Electrifying process heat changes what a factory buys. Instead of gas or oil, the site is now buying electricity, and the price of electricity changes throughout the day.
Wholesale prices rise and fall with demand and with how much wind and solar generation is on the system. Network charges are often highest in the late-afternoon and early-evening peak. At other times, particularly overnight or when renewable output is high, electricity can be considerably cheaper.
A heat pump connected directly to the process has to run whenever the process needs heat, whatever electricity costs at that moment.
Thermal storage changes that.
By storing heat in a buffer tank, a site can run its heat pump when electricity is cheaper, then reduce or pause electricity use while stored heat supports the process. The same store can also allow the site to respond to signals from the electricity system and be paid for doing so.
This article explains where those benefits come from, how storage should be sized to capture them and why storage is not usually a route to installing a smaller heat pump.
What does a thermal buffer tank do
A thermal buffer tank stores energy in hot water or another suitable storage medium.
When the heat pump is running and the store has capacity, surplus heat charges the tank. When the heat pump is turned down or paused, hot water from the tank supports the process.
The arrangement can perform several functions:
- Shift heat-pump operation into periods of lower electricity cost
- Allow the site to reduce electricity demand during peak-price periods
- Enable participation in grid-balancing and demand-flexibility services
- Reduce rapid changes in the load seen by the heat pump
- Allow longer and more stable operating cycles
- Reduce frequent compressor starts and stops
- Provide limited heat during temporary interruptions
The tank does not create additional energy. It shifts the time at which generated heat is used, and therefore the time at which electricity is bought.
Running the heat pump when electricity is cheaper
For most industrial heat pumps, electricity is the largest operating cost. Moving consumption away from expensive periods can have a direct effect on running costs.
Savings can come from several sources:
- Time-of-use or half-hourly-priced electricity tariffs
- Lower wholesale prices overnight and at weekends
- Periods of high wind and solar output, when prices can be very low and occasionally negative
- Avoiding the higher distribution network charges that apply in the late-afternoon and early-evening peak
- Reducing the site's maximum electrical demand during peak periods
Earning income from grid balancing
The electricity system must balance supply and demand second by second. As more wind and solar generation connects, the system operator increasingly needs flexible demand that can be increased or reduced on request.
A heat pump with thermal storage is well suited to this. It can turn down for a period while the store carries the process, or turn up to absorb surplus generation and charge the store.
Depending on the site and the service, this flexibility may earn payments through:
- The Balancing Mechanism operated by the National Energy System Operator (NESO)
- Demand-flexibility and demand-side response schemes
- Local flexibility services procured by Distribution Network Operators
- Capacity Market agreements for demand-side response
Industrial sites can access these services through an aggregator, which combines flexible assets from several sites, manages bidding and dispatch, and shares the resulting revenue.
The store is what makes participation possible. Without it, turning the heat pump down would mean interrupting the process. With it, the process continues while the heat pump responds to the grid.
How much heat can a buffer tank store
The useful storage capacity of a hot-water tank depends mainly on:
- The volume of water
- The difference between the charged and discharged temperatures
- The minimum temperature acceptable to the process
- Heat losses from the tank and pipework
- The amount of water that can be practically circulated
- The control strategy
A larger temperature difference allows the same volume of water to store more energy. However, the lower end of that temperature range must still be hot enough to serve the process.
Charging the store to a higher temperature increases its capacity, but a heat pump generally operates at a lower coefficient of performance when producing hotter water. The value of shifting consumption must outweigh that efficiency penalty.
The design should therefore be based on usable storage capacity, not simply the physical size of the tank.
Stratification also matters. In a well-designed tank, hotter water remains towards the top while cooler return water settles lower down. Poor hydraulic design can mix these layers, reducing the useful delivery temperature even when energy remains within the tank.
Thermal storage and waste heat recovery
The same principle can support industrial waste heat recovery.
Waste heat availability and process demand do not always occur at the same time. A kiln, dryer, refrigeration system or exhaust stream may release recoverable energy while the receiving process has little immediate demand.
Without storage, some of that energy may remain unused.
A thermal store can help separate the timing of heat recovery from the timing of heat use. Energy recovery from waste heat can continue while demand is low, with the recovered energy stored for later use, including periods when buying electricity would be most expensive.
This can improve industrial waste heat utilisation, but only within the temperature and capacity limits of the system. The stored temperature must remain useful for the receiving process, and losses from the tank and distribution network must be included in the assessment.
Thermal storage may therefore form part of wider industrial energy recovery systems that include:
- A waste heat source
- Heat exchangers or a heat recovery tower
- An industrial heat pump
- A thermal buffer tank
- Process heat exchangers
- Pumps and insulated distribution pipework
- Existing boilers or heaters for backup
- An integrated control system
These components must be designed as one system.
Understanding demand and price profiles together
To capture the value of storage, the heat-demand profile needs to be compared with the electricity cost profile over the same period.
The assessment should establish:
- The sustained heat demand and how long it lasts
- When during the day and week demand occurs
- Required flow and return temperatures
- Periods when production stops or demand falls
- The site's electricity tariff and network charging bands
- Times when electricity is typically cheapest and most expensive
- Available electrical connection capacity
- Seasonal changes in heat demand and heat-pump performance
- Production schedules and future changes in output
- Existing backup capacity
Temporary monitoring may be required where the site does not already record this information.
The most valuable opportunities arise where heat demand falls within expensive periods and there are low-cost periods beforehand in which the store can be charged, for example overnight before a daytime curing cycle, or across weekends and shift changes.
Controls determine whether storage delivers value
A buffer tank is not simply a vessel installed between the heat pump and the process.
The control system must decide:
- When the tank should be charged, based on price signals and production schedules
- What temperature it should reach
- When stored heat should be released
- When the heat pump should increase or reduce output
- How to respond to grid-balancing or flexibility instructions
- When backup heating is required
- How minimum process temperatures will be protected
- How the system will respond to a fault
Poor control can cause the heat pump and backup boiler to compete with each other, discharge the store too early or leave it empty when a high-price period begins.
A well-designed control strategy treats the heat pump, storage, backup heating, process and electricity price as one coordinated system. Where grid services are offered, the controls must also be able to receive dispatch signals and provide the metering and data the service requires.
Thermal storage in concrete-tile curing
At wienerberger's Sandtoft concrete roof-tile factory, five curing chambers had previously been heated by two 120 kW oil-fired boilers supplying hot water at temperatures of up to 80°C.
The existing hot-water system and controls were modified so that heat could alternatively be supplied by two 62 kW air-source heat pumps and two thermal buffer tanks.
The project required an understanding of the energy available from ambient air across different seasons and the changing thermal load of the curing process.
The buffer tanks formed part of the integration between heat generation and curing demand. They helped the system manage differences between when the heat pumps produced energy and when the chambers required it.
That same separation between heat generation and heat use is what allows a heat pump to respond to electricity prices and grid signals. Once heat can be stored, the question of when to generate it becomes a commercial decision as well as an operational one.
When thermal storage may not pay back
The case for storage is weaker when:
- Heat demand runs continuously, leaving no low-demand periods to charge the store
- The site's electricity tariff offers little difference between peak and off-peak prices
- The heat pump has no spare running time in which to charge the store
- The process requires a tightly controlled constant temperature
- The site lacks space for an adequately sized tank
- Stored temperatures are too low to remain useful
- Heat losses would be excessive
- The efficiency penalty of charging to higher temperatures outweighs the price saving
In these cases, the project may be better served by a direct heat-pump installation with a smaller buffer tank used only for hydraulic stability and controlled operation.
Thermal storage is one design option, not a universal answer.
Size the heat pump for the process and the store for the market
The heat pump should be sized to meet the sustained demand of the process. Thermal storage will not usually change that.
What storage can change is when the heat pump uses electricity. That can reduce energy costs, avoid peak network charges and open up income from grid-balancing and flexibility services.
The first priority is always to provide reliable process heat. Tariff and flexibility benefits should strengthen a sound design rather than rescue a weak one.
The most useful question is not simply:
How big should the heat pump be?
It is:
When does the process need heat, when is electricity cheapest and how much storage is needed to connect the two?
Heatcatcher assesses heat sources, process demands, storage opportunities, electricity tariffs and existing infrastructure to develop industrial energy efficiency projects that are designed around how factories actually operate.