Power Packed!

One of the key criticisms of renewable energy is the unreliability of natural sources, and the fact that the sun has to be shining or the wind blowing for energy to be produced. While traditional power generation typically consists of a ready fuel source which can be consumed on demand, renewable energy must be stored to cover times when it cannot be generated. This mismatch is one of the two central challenges with energy delivery, namely how to make it available when it is needed and how to get it to the right place.

There are many ways the storage challenge can be addressed with the most established being large scale projects to store potential energy by pumping water to a higher elevation for use in hydroelectric generation, or by storing compressed or liquified gas using dedicated storage tanks or caverns (see Hydrostor or Highview Power). A recent alternative comes from Energy Vault which follows a similar approach by proposing to lift and stack heavy concrete blocks. The attraction of these methods is that energy can be stored for long periods, the drawback being that large infrastructure investment is typically required and flexibility is limited. An obvious alternative is batteries which allow concentrated and portable energy storage. This is clearly an important part of the solution and technology continues to advance extremely fast in this area but batteries present drawbacks in terms of limited lifespan, toxicity of materials and the high energy required for manufacture and recycling.

Another answer is thermal energy storage which has an unglamorous image, but offers increasingly attractive options. A common approach is to use molten salt or silicon as the primary heat medium, however this also requires significant infrastructure when compared to storing energy in solid form. The principle of using dense blocks to store energy has been around a long time and is based on finding materials with a very high specific heat capacity and insulating them effectively until the heat is needed, at which point it can be released either directly or used to generate power. While heat isn’t the best solution for long term storage, it can be used effectively to smooth demand peaks. Examples include directly storing heat from solar generation for use at night or repurposing existing coal-powered infrastructure to use stored thermal energy instead of fossil fuel at times when renewable sources are not available.

Recent advances in this area include MGA Thermal which is building on work from Australia’s Newcastle University. Under their patented method, which they call “miscibility gap alloys”, graphite is combined with aluminium particles to form solid modular blocks. The graphite powder provides a solid conductive framework which remains stable at high temperatures while the aluminium particles within the block melt at 600c capturing latent heat. This not only allows energy capacity to be increased but flattens the release curve to deliver at a constant temperature and can be repeated over thousands of cycles. In late 2021 the group raised AUD 8m in a VC round to fund their next leg of growth.

Other teams such as Antora in California are experimenting with more traditional solid blocks made from graphite or ceramics which can be heated to as high as 2000°C as a means of increasing capacity. Another of Antora’s advances has been improving the efficiency of conversion back into electricity, a key consideration when looking at overall viability, and has demonstrated thermovoltaic electricity generation at a record 30% efficiency.

As a result of these and other similar innovations thermal energy blocks can now achieve similar energy density to conventional lithium batteries at a fraction of the cost. Moreover, the predicted lifespan for a single block can be 30 years or more while production can be easily scaled using existing infrastructure, and recycling is far less challenging. Although the ‘full cycle’ efficiency is significantly lower than for battery or pumped energy storage where round trip efficiency is around 80%, these additional factors make energy blocks an attractive alternative.

Thermal storage is not the only answer but it certainly has an important part to play in the transition towards clean energy. While battery technology is in vogue, interest in overall energy storage generally has fallen far behind the rapid increase in renewable energy production and capacity is very limited. This balance should reverse over time and as it does we believe the energy storage sector will provide fertile opportunities for innovation and investment.