By Ntsako Khosa
As costs increase and uncertainty regarding sustainable power provision remains, thermal ice storage is considered one of the viable solutions to generating cooling power.
Thermal ice storage provides many environment-friendly opportunities that are a result of reduced peak electrical demand. Andre van der Merwe, managing director at Evapco says, “Anyone who has a desire to be environmentally responsible and save energy would find using a thermal storage system beneficial.”
WHAT IS THERMAL STORAGE?
“Thermal storage as it pertains to HVAC is a way to store energy in various forms to be used later,” says Wayne Muller, national sales manager at Airco. Garid Glenn, head of marketing and refrigeration sales at Baltimore Aircoil Company (BAC) adds that a thermal storage system creates ice to build up and store cooling when cooling demand and/or energy rates are low and the system then uses the ice for air conditioning or process cooling purposes when energy rates are high, for instance, during the day. According to Evapco, this then reduces overall kilowatt demand and energy usage.
“Thermal ice storage has been around since the 1930s,” says Andre van der Merwe. “Ice thermal storage has been used for airconditioning and process cooling for over 60 years. In the early years of air-conditioning thermal energy storage was used for high diversity loads such as churches and theatres to reduce the cost of equipment,” says Bill Bartley, Evapco president and CEO. Christo van der Merwe from Marine & Refrigeration Engineering (MRE) notes that thermal storage has been used as:
- a means to shift a cooling requirement away from peak electrical tariff periods to times of lower demand, and
- a means of reducing the peak cooling load on a process plant that has peak loads caused by pasteurisation (hot processes) or cooling demand from receiving heated products.
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DIFFERENT FORMS OF ICE STORAGE Step 1: During night-time and off-peak hours, water that contains 25% ethylene or propylene glycol is cooled by a chiller. That solution circulates inside the heat exchanger within the Ice Bank tank, freezing 95% of the water that surrounds the heat exchanger inside the tank. The water surrounding the heat exchanger never leaves the tank. Step 2: Ice is created uniformly inside the Ice Bank tank via counter-flow-heat exchanger tubes. As ice forms, water still moves freely which prevents damage to the tank. To fully charge an Ice Bank tank usually takes from six to 12 hours. Step 3: During the daytime, in peak hours, the glycol solution circulates through the ice storage tanks and then through cooling coils in air handling units. Step 4: Fans blow air over the coils to deliver cooling to the occupant spaces. People feel cool and comfortable and never know ice storage is being used to save money on cooling costs. Source: Calmac Manufacturing Corporation |
THE SYSTEM
Equipment that makes up a thermal storage system “is basically a chilled water system with chillers, air handling units (AHUs) or fan coil units, circulation pumps, piping, valves and controls.
The Calmac ice tanks are added to the system almost like an additional chiller as they increase cooling availability through a three-way valve, which controls the system,” says Muller.
Thermal storage systems can be installed in new construction and retrofits (replacement and / or expansion of systems or improvement to existing systems). Some of the applications include manufacturing / process cooling, office buildings, retail (for example banks, malls), schools, hospitals, district cooling plants, stadiums, hotels, emergency cooling, breweries, meat processing, wet-air pre-cooling for storage of fruits and vegetables and cooling for mines. “Although this is a design decision ultimately it is an investment with return on investment pay back, so the additional cost of the ice system will be directly compared to the savings to generate years of return. With a lifespan between 20 and 30 years – with no moving parts and on most buildings, this is a great investment into running costs for the company,” says Muller. Andre adds that such a lifespan is determined by a quality design and well-maintained system. Additionally, “regular maintenance inspections of coil and casing conditions and ensuring the water quality is within specification will add to a preserved system,” says Glenn.
Most installations are unique from client to client, mainlydepending on their needs. Andre states, “depending on the application a thermal storage design strategy can be a ‘full storage’ or a ‘partial storage’. Calmac explains this further, “full storage, fully shifts all of a building’s cooling load to off peak hours. While initially more expensive to install, full storage normally offers the greatest savings over time. Partial storage employs a chiller which is approximately half the size of a conventional installation. During the night it stores cooling; during the following day, that stored cooling helps meet a portion of the building’s load while the downsized chiller handles the rest. Partial storage is often chosen for many new projects because it is very cost-effective”.
Christo shares some considerations for opting for such a system in office air conditioning and process factories. “For office air conditioning, if you have to handle the peak heat load that can occur then the plant room equipment and power supply has to be sized for the peak load that occurs only for around four to five hours a day. If a thermal storage system is installed, then the peak heat load is reduced, and the plant room equipment can be smaller as it works harder for most of the day. In process factories peak loads occur when either warm product is received and has to be chilled quickly or if a pasteurisation process needs maximum cooling to cool the product after it has been warmed up. If there is no thermal storage system, the plant has to be designed to handle the peak heat loads which would only occur at certain times during the day. The thermal storage system reduces the peak load and ensures that the plant works at a consistent but reduced level throughout the day. The thermal storage system can allow the plant to switch off during peak electrical tariff periods thereby saving on electrical costs. But the equipment has to be increased in capacity so that it can deliver the same capacity in a reduced time and still reduce the peak heat load.”
Barry Watts, team leader at Johnson Controls highlights, “any application can find a thermal ice storage system beneficial, however, its installation is dependent on space and the area a client has for the tanks.”
HOW A THERMAL STORAGE SYSTEM WORKSThe TSU-C/D is an external melt system that produces and builds ice (1) around a coil (2) submerged in water (3). A refrigerant or cold glycol (4) circulates through this, while ice accumulates in the outside. The ice is melted by circulating warm water (5) from the load over the coil, which cools the water (6). Low pressure air (7) from an air pump (8) is distributed below the coil for water agitation.
The TSU-M is an internal melt system that produces and builds ice (1) around a coil (2) submerged in water (3), most of which freezes within the tank. The warm glycol (4) from the load circulates through the coil and melts the ice from the inside. The newly cooled glycol (5) is then pumped through the building cooling system or used to cool a secondary refrigerant that does the same. Internal melt is ideal for air-conditioning involving cooling at higher temperatures than external melt. Source: Baltimore Aircoil Company (BAC) |
LOAD SHEDDING AND THERMAL STORAGE
As reports suggests the country will experience load shedding over the next few months, is thermal storage one way of ensuring survival during the blackouts? “If the system is designed correctly to have ice available whenever required, then it is definitely a viable solution for cooling during load-shedding,” says Glenn. Andre explains, “The only standby generating energy needed will be to run some pumps as well as the system air handling units as opposed to installing full plant energy requirement for chillers, etc.”
The ice found in the coils that is melted during the day.
Image credit: Evapco
Christo Vermeulen from Intramech says, “An interruption in indoor climate control can cause HVAC havoc – sweltering offices, spoiled produce, a decline in productivity.” How an ice thermal storage system would work during load shedding is; “During Stage 4 load shedding conditions the ice will keep the chilled water at the design temperature of 6°C instead of the chillers, without using any power to produce cooling. The only power required will be to run the chilled water pumps and air handling equipment fans. However, the pumps and fans use a fraction of the chiller power requirement and can therefore comfortably be supplied by a renewable source such as solar panels.”
The size of a thermal storage system can start from
200 tons generating about 700kW. There is no limit to the size.
Image credit: Evapco
Vermeulen says that if it is designed for such at the time of construction, then, yes it could provide a good solution. However, “if it is designed for Stage 2 and then, for a period, stage 4 becomes common occurrence, the cooling will be compromised so it has be designed for a worst-case scenario. In theory, load shedding is with us for another year so to invest additional capital for such a short-term benefit would not be practical.” Muller says that a thermal storage design shouldn’t be focused on load shedding, “the key is return on investment with regards to energy saving.”
