In the previous article we analysed the problem of renewable energy fluctuation and I came to the conclusion that if we really want the proposed zero emission commercial building to be totally independent from the electric grid, we should be able to store about 2 MWh of electric energy to cover a 5 day period of time without electricity produced in real time by on-site wind turbines and PV solar panels.

What are the available technologies out there that can satisfy this requirement?

In fact, energy storage solutions do exist in many electrical power systems. In 2007, 2.5% of the electricity that passed through the US network was stored. However, some of the technologies that are used to store electricity are suitable mainly for large power systems while others can work well with on-site micro-generation.

Let's go through some of the proven technologies but without excluding some promising new ideas.

The most widely used large-scale technologies are pumped-hydro and compressed air energy storage (CAES). They may be used to store large amounts of energy generated from wind farms and other renewable sources, allowing the energy to be dispatched when it is needed. The downside is that they require big investments and a lot of space.

Pumped hydro-electric storage works by using surplus electricity to pump water to a high level and then letting the water fall through hydroelectric turbines to generate power when needed.

Compressed air energy storage works by using renewable power to pump air into underground caverns, then releasing it to augment a natural gas-fired turbine when power demand peaks.

Both technologies need large natural spaces such as water reservoirs, lakes or underground caverns. Even though hundreds of hours of output can be stored, providing the ability to cover very long periods without sun or wind, these solutions are not quite suitable for the scale of our project where we have to backup the energy needed by a single commercial building and not by a whole town.

Though we can't reasonably use traditional pumped hydro and CAES for a small site like the one under analysis, we may consider an alternative option.

Isentropic is a Cambridge based company that designs and develops an innovative system called Pumped Heat Electricity Storage. According to Isentropic, this storage system is much smaller and cheaper that a standard pumped hydro facility.

As its name suggests, instead of storing the energy in potential energy form (a mass of water brought up to a high level), this solution stores the energy in thermal form. The storage is formed by two large containers of gravel, one hot (500°C) and one cold (-150°C). The electricity we want to store is used to compress/expand air to +500°C on the hot side and -150°C on the cold side. The air is passed through the two piles of gravel where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the Isentropic machine as a heat engine. Figure 1 describes a 30 MWh Isentropic Pumped Heat Storage system formed by two 7m high gravel containers with a diameter of 8m.

Figure 1 - Isentropic 30 MWh Pumped Heat Storage system

According to Isentropic, the efficiency of the system is 70% - 80%. Because gravel is very cheap and readily available, the cost per KWh can be kept very low - $55/KWh - and $10/KWh at scale, which is about £36/KWh at the time of writing.

Scaling down the system to 2 MWh (which is what we need for our project) the containers would be much smaller in size and the cost would be about £70,000, assuming £35/KWh.

Ultracapacitors and flywheels are short-duration storage technologies that can be used to address power-quality disturbances and frequency regulation, applications in which only a few kilowatts to megawatts are required for a few seconds or minutes. In this case the amount of energy that can be stored would be enough but we need to store energy for more than just minutes and for this reason we have to rule out these technologies.

What else can we use to store electricity? We can't forget another option widely available, actually the electricity storage solution which we are most familiar with: batteries.

There is still a lot of ongoing research about batteries. Experiments have been carried out with so-called flow batteries, which contain large chemical tanks of liquids that can absorb and release electricity due to electrochemical reactions. Unfortunately, they are environmental nightmares and engineering headaches to build and operate. Most technologies rely on bromine (the most dangerous element for ozone depletion) or vanadium (thought to be toxic to fish) compounds or contain large volumes of highly corrosive liquid. Moreover, flow batteries are also relatively inefficient: less than 80 per cent of the energy stored can be released. I have to say that they don't look like the best solution for an eco-project!

What about hydrogen batteries? Although hydrogen batteries are often passed off as the definitive solution for electricity storage, this technology is not widely accepted by many from social, technical and economic points of view.

Socially, because hydrogen is a colourless, odourless and highly explosive gas that has no use outside of chemical plants. Technically, because despite over 50 years of intensive research there is still no good way of storing hydrogen. Economically, because the whole process of producing hydrogen in an electrolyser and using it in a fuel cell is less than 50 per cent efficient.

Nonetheless, La Fabbrica del Sole (The Sun Factory), an Italian research centre funded by the Tuscan local administration, has built a showcase off-grid site called HydroLab where they have used PV solar panels for the on-site electricity generation, hydrogen generated by the surplus electricity and used in fuel cells, thermal solar panels and a rain harvesting system. It is an intriguing project, very similar to Zero Emission Workspace but very much focused on hydrogen as means of energy storage.

Probably, the most promising battery technology is the one that many of us have been using for ages: Lithium-ion batteries, which are likely to power your mobile telephone and laptop computer. This is a very old technology but recent research has shown that batteries large enough can interface with wind turbines with efficiencies very near 100%.

According to Peter J Hall, who is Professor of Chemical Engineering at the University of Strathclyde and leads the government-supported Supergen Energy Storage Consortium, since batteries only take up and release energy slowly, when sudden surges of power are required, batteries of the future will be backed up by supercapacitors. These are very simple devices, in which energy storage involves no chemical changes, so they are able to store and release energy very quickly. As with Li-ion batteries, they have an efficiency of 100 per cent and very long cycling times.

But is this just science-fiction? No, it isn't. At the end of 2009, the Japanese electrical giant SANYO Electric has announced the launch of large capacity, high-voltage Li-ion batteries suitable to be incorporated into existing systems as part of hybrid schemes using solar cells. They can also store electricity generated by wind power, or used for electrical output stabilization. The mass production of these batteries started in March 2010.

SANYO is not the only company that invests in batteries. SAFT, a Paris based world specialist in design and manufacture of high-tech batteries for industry, produces a range of modular Li-ion batteries that can store up to 50 MW for 60 minutes and beyond. Even if this solution is suitable to satisfy a short electricity generation drop, it seems to be an appealing technology. One of the reasons is that it will be deployed by the energy consulting company ABB within their SVC Light with Energy Storage technology, where active power control is enabled with dynamic energy storage by levelling out power fluctuations from intermittent generation sources such as wind and solar energy. This is actually a large-scale implementation which is mainly aimed at improving the quality of the electricity delivered through the grid, especially for big industrial consumer. Nonetheless it is interesting to see that a lot is going on regarding Li-ion batteries for many different applications.

Also, we cannot ignore the fact that in the proposed implementation of Zero Emission Workspace we have already introduced a number of Li-ion batteries, which are  the batteries on board of the six electric vehicles available in the eco-car park. In fact, we may want to take advantage of these batteries to help manage the energy fluctuation problem. For obvious reasons, we have already decided to recharge the batteries overnight, when the consumption of the commercial building is at its minimum. Furthermore, when the on-site electricity production is running low, we may want to implement a smart charging scheme, under which we stop charging the batteries to satisfy the overnight electricity demand and start charging them again when there is more electricity available. Whenever it is required, we could even decide to sacrifice the primary use of these batteries (powering the vehicles) and instead use them as electricity generators. This means that we could run the chargers in reverse, releasing 64 KWh per day - the equivalent energy that we should use to power all 6 electric vehicles for 100 Km per day.

All the above solutions need careful analysis. Many of them are cutting-edge technologies and their application in a project like Zero Emission Workspace has probably never been tested.

In particular, we need to answer many questions, like:

  • in which scenarios have these technologies been already used?
  • what would it be the size and the cost of their application?
  • what are their reliability and performance on a short and long term?
  • who would be responsible for the design and installation of the energy storage system?

These uncertainties shouldn't stop us from trying out new things though: we should bear in mind that usually grants and funding in general are given to innovative ideas. Also, companies that have developed new technologies are keen to try them in practice with pilot projects and if someone gives them this opportunity, they should seize the moment.