Written by Eamonn Ryan

Today, with the rise of green energy sources like solar and wind, the need for industrial-scale energy storage is becoming ever more vital to make sure there’s power even after the sun sets or the breeze dies down.

As night falls over the city, at a gleaming Sandton office tower employees shut down their laptops, grab their mobiles and flood out of the building as the workday ends.

But in the basement, the action is just getting started. Using cheaper overnight electricity from the grid, a large refrigerator chills water mixed with glycol (a component of antifreeze) below the freezing point. The system then pumps the mixture into tubing coiled inside each of the tanks full of water. Hovering near zero, the glycol solution freezes the water, effectively storing energy in the form of ice.

The next day, when the building’s cooling capacity is about to be breached, the glycol mix flows out of the coils and into a closed-loop air-conditioning system. Combining with water and air, it helps to chill the building for hours during the day — when power is typically pricier.

Ice storage systems apart, the electric grid is changing, as concerns about climate change and renewable resources increase. Thermal energy storage is becoming more important to building owners and utilities for their ability to enable growth of renewable energy resources.

An Evapco thermal storage unit in a commercial building

An Evapco thermal storage unit in a commercial building. Image credit Evapco

Thermal energy storage:

  • Supports renewable energy. A thermal battery system can increase renewable energy use by up to 50% making renewable resources more effective and affordable1.  That helps with zero net energy design.
  • Supports high performance: To have lowest cost of operation, a building must be agile. A thermal battery system can store clean inexpensive energy, when available, to be used during periods of high demand to help lower the cost of operation.
  • Supports grid resiliency: A thermal battery system helps overcome the intermittency of renewable energy. It can store excess energy to be used during times when the sun does not shine or wind does not blow. Additionally, a recent study suggests that these systems do more to help the grid during heat storms than previously thought2. This is important for both grid resource adequacy planning and providing proper financial compensation which in turn means more incentives.

Harnessing thermal ice storage: a sustainable solution for energy efficiency

As energy demand surges at an alarming rate, individuals and organisations face mounting pressure to reduce their carbon footprint. A significant contributor to energy consumption is commercial buildings, accounting for 37% of total energy and over 68% of electricity usage in the US alone. This issue extends beyond borders, with buildings in China responsible for nearly 30% of the nation’s energy use and a quarter of its greenhouse gas emissions, according to the China 2007 Blue Book. The global impact is evident, necessitating solutions that address energy demands that unreliable renewable sources cannot meet, while preserving the environment and ensuring economic feasibility.

Fortunately, a practical and environmentally friendly solution exists: thermal ice storage. This proven technology has been around many years and efficiently stores frozen thermal energy for later use. By utilising electric power during off-peak hours when energy is abundant and less expensive, thermal storage systems create ice. This stored ice is then melted during peak daytime hours when electric energy demand is high and costly – supplementing the cooling systems of buildings.

A mine’s thermal storage structure, ultimately buried underground.

A mine’s thermal storage structure, ultimately buried underground. Image credit: Evapco


A thermal storage structure under construction.

A thermal storage structure under construction. Image credit: Evapco

To cater to various applications, three ice melt system designs are available:

  • Internal melt
  • External melt, and
  • External melt with air agitation.

Internal melts are suitable for single-building or small-scale applications, providing a 3°C supply of glycol. External melts offer a quiet melt process without air agitation and can achieve a 2°C to 3°C supply of glycol. External melts with air agitation are commonly used in larger systems or district cooling applications. This design can provide a 1°C supply of water and utilises power during the night.

Ice storage systems offer an energy-efficient cooling solution that aligns with fiscal responsibility and environmental commitments. By reducing demand on electrical infrastructure, ice storage eases the strain on utilities and decreases the cost of electrical and HVAC system components in buildings. Consequently, this aids utilities in avoiding the construction of new power plants, resulting in reduced carbon emissions.

Analogous to the cooling load in buildings, electric utilities’ demand experiences low points at night and peaks during the afternoon when people are present and energy consumption is high. Implementing ice storage can help flatten the utility’s demand profile, enhancing its efficiency with existing equipment. For every four buildings equipped with ice storage, utilities can serve five effectively.

Ice tank production prototype.

Ice tank production prototype. Image credit: Evapco

Significant cost savings

Beyond environmental benefits, ice storage systems generate substantial cost savings. By minimising the size of equipment components in air conditioning systems, the initial costs of equipment and installation are reduced. Smaller component sizes also result in lower electrical kW requirements, saving on ‘first cost’ of electrical distribution, wiring, starters, and transformers. When combined, these mechanical and electrical savings enable the implementation of a lowest-cost chilled water system as you start construction of your building.

Another benefit is a lower operational cost: power at night is much cheaper than during the day and when employed during the day can dramatically reduce the operating cost throughout the life of a building.

The efficiency of an ice air cooled chiller is also much higher because the nighttime ambient delta is at least 10-15°C. Chillers work better during the night as the typical utility most efficiently produces power in the middle of the night, which simultaneously translates to lower air emissions (GHGs).

Thermal ice storage is recognised for its sustainability by esteemed organisations such as the US Green Building Council, ASHRAE, and the International District Energy Association. The US Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system grants credits for thermal ice storage in energy and atmosphere categories. ASHRAE’s Green Guide recommends thermal energy storage and district energy plants to reduce energy consumption, lower greenhouse gas emissions, and enhance system efficiency.

Source: Evapco promotional film on thermal storage.

History and background of thermal storage in the HVAC industry

Andre van der Merwe, Evapco managing director, explains that the concept of thermal storage has its roots in the 1930’s dairy farming industry. “Farmers faced the challenge of cooling large quantities of milk quickly during specific periods while avoiding the installation of oversized refrigeration systems that would remain idle for most of the day. To overcome this, they developed a smaller cooling system that operated during the day to manufacture ice and stored it for later use when the cooling demand peaked.”

The dairy industry, including farms and manufacturing plants, benefits greatly from thermal storage systems. Cooling milk is a critical process to prevent spoilage, and thermal storage provides a batch processing solution that effectively manages the pasteurisation and cooling stages. From the initial stages at the farm with the cows to the storage and packaging processes, thermal storage plays a pivotal role in maintaining the quality and safety of dairy products.

Over time, thermal storage technology expanded beyond its dairy farming origins and found applications in commercial HVAC applications. The ability to shift power usage from peak to off-peak hours became an attractive proposition for utilities around the world seeking to optimise their operations – though this has not occurred in South Africa. In countries with time-of-use tariffs, thermal storage systems allowed building owners to take advantage of lower electricity rates during off-peak periods, resulting in cost savings.

Sufficient incentives are currently not available in South Africa, but Van der Merwe notes that it has nonetheless found resonance locally for ‘cooling back up systems’ in the wake of loadshedding. What limits the market from exponential growth in this regard is uncertainty as to how bad loadshedding will get. There is no point in installing a thermal storage system which produces two hours’ back up if the power is off for four hours.

Thermal storage system schematic.

Thermal storage system schematic. Image credit: Evapco

It would be the wrong solution in that case, he explains.

“Internationally, thermal ice storage primarily focuses on cooling applications rather than heating. The technology involves creating and storing ice to meet cooling demands during peak periods. Municipalities play a crucial role in determining energy rates, including peak demand tariffs and time-of-use benefits. The availability of such incentives can influence the adoption of thermal storage technology in different regions.”

Van der Merwe sums up the attraction of thermal storage technology as enabling any business not to install their maximum capacity, but only 60% of peak chiller demand and supplement it on demand with stored cooling according to the 60/40 rule. “Whether a bank or a shopping mall or any commercial building, peak demand typically occurs at lunchtime (bank) or mid-afternoon (office) for an hour or two.”

That is when stored cooling is required. They run their chiller all day, and when it runs out of capacity, the storage kicks in at low-cost rather than premium-cost rates. Then the system has the whole night to recharge for peak the next day.”

It is not a separate system – the chiller that is in any case there, creates ice and stores it like in a battery for the next day. Chillers can be retrofitted to accomplish this, he explains.

“Until a few years ago – with the emergence of loadshedding – people never saw thermal storage as ‘backup.’ That concept gained traction as South African industries requiring continuous refrigeration for critical cooling – such as theatres, hospitals, clinics and data centres – sought alternatives to expensive diesel generators or less reliable renewable energy plants. These establishments often experience significant fluctuations in their heating and cooling demands. For instance, office buildings may have varying occupancy levels throughout the week, while theatres and cinemas might see a surge in visitors during specific hours or days. Thermal storage provides an effective solution by allowing excess thermal capacity to be built during off-peak periods and released quickly when required.”

This led to a shift in perception, with clients exploring the feasibility of integrating thermal storage systems as backup cooling during power outages. “We are now getting many enquiries for this usage as opposed to taking advantage of time-of-use tariffs.

“Compared to alternative solutions like solar and diesel generators, thermal storage presents an economical option. Although no system is 100% tailored for universal efficiency, building ice as a form of energy storage provides a high-efficiency return. Moreover, the design of the thermal storage system plays a crucial role in optimising its cost and operational effectiveness. By supplying cooler glycol at a lower temperature range, smaller piping, air handling units, and electrical components can be used, resulting in reduced first costs and operational expenses.”

Van der Merwe explains that in the realm of refrigerated distribution centers, thermal storage has found its niche in South Africa. “These centres are vital for ensuring the freshness and preservation of perishable goods, and thermal storage technology enhances their efficiency. This demonstrates the capability of thermal storage in large-scale commercial applications, further highlighting its potential benefits for the South African market.”

How thermal storage beats traditional cooling

With thermal storage, businesses can lower cooling costs dramatically and reduce environmental impact. There are also at least four built-in advantages over traditional air-conditioning: flexible control, stored emergency cooling or redundancy, resiliency, and future proofing for smart grid preparedness.

With flexible control you can decide to use energy when it is most cost effective.

Thermal storage offers six modes of operation for various times of the day or year. During on-peak daytime hours, occupants and buildings may be kept cool and comfortable by the cooling stored within Calmac IceBank tanks. Occupants can be:

  • Cooling with the chiller only.
  • Cooling with ice storage only.
  • Cooling with chiller and ice.

Charge and store ice with cheap night-time energy for daytime use. These factors all depend upon electrical rates, demand charges and demand response events. During night-time off-peak hours chillers can charge and store ice inside the energy storage tanks. During spring and fall when temperatures are milder, and cooling demands are lower, the ice can exclusively cool the facility.

Plan ahead with emergency cooling: With energy storage, you always have extra cooling on standby. During chiller maintenance, or for a quick several-hours cool down for your building, ice storage is available to meet the need.

Make a building resilient: In a power outage, people in a building can keep cool, productive, and comfortable. A backup generator can run controls, pumps and fans while staff evaluate the situation. With stored cooling, there’s no need for oversize backup generators to run chillers—you can get by without.

Airco national sales manager Wayne Muller lists the points to be taken into account when considering thermal storage for a building: “The size of the building’s energy consumption – generally for a building over 250kw – determines whether one can consider thermal storage. The hours of operation are important as you need time to build ice at night when electricity is cheaper, as well as issues of peak cooling in summer versus winter conditions, location, the current plant and HVAC infrastructure.

“As an example, you can consider a 1 000kw plant, chilled water in an office park application, nine hours of operation using a 1 000kw chiller drawing 400kw (air-cooled chiller). We can consider seven  Calmac tanks 500kw/hours of storage per tank for a total of 3 500kw stored energy, which would result in a reduction with only 550kw of cooling required from the chiller (half of the chiller size required) with partial storage of seven Calmac tanks,” explains Muller.

He adds that return on investment (ROI) is the most important element: noting that at the current electrical tariffs the ROI in the case of Calmac thermal storage is around four to six years, “which is very quick considering the electrical savings for the client and convenience of having cooling during load shedding”.

He explains that the essential element of the Calmac Ice Bank System is a modular, insulated, polyethylene tank containing a spiral wound plastic tube heat exchanger surrounded with water. The tank is available in many sizes ranging from 160 to 1 760kW-hours. At night, water containing 25% ethylene glycol is cooled by a chiller and is circulated through the heat exchanger, extracting heat until eventually 95% of the water in the tank is frozen solid. The ice is built uniformly throughout the tank by the patented temperature averaging effect of closely spaced counter-flow heat exchanger tubes. Water does not become surrounded by ice during the freezing process and can move freely as ice forms, preventing damage to the tank. Calmac tanks are virtually maintenance free and have no moving parts, resulting in extended life span of the tanks.

“Thermal storage assists clients with large reductions in the capital cost of chillers – so smaller chillers may suffice. Other benefits include: lower maintenance costs with smaller chillers and tanks; large reductions in running costs; longer life span on thermal storage plants. Thermal storage allows clients to build thermal energy at night when electricity is 25-50% of the cost the next day, thereby reducing electrical loads during peak times. By having additional cooling in winter there is further savings during winter, autumn, and spring months,” says Muller.

How do you determine if thermal storage is right for a building?

Ngonidzashe Kundhlande, development engineer at Baltimore Aircoil Company SA, says: “Traditionally, ice thermal storage has been considered as a way to shave the peak load off a building and shift the load to lower load periods of the day and periods where off-peak electricity rates would apply. So, in essence ice thermal storage reduces energy costs, minimises peak electrical demand, and lowers system costs. By producing low process fluid temperature during off-peak times, this environmentally friendly cooling solution reduces energy consumption and greenhouse gas emissions.

Image credit: Evapco

“South Africa finds herself with the low energy security situation which will most likely persist for the foreseeable future, and it is incumbent on everyone to do their bit in reducing energy consumption and find viable alternatives to how things have traditionally been done. We, at Baltimore Aircoil Company, believe that the ice thermal storage provides a unique opportunity to contribute to this as all the benefits mentioned above would apply and should be considered as one of the possible solutions,” says Kundhlande.

He notes that the largest portion of a building’s energy consumption goes to cooling, and ice storage will not only help by reducing peak energy demand, thereby reducing pressure on the grid, but also provide the much-needed cooling during loadshedding periods at a much lower cost than what is being seen at present.

“Currently, back-up systems (diesel generators) need to be sized to provide enough energy to power the whole cooling system, whereas ice thermal storage affords you the opportunity to build and store ice during the ‘grid up-time’ and then melt that ice during a loadshedding period. During the melt cycle, the energy users on the cooling system could be the chilled water pumps and AHUs with the biggest energy user, the chiller, being off completely.”

By considering ice thermal storage one could alter the energy usage profile of a building in a way that profoundly affects the energy costs, impact on South Africa’s fragile grid, and impact on the environment.


  1. ASHRAE Research Paper-1607: Design and Utilization of Thermal Energy Storage to Increase the Ability of Power Systems to Support Renewable Energy Resources, January 2018.
  2. Western Cooling Efficiency Center at University of California Davis
  3. Source: Calmac.com https://archive.nytimes.com/www.nytimes.com/2017/06/03/business/energy-environment/biggest-batteries.html
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