The BankEnergi Vision: Energy Storage

This is the default storage method. Most of the energy used on the South Bank is in the form of electricity, for lighting, power, air conditioning, heating and – increasingly – charging electric vehicles. Retaining energy as electricity reduces conversion losses – although no battery will give out as much energy as is put in. Although some stationery lithium-ion batteries claim a 90-95% efficiency, that figure is based on low-voltage DC electricity, and allowing for conversion from 230V or 415V AC to low voltage DC reduces the overall system efficiency to the 70-80% range.

Lithium-ion (or other high efficiency) batteries are no cheap. Although they would be our first choice for BankEnergi, it may be necessary to compromise and use end-of-life batteries from Electric vehicles (EVs), which require a greater reliability, or even to use old-fashioned – but low cost – lead-acid units.

Other issues to consider are space – and we are looking at possible sites in disused car parks, disused swimming pools and space beside railway tracks, and the need for cooling and avoid fire risk.

Vehicle to Grid is a great idea for storing surplus electrical energy and releasing it when needed.  Using electric vehicles (EVs) shares the capital cost of the batteries between the car owner (who would have it anyway) and the building.  However there are many issues that still need to be addressed.  These include:

• Ensuring vehicles are linked to the grid adjacent to the building when energy needs to be stored or released – if commercial vehicles are used, they are likely to be away from their base at times of peak demand;
• Regulatory issues that mean it’s not currently possible for a fleet owner to contract with a different building owner other than through a common energy supplier;
• Scale, with the total number of vehicles – a Tesla Model 3, for example, has a capacity of 50-75kWh maximum storage capacity, and a commercial system would typically have at least 1,000kWh (requiring a reliable pool of at least 20 cars to be equivalent);
• Discharge cycles – most EV batteries are only warranted for a number of charge-discharge cycles, and using them in V2G applications would lead to a need to replace the batteries much earlier in the life of the vehicle.

Currently BankEnergi is not intending to trial any V2G storage, but it is keeping up with the market, and participating in the Elexon P379 working group, which will address the second point above.

This is the second technology that BankEnergi expects to implement. Essentially it works by uses electricity from cheaper times of day to heat up a storage tank of hot water, and then releases the hot water when it is needed.  Normally this is done in conjunction with a heat pump, as ground or water source heat pumps systems often incorporate a buffer tank for hot water in any case.  This allows the heat pump to operate at cheap times, recognising that they often need to run for several hours before an office or shop is opened.  There is also a lot of inertia in relatively low temperature systems fuelled by heat pumps, which enables them to maximise short-term advantages in pricing – switching on or off to run only at the very best tariffs. Compared to other energy storage, this is the low-tech solution with few implementation risks, and relatively low costs, though it does requires space for water storage.

The main downside is that thermal energy cannot be sensibly turned back to electricity, so this only works where there is a demand for hot water or a “wet” central heating of HVAC system.  This restricts it to seasonal applications, or premises with a high year round heating and hot water demand, such as hotels, leisure centres and hospitals.  No thermal store can be perfectly insulated, so it works best taking advantage of a 24-hour demand cycle, and cannot store energy for long periods. With several large hotels and two major hospitals on the South Bank, we see this as a promising application.

These are two separate applications:

The warm soil approach simply reverses the heat transfer from a heat pump, pushing heat back into the ground when energy is very cheap; when the heat pump is in heating mode it can recover some of this energy and will generally operate at a slightly higher efficiency (CoP) owing to the warmer input.  It has a very low system efficiency and most of the heat leaches into surrounding ground, and is not currently a realistic alternative, even on a short diurnal time frame.

A higher tech alternative is to pump energy into a special phase change material, taking advantage of the latent heat of freezing to absorb or release low temperature heat.  Although these have been trialled in applications such as a trombe wall, the BankEnergi project does not see this as a readily replicable technology.  Molten salts, which operate at a much higher temperature have also been proposed as a workable storage solution.

One alternative that has generated a lot of interest is storing energy kinetically, in fast rotating flywheels.  Although a couple of test installations have been made in the UK, for example at the University of Birmingham, this technology is not sufficiently proven to be applied to large buildings, and is more likely to have applications in transport or as small-scale storage for computer servers.

Another storage method at the test stage is to use energy to compress air, and then generate it by controlled release.  A couple of large scale installations both rely on underground caverns; a smaller scale installation in Bedfordshire generated useful results but has not led to commercialisation.

Most commonly seen as pumped hydro, where huge volumes of water can be pumped back uphill into a hydro-electric reservoir, the same approach can be used where there are water towers or – in theory, at least – tall buildings.  While it might be conceptually attractive to convert a swimming pool high up in the Shard into a store for energy, it is unlikely to prove cost effective.  In practice, this technology works best where there is a need for unpredictable balancing at a very short notice.

As an alternative to converting mains voltage electricity into low voltage DC for battery storage, it is possible to use it to produce hydrogen that can then be stored and used to generate more electricity through a fuel cell (or potentially blended with other gases in a conventional boiler).  In the long term, this may well prove to be a favoured method or energy storage, although it currently suffers from both high cost and relatively low system efficiencies.  Hydrogen needs careful storage, usually under quite pressure, and a significant amount of energy is lost in the compression/decompression cycle.  Ultimately, the best energy carrier may turn out to be a different medium, such as synthetic natural gas (methane), or even a liquid hydrocarbon that can be readily stored but not used immediately in a fuel cell.