Thermal Energy Storage

A Closer Look At The Options

15/01/2024

The need for an increased reliance on renewable energy regularly surfaces as we try to combat climate change. The latest COP28 agreement spelt it out clearly, calling for a tripling of renewable energy capacity and doubling of energy efficiency improvements by 2030. It is a bold but necessary ambition to get anywhere close to achieving net zero goals in the timescale needed.

A subject that is often overlooked is how best to manage the unpredictability of renewable energy supply. And, when it is discussed, it often focuses on issues at a high level, like grid distribution and national power supplies. However, it can take up to 15 years for expansions to electricity grid transmission and distribution networks to come into effect. With the need for immediate action to limit the impact of climate change, there is an urgent need to look beyond centralised power generation, and towards localised heat generation.

The peaks and troughs in supply from wind and solar resources, and the considerable increase in demand as heat is electrified means it makes sense to look at opportunities for new synergies between the power and heat sectors.

Thermal energy storage (TES) and other forms of long-duration energy storage (LDES) are two promising avenues to maximise the potential of an evolving situation.

The need to adopt methods of TES as we continue the journey towards a more sustainable future is clear. And, as technologies evolve to meet this demand, it is worth considering the wider impact these options might have on our environment, beyond factors like capital costs, efficiency, and energy output. Here we look at two alternatives and consider some of these issues.

Storing energy for heat: conventional batteries

Today the most common forms of energy storage for heat are thermal storage via sensible and latent heat storage using phase-change materials (PCMs), and thermochemical storage. Electrochemical storage options are divided into two categories; capacitors and batteries. Whilst capacitors offer higher efficiencies and increased lifespan compared to batteries, they carry far less charge per unit per mass in comparison.

Batteries have also been the subject of much research on their use in energy storage systems, including integration with renewable technology systems.

Lithium Iron Phosphate batteries (LIPB) have been the subject of several studies evaluating their use, such as on wind farms to store energy for use when the wind isn't blowing. Their efficiency improves dramatically when more than one battery is used, allowing for complete charge and discharge cycles.

Other papers have focused on improving their efficiency, for example by controlling their operating temperature by using heat mats, or have looked at their impact on the environment through life cycle assessments (LCAs).

A new alternative: the SteamBattery

At Spirax Sarco, together with colleagues at Chromalox, we have developed an innovative form of TES: the SteamBattery. This stores heat generated by an immersed electrical heater as high-pressure hot water in a well-insulated vessel.

When steam is needed from the SteamBattery, it is taken from the ullage (gas) space of the vessel, and is either used directly as steam, or indirectly through means of a heat exchanger to connect with a "wet" heating system. The condensed steam is returned to the vessel. As the steam is used, the pressure lowers to the point where the SteamBattery is fully discharged.

It is recharged by the immersed electrical heater, which is able to use electricity from direct renewable sources or from the grid when low-cost renewable power is available. It can both discharge steam and be charged simultaneously, giving flexibility in how it is employed, and as buffer storage. Able to fully charge within 8 hours, it is able to do so overnight.

steam battery

Considering the wider environmental impact

Using current literature on LIPBs alongside our model, and existing studies for the SteamBattery, we aimed to compare the environmental impact of these two energy storage solutions. There were some limitations, due to the boundaries set by the LIPB studies; notably a cradle-to-gate approach that doesn't consider either their transportation or disposal at end-of-life.

Once the system boundary was established, a range of comparative environmental impacts could be assessed. Due to differences in the models used between the LIPB study and that for the SteamBattery, we found 10 of the 18 in the LIPB study offered a direct comparison.

Greenhouse gases (GHG):

These are the most relevant to climate change impact, and are measured in kg of carbon dioxide equivalence. The results shows that the SteamBattery would emit 8.58 kg/1000 kWh of energy stored throughout its lifetime, whereas the LIPB emitted 16.10/1000 kWh throughout its lifetime. Effectively, the SteamBattery has half the CO₂ emissions of the LIPB throughout its useful lifespan.

Effect on ecosystems:

We examined six environmental impact categories, including those that cover ecotoxicity and eutrophication in marine and freshwater environments, plus acidification and ecotoxicity in terrestrial ones. For both freshwater and marine environments, the SteamBattery was found to be 95% less impactful compared to the LIPB. This was largely accounted for by the cathode plate manufacturing process needed for the LIPB.

When looking at the terrestrial impacts, a different picture emerges. The SteamBattery's sulphur dioxide production was 83% less than the LIPB. However, its dichlorobenzene equivalent was higher than the LIPB. A closer examination, considering the impact loads of both products across the different environmental categories, concluded that this was an area for potential improvement rather than a serious flaw.

graphical battery comparison

The assessment further highlighted SteamBattery's reduced impact on natural resources, such as fossil fuels and water. Notably, the highest environmental loads were predominantly associated with the LIPB, particularly in marine and freshwater ecotoxicity, whereas the SteamBattery's most significant impact was considerably lower in terrestrial ecotoxicity.

As the need for sustainable steam systems grows, there is a clear imperative to consider more than simply avoiding fossil fuels. The planet's resilience and future depend on a host of other factors, with environmental considerations high on the list.

This initial study shows a more holistic survey of potential options should always be considered before final decisions are made.

Source:

^{Borbala Rebeka David, Sean Spencer, Jeremy Miller, Sulaiman Almahmoud, Hussam Jouhara:}^{(Comparative environmental life cycle assessment of conventional energy storage system and innovative thermal energy storage system, 2021).}

Towards Sustainable Steam

Our need for steam, in so many sectors, is undiminished. We are driven to ensure its future is a positive one, for people and planet. Innovation, optimisation, and best-in-class steam system management will be vital for its sustainable future.