The overlooked resource in industrial heat

British industry is very good at making things, but it is not always good at keeping the heat those processes produce. Flue gases, cooling water, steam condensate and hot air from compressors all carry useful energy. Too often, that energy drifts into the sky or down the drain. Thermal storage systems change that. They capture waste heat and hold it in an insulated store until a process needs it, whether that is minutes later or the following morning. The result is lower fuel demand, smaller bills and a more flexible way of running a site. It is not a silver bullet, but for many industrial operations it is one of the most practical decarbonisation measures available.

How thermal stores capture and release heat

A thermal store is, at its simplest, an insulated vessel filled with a material that can absorb heat and give it back later. The engineering lies in matching the store to the temperature and duty of the waste heat. There are three broad families:

• Sensible heat stores: these raise the temperature of a material such as water, thermal oil, molten salt or a packed bed of ceramic or gravel. Water is cheap and effective up to about 95°C, or higher in pressurised systems. Thermal oil suits medium temperatures, often up to 300–400°C. Molten salt can reach 565°C but needs careful freeze protection.
• Latent heat stores: these use phase change materials (PCMs) such as salt hydrates, paraffin waxes or sugar alcohols. The material melts as it charges and solidifies as it discharges, storing heat at a nearly constant temperature. That makes PCMs excellent for matching a specific process demand, such as hot water at 80°C or steam at 120°C.
• Thermochemical stores: these rely on reversible chemical reactions. They can hold heat for long periods with very little loss, but they are still less common in mainstream industrial settings. For most sites, sensible and latent stores offer the better balance of cost, reliability and simplicity.

Charge and discharge usually happen through heat exchangers. Good controls decide when to send waste heat to the store, when to draw from it, and when to bypass it altogether. Insulation, pipework and valve selection matter as much as the store itself.

Matching storage to your process temperature

Temperature is the first question to ask. Low-grade waste heat below 100°C is common from compressors, chillers, drying and washing. It can charge a hot water store for space heating, preheating or wash-down. Medium-grade heat between 100°C and 400°C comes from baking ovens, steam systems, kilns and some furnaces. Here, thermal oil or a pressurised water store may fit. High-grade heat above 400°C appears in metal treatment, glass and ceramics, where molten salt or a packed bed could be appropriate.

Consider a bakery with a flue gas stream at 250°C. A thermal oil store can capture that heat during baking and release it later to preheat combustion air or wash water. A food factory with a pasteurisation line may need 85°C water for several hours, but its refrigeration plant rejects heat continuously. A PCM store can bridge that mismatch. Batch processes often benefit most because the store acts as a buffer, letting the heat source and the heat demand run on their own schedules. Continuous processes can still gain, but the store must be sized for steady, realistic duty rather than a best-case peak.

Flexible electricity and grid support

Thermal stores do more than save fuel. They let a site decouple heat generation from heat use, which opens the door to flexible electricity consumption. Instead of burning gas on demand, you can charge the store with electricity when prices are low or when renewable generation is abundant. Heat pumps can upgrade low-grade waste heat into a store at 60–90°C. For higher temperatures, electric boilers or immersion heaters can charge thermal oil or molten salt stores.

That stored heat becomes a thermal battery. A site can preheat during off-peak hours and discharge during peak periods, cutting both energy bills and strain on the local network. In the UK, where wind generation is sometimes curtailed when the grid cannot absorb it, industrial thermal stores offer a way to soak up excess renewable electricity. They can also support demand-side response and grid balancing services, turning a heat recovery project into an income stream as well as a saving.

Practical design and integration tips

Thermal storage is mature technology, but it rewards careful design. A few principles make the difference between a store that performs for decades and one that underdelivers:

• Insulate thoroughly: every extra centimetre of insulation reduces standing losses. Pay attention to flanges, supports and valve bodies, not just the main vessel.
• Preserve stratification: in water stores, keep hot and cold layers separate. Diffusers and low flow rates help. Mixing destroys the useful temperature difference.
• Size for realistic duty: oversizing wastes capital and increases heat loss. Undersizing means the store cannot cover the demand period. Model the actual load profile.
• Plan for fouling and corrosion: dirty waste heat streams can coat heat exchanger surfaces. Include filtration, cleaning access and corrosion-resistant materials where needed.
• Integrate controls: link the store to the process, the heat source and any electric charging equipment. Simple sequencers often work better than complex algorithms.
• Think about safety: high-temperature stores need pressure relief, thermal cut-outs and clear operating procedures. PCMs may need freeze protection or containment.

Monitoring is essential. Meter the heat going in and out, track storage temperatures and log how often the store is fully charged or empty. That data proves the savings and shows where to improve.

Assessing your site for heat recovery

Start with a heat map of your site. Measure temperatures, flow rates, operating hours and contamination levels for every waste heat source. Then list every heat demand: hot water, steam, drying, space heating, preheating. Match the two lists by temperature and timing. Look for continuous sources that can charge a store and regular demands that can draw from it. Check available space, floor loading, access for maintenance and any permitting requirements.

A pilot project is often the sensible first step. Install a small store on one process, measure the results for a few months, then scale up. Grants and advisory services exist for UK industrial decarbonisation, so it is worth checking what support is available. Not every site needs molten salt or a bespoke PCM. A well-insulated water tank with good controls and a clear understanding of the load profile often delivers a strong return. Thermal storage is a practical, proven tool for keeping valuable heat in the system, cutting fuel use and making electricity demand more flexible. For many industrial sites, it deserves a place near the top of the decarbonisation list.