Battery Limits

Efficient energy storage has always been one of the biggest challenges in engineering, which is why even in the 21st Century, we are still reliant on the chemical energy stored in fossil fuels for the majority of our heat, power and transport. In power generation, the solution has always been to only generate electricity when it is needed, because storing it is too hard and generating too much of it disrupts the power network. Batteries are large, expensive and inefficient compared to fossil fuels, but that is changing.

Battery technology has advanced tremendously since the 1980s, largely as a result of miniaturization associated with mobile phones and other portable electronics. But can we expect that to continue, and if so, how quickly? It is often quoted that the speed of microprocessors will double every two years, a phenomenon known as Moore’s Law. There is a lively and seemingly never ending debate about the death of Moore’s Law, but to date it is still holding up. Batteries also improve over time, but much more slowly. Tesla have stated 5-8% improvement year on year. That pattern holds for the last 30 years, but for the most part it has come about in a series of steps, when new chemical combinations are used in batteries. Whereas processor speed increases as ‘switches’ get smaller and frequencies higher, the energy capacity of a battery has a more fundamental limiting factor. Chemistry limits the amount of electrical energy that can be stored to about 1 eV per atom and the best batteries today can hold about a quarter of that. So if Tesla’s improvement rate is correct, we will be approaching that limit in 15-25 years.

The way we use batteries is also starting to change, with more electric and hybrid vehicles coming to market. Batteries are also starting to feature in buildings and as part of the electricity grid, balancing supply and demand and providing resilience. When combined with a renewable energy supply such as wind or solar, they turn the intermittent and unpredictable power output into one that is reliable and therefore more valuable. When installed in a building ‘behind the meter’ they charge when the building energy demand is low and discharge during busy periods. This means that the grid connection to the building can be much smaller, which reduces infrastructure costs and frees up capacity in the network.

Batteries can also be combined with onsite generation, to great effect. For example, the correct sizing of a combined heat and power unit is critical to making it economically viable. Too big and its expensive and operates inefficiently. Too small and economies of scale are lost. But if it is combined with battery storage then it can operate like the engine in a hybrid vehicle; running for longer periods at its most efficient output and then switching of and waiting to run again. Add smart control, demand prediction and the analytics that are possible with big data and the mix becomes very potent.

Tactical, modular battery units combined with onsite generation can provide a scalable, interchangeable energy system for buildings. As battery costs reduce, this offers an interesting alternative to the standard grid connection model.