “off-the-shelf” components. The primary reason would seem to be that they thought they could implement a marked reduction in container costs compared to leading manufacturers, and a recognition that building high pressure vessels was certainly possible, as these type of containers do exist and have a market. This would have potentially allowed them to make incremental scientific progress and make a better or cheaper version of a product that already exists rather than the more ambitious aim of a game-changing energy storage technology. The problem with this turned out to be that they couldn’t make a product that was enough of an improvement to ship many orders. It is very difficult to work out exactly what went on at Lightsail – I would love to talk to someone who worked there as I’m sure they were full of talented employees. There were also various reports about questionable spending habits on behalf of the senior management – again a look on glassdoor also suggests this. At least they were probably good to their employees, even offering pet insurance as a benefit… In any case, it’s a huge shame for thermo-mechanical energy storage, and what’s most frustrating is that there is almost no ability for anyone to learn from the technical failures that undoubtedly sank the company. Picking through the Lightsail patents (now owned by the Silicon Valley Bank) is difficult, and they just seem to be full of quite generic ideas, with no indication of any ingenuity in the designs. This reinforces why these early stage companies are so frustrating, and can ultimately be bad for the technology development, as they simply put-off would be investors while no new scientific knowledge can be gained. Maybe I will email Bill Gates and see if he can push for any knowledge to be made public… # CAES: A simple idea but a difficult practice In the mainstream there are two main branches of Compressed Air Energy Storage (CAES) – conventional and adiabatic. 1. Conventional CAES Conventional (also known as diabatic) CAES plants are essentially gas turbines in which air is pre-compressed using off-peak electricity, rather than running a turbine and compressor simultaneously. In these plants, off-peak grid electricity is used to compress air which is stored, and then mixed with natural gas and combusted during expansion. Compression is staged and the majority of the compression heat wasted (although some may be stored in a recuperator to pre-heat the air before combustion). Currently there are two commercial CAES plants worldwide; the Huntorf plant in Germany and the McIntosh plant in Alabama. • Huntorf CAES plant: Data from [1]. 310,000m3 cavern at a depth of 600m, pressure tolerance between 50 – 70 bar, converted from a solution mined salt dome. Daily charging cycle of 8h, output of 290MW for 2 hours. 0.8kWh of electricity and 1.6kWh of gas required to produce 1kWh of electricity. Notably, built when the price of gas turbines was historically high. • McIntosh CAES plant: Data from [2]. 538,000m3 salt cavern at a depth of 450m, pressure tolerance between 45-76 bar. Originally it provided an output of 110MW for 26 hours but in 1998 two extra generators were added and its total output capacity is now 226MW. 0.69kWh of electricity and 1.17kWh of gas to produce 1kWh of electricity. Both plants are commercially viable and still running in their respective markets! Figure 1: Schematic of diabatic CAES system. As with Pumped Hydro Storage (PHS), CAES also requires favourable geography to provide the underground air storage caverns. However there are many more suitable sites worldwide than for PHS, although the costs are highly site specific. The costs of mining a suitable underground cavern where suitable geology doesn’t exist or creating an above-ground equivalent storage container are potentially prohibitive, whereas alternatively a naturally occurring cavern or somewhere easily minable may offer a very attractive price of storage in terms of$/kWh (or dollars per metre cubed of air storage).

Caverns can be created in salt geology (typically using salt solution mining techniques) or existing caverns can be exploited provided that they are capable of housing the desired pressure. Geological formations such as aquifers and salt formations (bedded salt and domal salt) offer potential locations. Costs can also be reduced if existing well infrastructure is in place from previous underground drilling operations. While specific geology is required, this geology is relatively widespread. For example, the EPRI suggests that up to 80% of the US could have favourable geology [3] (see Figure 2).

US CAES map with wind resources marked

Figure 2: US geology for compressed air caverns. Regions with high wind resources are also indicated with the idea that CAES sites and wind turbines could be co-located [4].

Estimates for the costs of cavern mining can be as low as $1/kWh of storage capacity if solution mining techniques can be used [5]. In solution mining, fresh water is pumped in a salt deposit, becomes saturated with salt and is then removed. One problem however is that disposal of this brine can cause environmental issues. 1.1 CAES Performance Characteristics and Applications CAES systems have traditionally been designed as centralised storage facilities which are intended to cycle on a daily basis and to operate efficiently during partial load conditions. This design approach allows CAES units to swing quickly from generation to compression modes and means that they are well suited to ancillary services markets, providing frequency regulation. Their ability to operate on a (intra) daily cycles means that they are also useful for load-following/peak shaving. The air storage caverns can also be very large, allowing for multiple days worth of electricity storage. It should be noted that the inlet pressure (45-76 bar) for the CAES high pressure turbine is much higher than the equivalent for a typical gas turbine (about 11 bar) so a typical gas turbine can only be used as the low pressure expander. The high pressure turbine at Huntorf is based on a small-intermediate steam turbine design. 1.2 Table of Cost Estimates  Typical Capacity Typical Power Efficiency Storage Duration$/kWh /kW Lifespan Cycling capacity 500MWh – 2.5GWh 50 – 300MW n/a Hours – days 4-7 [6], 2-50 [7], 60 – 120 [8] 300-600 [6], 400-800 [7], 1000-1250 [8] 20-40 years High Table 1: CAES cost characteristics 1. Adiabatic CAES Adiabatic CAES is an energy storage concept that removes the natural gas combustion from conventional diabatic CAES. In adiabatic CAES the heat generated by the compression of air (the charging process) is stored in a Thermal Energy Store (TES) which is separate from the ambient temperature high pressure air store. When the system is discharged the high pressure air is reheated using this stored heat and then expanded. Without the stored heat, the process has an unacceptably low efficiency – this is because significant exergy is stored in the heat as well as the cool high pressure air. When the heat is recovered, the expected practical efficiency of these systems is debated – though the second law of thermodynamics does not pose a ceiling on the efficiency as for heat engine – it just means that the real process has to be less than 100% efficient. Pragmatic estimates of the real efficiencies of this type of system are debated; most of the academic literature estimates practical efficiencies in the range of 60-75% [9,10]. If a plant could be constructed with no inefficiencies in any process – the theoretical efficiency would approach 100%. 2.1 Status As no demonstration plant has ever been successfully constructed, Adiabatic CAES must be considered as an unproven technology. It does however have significant promise for use with renewables integration, energy management, peak shaving and grid reserves. The largest planned demonstration ACAES facility is a 290 MW adiabatic CAES project based in Germany called project ADELE [11]. It is a consortium between German utilities RWE and GE, the German Aerospace Center DLR, construction company Zublin, the Fraunhofer IOSB and the Unversity of Magdeburg. Figure 3: A simple schematic of an ACAES configuration. There is a thermal store for each compression stage. A schematic diagram of an ACAES system is shown Figure 3. In this configuration, air is compressed and then cooled using counter-current heat exchangers that transfer the heat from the air into a thermal fluid. This thermal fluid could then be stored in an insulated tank and used to reheat the air prior to each expansion stage. Several people have also suggested the use of Packed Bed regenerators to store the compression heat in the air. 2.2 Underwater CAES Underwater CAES is a sub-type of ACAES which exploits an underwater Compressed Air Store at a depth of typically around 400m. The ambient pressure at this depth is approximately 40 times the atmospheric pressure, and the air store is either a flexible bag or a dome structure open at the bottom. As air is pumped into the storage container it displaces water and thus the store can operate at a constant pressure. This idea was pioneered by Prof Seamus Garvey and Dr Andrew Pimm at the University of Nottingham, as well as by researchers at the University of Windsor Ontario and Canadian startup Hydrostor (whose work is ongoing at the time of writing). 2.3 Fuelless CAES The usage of the term “adiabatic CAES” is also somewhat ambiguous, as the term “adiabatic” is sometimes used to refer to the compressions and sometimes to refer to the overall process – i.e. the energy storage process aims to be adiabatic in the sense that ideally, it would exchange negligible heat with the surroundings. Therefore some authors therefore prefer the use of the umbrella term Fuelless CAES. This then clearly encompasses all compressed air processes which aim to store and return energy without the use of fossil fuels. This includes systems which have typically been labelled as isothermal CAES. 2.4 Isothermal CAES In isothermal CAES the compressions aim to be isothermal and reversible. This is theoretically achieved by minimising the temperature differences which drive heat flow from the compressors to the environment (which is at a lower temperature). A huge challenge here is to make an isothermal compression process which operates sufficiently quickly to be of practical industrial importance but which is still slow enough to maintain the small temperature differences required for high reversibility. One idea for near-isothermal compression which has been suggested by LightSail (a start-up company in California) involves a water spray into the compression chamber of a specially designed reciprocating compressor/expander unit (see Figure 4). The water droplets absorb the heat of compression and their high specific heat capacity causes the temperature increase in the compression chamber to be much smaller. This warm water is then stored and on discharge is re-injected as a mist into the reciprocating machine which now acts as an expander. Figure 4: Illustrating a near-isothermal CAES concept [12] Isothermal CAES was also being pioneered by SustainX, however this company has ceased operations citing spiralling system costs. Lightsail Energy and SustainX had a similar goal of an efficiency above 60% for their first generation of machines and believe that 75% is achievable in the long term. The SustainX prototype was a 1.5 MW machine. 2.5 ACAES Challenges There are several challenges which must be overcome before adiabatic CAES can become a viable energy storage technology option. • Specialised compressor equipment must be developed, in which the heat generated during the compression procedure is stored in a highly reversible manner. This process seems most likely to consist of a series of adiabatic compressions in which heat losses from the compressor to the surroundings are minimised. The compressors must also operate with much higher compression ratios than current compressors which do not involve cooling during the compression. Each of the compressions is then followed by a cooling stage which aims to reversibly extract the compression heat. Possible options for heat extraction include packed bed regenerators or counter-current indirect contact air-to-fluid heat exchangers. This type of compression equipment is fundamentally different to industrial many industrial compressors. Why? Because the vast majority of compressors are designed to minimise the work required to achieve air at a given pressure. Most industrial compressions then typically involve trying to shed as much heat as possible from the compression process – as hot air takes more work to compress. The ACAES process is fundamentally different as reversibility should be maximised rather than work minimised. In fact, the greater the reversible work is per cubic metre of compressed air the higher the energy density of the storage system. • Specialised expansion equipment must also be developed. Air turbines which provide highly isentropic expansions and operate within the desired pressure ratios are required. The expansion process of an Adiabatic CAES system should aim to mirror as closely as possible the reverse compression process. Therefore it should include the same number of expansion stages and heating stages, and expansion stages must aim to minimise heat gain and return all heat reversibly during the heating stages. While these turbines do not currently exist on the industrial market, it is anticipated that their design can learn much from the current generation of gas turbines for power generation. The pressure ratios will likely be smaller than most current gas turbines. One specific advantage is that the material demands will be much less (in terms of temperature tolerance) than current gas turbines which operate with inlet temperatures up to 2200K. • Sliding pressures. Unless the system can be operated between constant operational pressures, both the compression and expansion machinery must operate at maximum efficiency over a range of pressure ratios. A single constant high pressure air storage is a primary advantage of UnderWater CAES. • High pressure air storage. Depending on the chosen method of storage high pressure, air storage tanks must be developed which have minimum cost. This has apparently been a problem area both for SustainX and LightSail, however LightSail have released statements which hint that they may have found a method of lowering the costs. • highly reversible heat exchangers will also be required which can minimise the temperature difference between the working fluid and the thermal storage medium while introducing minimal pressure drops. 2.6 Notable experimental ACAES development Lightsail (California) – startup. http://www.lightsail.com/ Hydrostor (Ontario) – startup. https://hydrostor.ca/ SustainX (Massachusetts) – startup (liquidated) Project Adele (Ongoing utility/academic collaboration – big unexplained delays??) University of Windsor – Prof. Rupp Carriveau and Dr. David Ting University of Nottingham – Prof Seamus Garvey and Dr Andrew Pimm References [1] BBC Brown Boveri. Huntorf Air Storage Gas turbine Power Plant. https://www.eon.com/content/dam/eon-content-pool/eon/company-asset-finder/asset-profiles/shared-ekk/BBC_Huntorf_engl.pdf [2] M. Nakhamkin, L. Andersson, E. Swensen, J. Howard, R. Meyer, R. Schainker, R. Pollak, and B. Mehta, J. Eng. Gas Turbines Power 114, 695 (1992). https://doi.org/10.1115/1.2906644 [3] Compressed Air Energy Storage: Renewable Energy (2010, March 17) retrieved 22 April 2017 from https://phys.org/news/2010-03-compressed-air-energy-storage-renewable.html [4] Succar, S & Williams, R.H.. Compressed Air Energy Storage: Theory, Resources, and Applications for Wind Power, Princeton University (published April 8, 2008) [5] De Samaniego Steta, F. Modeling of an Advanced Adiabatic Compressed Air Energy Storage (AA-CAES) Unit and an Optimal Model-based Operation Strategy for its Integration into Power Markets. EEH – Power Systems Laboratory. Swiss Federal Institute of Technology (ETH) Zurich [6] Kaldellis, J. K. & Zafirakis, D., 2007. Optimum energy storage techniques for the improvement of renewable energy sources-based electricity generation economic efficiency.. Energy, Volume 32, p. 2295–2305. [7] Chen, H. et al., 2009. Progress in electrical energy storage system: A critical review. Progress in Natural Science, Volume 19, pp. 291-312. [8] EPRI, 2010. Electricity Energy Storage Technology Options. http://large.stanford.edu/courses/2012/ph240/doshay1/docs/EPRI.pdf [9] G. Grazzini, A. Milazzo. A Thermodynamic Analysis of Multistage Adiabatic CAES. Proc IEEE, 100 (2) (2012), pp. 461–472 [10] Barbour, E, Mignard, D, Ding, Y, Li, Y. Adiabatic Compressed Air Energy Storage with packed bed thermal energy storage, Applied Energy, Volume 155, 1 October 2015 [11] RWE Power. ADELE – Adiabatic Compressed Air Energy Storage for Electricity Supply. https://www.rwe.com/web/cms/mediablob/en/391748/data/364260/1/rwe-power-ag/innovations/Brochure-ADELE.pdf [12] Fong, D. Insights by Danielle Fong. https://daniellefong.com/ # A new high in renewable electricity for Germany – and a low in electricity prices! On Sunday 8th May Germany hit a new high for electricity generation from renewables, with renewable plants supplying 87% of the demand. The result was several hours of negative electricity prices – to such an extent that commercial electricity consumers operating through the wholesale market were paid to consume electricity. This has been reported by articles in Quartz and The Independent. German consumption, renewable energy supply, conventional supply and prices for 7/8 May 2016. Figure from Quartz article. As I reported in an earlier blog post, negative electricity prices are generally a reflection of insufficient flexibility in the power network. In the case of Germany on the 8th of May 2016, negative prices occurred due to a combination of high wind and solar production, and low energy consumption (due to it being a Sunday). Accordingly, there was significantly more electrical supply than demand and the electricity prices were negative from 7am – 5pm. It is interesting to note from the Figure (taken from the Quartz post) that there doesn’t seem to be a huge difference between the conventional production on Saturday compared to that on Sunday, but this small increase in the ratio of renewable to conventional power resulted in a hugely negative price spike. Additionally we also see that on both days the production was higher than consumption – I assume that this is a reflection of German exports to other electricity markets. It only seems likely that as Germany strives for 100% renewable electricity this type of situation will occur more and more frequently and will make the situation for storage more favourable. It would be ironic if the low daytime electricity prices that have eroded the market for energy storage in Germany could become so low that they actually began to favour storage again. Also, for interest, an article I wrote about negative electricity prices and energy storage. # Storage and the duck The California duck curve is now infamous and is very often features in discussions around storage. The duck phenomenon is a result of several factors coming together at once to create a scenario in which there is significant strain on the electricity generation system. The now infamous California duck. Typically the output from solar panels is well-aligned with times of high electrical demand, especially in systems which have large cooling dominated loads. This is because it often gets hot when the sun is shining and people tend to be most active during the daylight hours. Solar generation typically occurs when demand for electricity is high – during the middle of the day. However when there is a cool sunny day in systems which have a lot of solar panels that are typically used to meet cooling-driven loads, then the situation can arise in which the net demand for electricity which must be generated by conventional powerplants (i.e. coal, nuclear, gas) becomes depressed, as most of the demand can be met by the solar. This is a problem for utilities in itself as turning down the output on some of these plants (especially nuclear, to a lesser extent coal) is difficult and costly, so instead they sometimes opt to sell their electricity very cheaply (or even pay for it to be used when prices go negative). For utility-scale renewables this is also a problem, as they can end up in the situation where they simply have to stop producing electricity. On top of this, the power output from all the solar panels in a local region is very well correlated. Therefore they all start and stop producing power at close to the same time (there is some spread due to orientation and location). This leads to a sharp increase in the net demand leading up to the evening peak which typically occurs after the sun goes down. There are only certain types of plant which can react to changes in demand quickly (they have high ramp rates), for example gas and hydro and only hydro can do it cheaply, as conventional gas plants must already be running for some time at their Minimum Stable Generation levels before ‘ramping up’, which is often less economic and more polluting per unit of electrical output. Solar Panel outputs from the Pecan Street project (https://dataport.pecanstreet.org/) all producing electricity at the same time. Red line is the average The concern about the duck is a prime driver for energy storage development. This storage can come in several forms – i.e. not just batteries coupled with the solar panels. Some of these are highlighted in this NPR discussion which includes fuelless Compressed Air Energy Storage, Concentrated Solar Power with thermal storage in Molten Salts and Ice Storage for cooling. Ultimately it is all down to the economics. If the costs of storage are less than the increased costs of utilities as a result of having to provide the additional flexibility the duck requires, or if storage can increase the value of renewable energy sufficiently then it will become a viable option. At present the costs of curtailment are likely to be less than storage, but as the amount of curtailment increases and storage costs fall then this could rapidly change. # Capacity markets and energy storage I’ve been meaning to get my thoughts together about capacity markets and energy storage ever since the inaugural UK capacity market last year. So here it is, I start by talking about what a capacity market is and aims to do and then think about how it can affect energy storage economics. The purpose of a capacity market is to ensure that there will be enough powerplants installed and available to generate (sufficient generating margin) for the future operations of an electricity system. The capacity market aims to do this by providing stable and regular payments to market participants who agree to guarantee capacity which can be used to meet peak demand at some point in the future, over and above the payment they receive for the energy that they sell. Historically the electricity industries in most countries were developed as government owned monopolies. One of the legacies of this is that the wholesale price paid for electrical energy in today’s restructured markets doesn’t usually include the cost of building the powerplants themselves. Although in today’s restructured, re-regulated (liberalised) markets the costs of electricity are nearly always much higher than the marginal production cost (due to a number of factors including utilities’ market power), often these costs are not high enough to justify investments in new powerplants. This is known as the ‘missing money’ problem in electricity markets. Because electricity demand is so variable, and the highest peak demand only covers a short time span and occurs infrequently, electricity systems often have more than sufficient capacity for normal demand levels but insufficient capacity to reliably cover the highest peak demand spikes. If electricity were a more normal commodity, at times of high demand, high electricity prices would cause some consumers to decide they didn’t want to buy electricity, which in turn would cause the demand to fall. The ‘market price’ reached in this situation may then persuade other potential generator operators to enter the market (by building new powerplants). However as the majority of consumers do not see and thus cannot respond to real-time electricity prices, during times of power scarcity, administrative controls often limit the market price to stop it becoming unreasonably high – the demand for electricity is very inelastic. The missing money problem then occurs when the market prices are limited by administrative actions such as price caps. This means that large potential rewards to generators are forgone, and this can in turn result in little investment in new plants which would provide electricity during these times of scarcity. In an attempt to get around this issue, the capacity market then provides supplementary income to powerplants, in addition to what they earn in the energy market, to cover the costs of ensuring that they are available to generate in future. Capacity (in kW per year) is traded in the capacity market while wholesale electricity (in kWh) is traded in the energy market. The capacity payments are received by the capacity providers in the relevant delivery year and are typically paid for by a charge levied on all electricity suppliers (which in turn is passed on to consumers). There are also penalties for failure to meet a provider’s agreed capacity. Capacity markets usually include a large primary auction at some specified interval (4 years in the UK) before real time followed by smaller incremental auctions at closer time(s). The price paid for all the accepted bids is the market clearing price – representing the price of the most expensive unit of capacity required. This is known as pay-as-clear. It aims to encourage plants to bid in at their actual marginal cost, rather than speculating what the market price may be. The plant with the lowest marginal cost should then make the largest profit as all the capacity providers receive the same payment. Figure 1 illustrates how the capacity market bids are evaluated. Illustrating how the “pay-as-clear” capacity market auction works. The red line illustrates the target capacity. All bids below this are accepted and paid the price of the last kW of capacity. Source the energy collective. One big risk with capacity markets is that they rely on accurately forecasting future demand levels, to determine the optimal level of capacity in future years. If future demand levels are over-predicted, this can result in a large oversupply of capacity and uneconomic plant retirements, burdening consumers with very high costs for reliability. This has happened in Western Australia. The inaugural UK capacity market was run in 2014 to secure capacity for the winter peak season 2018-19. This is a T-4 auction (four years ahead of delivery) and there will also be a T-1 auction. Capacity providers are free to adjust their positions in private markets from one year ahead of the delivery year and throughout the delivery year, subject to some restrictions. The capacity providers must then be prepared to provide capacity when the system operator issues a capacity market warning. The market rules state that the “System Operator will issue a Capacity Market Warning (CMW) when the anticipated system margin in four hours’ time is less than 500MW. In the event of a System Stress Event starting which was not forecast, a CMW will also be issued. The CMW remains inforce until the forecast available margin is greater than the trigger level of 500MW.” Once the CMW is issued, providers must deliver their capacity obligation in four hours’ time to avoid CM penalties, should a System Stress Event be active at that time. Where does energy storage fit into capacity markets? Capacity markets are often touted as a good potential way for energy storage operators to gain revenue, which when combined with revenues from the energy market, could be sufficient to encourage investment. It has often been observed that the rewards open to energy storage devices in energy-only markets are generally not sufficient (certainly not at present) to justify investment in storage technologies, and the extra income from capacity markets is designed to justify investment in new infrastructure (i.e. generation or storage). Importantly, not only for storage but for all capital-intensive electrical infrastructure, the payments for providing capacity also have the advantage that they represent a reliable source of future income. However in order to justify investment in new technologies like energy storage, the price reached in the capacity auction must be high enough, and if the price is to reach these high levels then either the level of required capacity must be set high by the system operator (the auctioneer), or some existing resources should not be allowed to enter into the market. Last year, the UK held its inaugural capacity market auction. The auction aimed to be “technology neutral” and nearly all existing generating resources were able to compete, including storage and demand response. The providers were only evaluated at their bid price rather than any other factors, for instance, emissions. The result of this was a price of £19/kW which was too low to for any new energy storage projects. The real winners of the auction were the incumbent large utilities, whose existing plants got most of the capacity. These plants will be paid the result of the capacity auction on top of what they already earn in the energy market, provided they are still available to operate in the delivery year. This has frustrated many people as it is highly unlikely that many of these plants wouldn’t have generated anyway, so the net outcome of last year’s UK capacity market seems to have just been a significant increase in consumers’ electricity prices for the services they were already getting. What many people see as a problem is that the capacity market did not take into account any environmental factors that would remove, say, coal generation from the auction. DECC had originally stated that nuclear and coal would not be allowed to participate in the capacity auction, but after lobbying from the industry, changed this position. It seems likely that if capacity markets are to be truly relevant to energy storage, then these markets will have to take into environmental factors like emissions into account. It is fairly obvious that in the short term the costs of meeting our capacity needs are lowest with fossil fuels, however it is the long term cost on society and the environment that these markets should aim to minimise. If we believe in decarbonisation then existing coal plants in particular should not be competing for new capacity payments. In order to encourage storage or other new low-carbon technologies, capacity markets should be further in advance and more ambitious – they should only be open to technologies whose emission levels comply with our decarbonisation targets. The results of the inaugural UK market imply that the current UK government is much more concerned with security of supply than decarbonisation (although many sceptics say that security of supply has decreased due to our extended dependence on fossil fuels and the result of the auction will be an increase in electricity bills of £1 billion for the same service). Undoubtedly, security of supply is important, but it is, at best, overly-pessimistic to think that security of supply cannot be achieved simultaneously with decarbonisation. An ambitious capacity market would seek to address both of these challenges. Smaller capacity auctions with ambitious environmental targets and higher rewards that look further into the future may be one way to simultaneously achieve both of these aims. In this way these markets could drive up innovation in renewable-storage projects. # Tesla enters residential battery market with the Powerwall The big storage news of the week has been the unveiling of Tesla’s new stationary energy storage battery project – the Powerwall. This has inspired me to think a little out-loud about the economics of residential batteries… The Tesla powerwall: could it become the iphone of residential batteries? The powerwalls units come in two sizes, 7 kWh and 10 kWh and cost3000 and $3500 respectively. Tesla gives the specs as follows: • Mounting: Wall Mounted Indoor/Outdoor • Inverter: Pairs with growing list of inverters • Energy: 7 kWh or 10 kWh • Continuous Power: 2 kW • Peak Power: 3.3 kW • Round Trip Efficiency: >92% • Operating Temperature Range: -20C (-4F) to 43C (110F) • Warranty: 10 years • Dimensions: H: 1300mm W: 860mm D:180mm Firstly it should be noted that the costs are wholesale costs, and don’t include the price markup – ignore this for the moment. Secondly, and more importantly, you need an inverter, so let’s conservatively add$1500 on to the capital of the battery. So for a 7 kWh system let’s say \$4500.

So what can you do with 7 kWh?

The average UK house used about 4200 kWh of electricity in 2013, giving an average demand of very roughly 500 kW (4200 kWh divided by 8760 hours in the year). This equates to around 14 hours of power at this average usage. Of course, sometimes you are asleep or out, so let’s assume that when you are in the average demand is double this at 1 kW (about the power of a medium kettle). Here is a page with estimates of the average power for household appliances in 2008 (they may have got marginally more efficient). So you can probably expect to run your dishwasher and washer in the evening and you’ll have enough juice for lights and TV watching, but you’ll struggle to tumble-dry your clothes too. The peak power is also slightly limiting – you may struggle to run your electric shower, dishwasher and washing machine at the same time.

You would have to be a very frugal user of electricity to consider going off-grid with this battery.

Economics (in UK context)

So how does it stack up economically? Well, obvious things first, you aren’t going to get a saving even if the battery is free unless the price of your electricity varies with time – for example with a time-of-use tariff or if you have your own solar installation that has a different cost associated with the electricity it generates.

Working with a solar PV installation

Let’s say the cost of a solar installation in the UK is roughly £4000 for a 2 kW system, and this produces roughly 2000 kWh per year (that’s about half the yearly average demand for a UK household and equates to roughly 40 kWh a week). Let’s also assume that you get a generation Feed in Tariff of 13.4 p/kWh and an export tariff of 4.8 p/kWh (you get paid 13.4 p/kWh of electricity that you use and 4.8p/kWh of electricity you export), and the price you pay for your grid electricity is 15 p/kWh. Again, if you are using all the solar electricity you generate rather than exporting it, then there isn’t going to be any economic case for a battery. But if you are exporting some to the grid then by storing it you’ll be able to earn the generation tariff and displace the cost of some grid electricity later, but you’ll forfeit any earnings from the export tariff. So, if the round-trip efficiency of the battery is 85% (Tesla say 92% but this will degrade over time so we assume 85% as an average and there will be small losses associated with the inverter), you’ll get an extra 13.4 p/kWh plus 0.85 X 15 p/kWh minus 4.8 p/kWh = 21.35 p/kWh for the electricity you would have exported. Using all of these estimates we conclude that if you exported 50% of the electricity generated by your solar unit, you could save 1000 kWh X 21.35 p/kWh = a princely sum of £214 per year.

Of course this assumes that the battery has sufficient capacity to store all the electricity that would have otherwise been exported to the grid. 1000 kWh per year is approx. 3 kWh per day and the battery holds 7 kWh, but there is also a huge variation in the daily electricity generated, accounting for seasonal and weather-related variations. But for simplicity let’s assume that out 7 kWh battery can hold all the electricity generated. Approximately then, over the course of 10 years you may be able to save about £2000 using the battery. This is approx. 2/3 the cost of the battery. The figure below shows the expected yearly saving against the percentage of electricity exported by the solar PV system. It also shows the saving associated with the standalone solar PV system.

Saving capability of battery (blue line) against percentage of solar electricity exported (assuming battery always has sufficient energy capacity) – the dotted line shows how I would expect this to vary accounting for the finite capacity of the battery. Estimated saving from 2 kW PV installation also shown (green line)

At 50% electricity export our standalone solar PV system gives us a yearly saving 1000 kWh X (15 p/kWh + 13.4 p/kWh) + 1000 kWh X 4.8 p/kWh = £332. Very approximately that yields a payback of 12 years, which isn’t too far off other estimates (usually around 10 years). The battery-plus-solar system increases the yearly saving to a maximum of ~£500 (with 50% electricity export) and increases the whole system payback in excess of 14 years. Including the inability of the battery to store all the energy exported on summer days I’d expect this to realistically be significantly in excess of 16 years.

Storage and variable grid electricity prices?

The other way electricity storage lets you save is by buying low cost electricity, storing it, and using it when you would otherwise have to buy high cost electricity. Most domestic customers in the UK aren’t on variable tariffs but as an academic exercise let’s consider an Economy 7 tariff, which gives 7 hours (12am – 7 am) at 8 p/kWh and 17 hours at 16 p/kWh (I think these numbers are reasonable estimates). Working at around 80% depth-of-discharge, the battery could displace 5.6 kWh of peak electricity, replacing it with 6.6 kWh of off-peak electricity. If this strategy was run 5 days a week for 52 weeks, then it could generate a saving of around £100 per year. This is a bit less than half of the saving associated with the battery-solar system.

It says quite a lot about the economics to note that at 16 p/kWh, the value of the electricity stored in the battery is ~£1.10. At 3000 cycles this equates to a value of £3300.

Using UK spot market prices from 2013 we find that the 7 kWh battery could have made a maximum of £65 from (wholesale) electricity arbitrage in the year 2013 (to calculate this I use MATLAB and an algorithm available here).

Do combined solar-battery systems reduce the net emissions of the electricity grid?

This is more tricky. Any energy storage device is a net consumer of electricity. From that perspective, unless the electricity would otherwise be wasted it’s better to use it rather than store it. So if you are exporting electricity to the grid and the transport process (to where that electrical energy is used) is more efficient than your round-trip storage efficiency, then storing this electricity instead will increase the global net electricity used. To understand the effect on emissions you’ll need to know what generation source the exported electricity would displace and what generation source the stored electricity would displace. Several other factors also contribute – the battery will contribute to grid reliability and thus reduce the operating reserve margin. If there isn’t much solar in the region then it’s likely that the reserve margins will remain unchanged, however with enough distributed renewable generation at some point another thermal plant will need to be brought online (to deal with the extra fluctuations in supply and demand associated with many distributed renewable generators). In this way, as more and more distributed generation (i.e. residential solar PV) is brought online then storage becomes more important for the grid and is likely to reduce emissions through a meaningful contribution to reliability.

What should be concluded from all of this?

Well firstly it should be pointed out that what Tesla is doing isn’t new – solar plus storage has been done for quite a long time. Traditionally Lead Acid batteries were used, and they still have lower capital costs but are bulkier, require maintenance to replenish the electrolyte and vent hydrogen gas during charge. There are also other residential Lithium ion battery systems out there. Having said that, what this move represents is a big, exciting & fashionable company throwing its weight into the residential storage market. Tesla has the potential to become the iphone of residential batteries.

In terms of the UK economics, the battery and solar option isn’t going to be more economical than using grid electricity. With the current subsidy levels, and given that our estimated system costs are probably on the low-side, I’d imagine that payback for a battery-plus-solar-PV system is in excess of 15 years at present. This is compared to a payback around 10 years for a standalone PV system. The economic case for based on variable prices is much weaker than the case for solar-plus-storage – we anticipate a max saving less than £100 per year.

In other countries I would expect a similar situation, however in regions where outages are more common, the batteries may add an Uninterruptible Power Supply (UPS) which could drastically increase its value. Though it should be noted that in these areas batteries are already used, and if these are lead-acid type batteries then they will be significantly cheaper. For UPS applications efficiency and cycling are much less important so it’s hard to see the Tesla batteries becoming a better option.

For people who want to use the battery to reduce global net carbon emissions then you’ll need to carefully construct your arguments on why you think this is the case. There are lots of inter-playing effects that, as discussed, can lead to an increase or decrease in global net CO2 emissions. In the UK at present, the grid’s CO2 emissions are fairly consistent at about 500 g CO2 per kWhel­ when the demand is above 25 GW, so it’s hard to imagine that battery use would do anything but lead to a net increase in emissions at the minute in the UK.

However, if you would like to be more independent of the grid, or take a big step towards what many experts believe is a likely possibility for a low-carbon future, and own what could be turn out to be a very fashionable product then this could be the battery for you.