The CSIRO has tested a solar-powered air-conditioning system which so far looks like a victory in the war against having to pay so much to keep cool.
Air-conditioning makes us all comfortable, sure, but it’s a luxurious drain on power bills. More than 20% of Australia’s electricity generation is used on air-conditioning and refrigeration, but on a hot summer’s day that can shoot up to 50%.
Large commercial spaces such as shopping centres and hotels have high energy consumption due to their heating and cooling energy requirements. It is estimated that 50-60% of total energy consumption in commercial buildings is for heating ventilation and air conditioning needs.
“It’s a big chunk, and if you can do it using solar heat instead of grid electricity you save on greenhouse gas emissions and reduce the stress on the grid,” says CSIRO research director of grids and energy efficiency systems Stephen White.
That day has arrived. The CSIRO has developed and prototyped a solar air-conditioning technology designed to provide high-efficiency, low-carbon heating and cooling for commercial buildings, specifically to operate using high temperatures. This increases the amount of cooling that can be achieved from the collected solar heat. Conversely, savings can be achieved through utilisation of a smaller PV module field to provide the same cooling. Collector fields are typically a major component of the overall project costs.
“It’s three times more efficient use of electricity,” says White, and electricity consumption is reduced by two-thirds.
The project is a world-first demonstration of the viability of a two-stage desiccant air-conditioning system that utilises high-temperature closed loop regeneration for improving the thermal efficiency.
“It’s a two-step process: dry out the air, and that makes it really effective when you put it through an evaporative air-conditioner,” says White. “Heat is doing the bulk of the job, but you still need a little bit of electricity to drive the fans to push the air through the desiccant wheel.”
The CSIRO system uses an indirect evaporative cooler which relies on a heat exchanger, so the air that is cooled down in the evaporative process hasn’t been humidified, White says. “You get dry, cool air coming in.”
Commercial partner Stockland Group hosted a demonstration of the system at its Wendouree shopping centre in Ballarat, Victoria. A solar collector field and heat storage system was installed by NEP Solar incorporating solar heat collection with parabolic trough collectors, and heat storage in a thermal oil tank.
HOW IT WORKS
The design consists of two desiccant wheels to provide maximum dehumidification of the process air. High-temperature air available from the parabolic trough collectors dries air in a high-temperature wheel, and the heat rejected from that is used to drive a low-temperature desiccant wheel.
“It’s two wheels with only one lot of heat added, which cascades down from the high temperature to the low temperature,” White says. “That means you get two bites at the cherry with the one source of heat, so you get much higher efficiency.”
In short, the system can dehumidify more air with the same amount of heat, although White concedes “we could go for extra dryness if we wanted to.”
The unit is expected to provide higher thermal coefficient of performance compared with a single stage desiccant air-conditioning system.
This enables a smaller solar field to be used for the same cooling capacity, or alternatively, more cooling can be provided from a given size field.
Heat delivered by the collectors is stored in a 2,000L thermal oil tank. Collected solar heat is used for providing part of both the air conditioning and space heating needs of the shopping centre.
By mid-June, data showed the collectors at the Wendouree shopping centre had been providing more than 1MWh of heat every month. During winter months, more than 50% of heat delivered from the collectors has been used for space heating.
WHEELS ARE TURNING
The two-stage desiccant system used high temperature heat at 150°C to deliver cooling to the facility. The system delivers cooling using the desiccant wheels if the process relative humidity is above 40%. Otherwise, the process air goes through only the indirect evaporative cooler.
The system delivered an electrical coefficient of 7 in desiccant mode and 10 in indirect evaporative cooler mode, “which means we get 10 units of cold for every one unit of electricity we consume,” White says. The technology has significant potential to improve the efficiency of solar heat usage in commercial buildings and reduced the required solar collector area. “But the fact you’re running at higher temperature means you need higher temperature solar collectors,” White says. “It’s not entirely a free lunch there.”
Development of a high-efficiency solar desiccant air-conditioning system is expected to overcome the low efficiency of conventional single-wheel systems, the CSIRO hopes. The installed collectors at the Ballarat site delivered more than 1MWh of solar heat a month. The system was used for winter heating last year and the air-conditioning component was tested during the recent summer. The desiccant air-conditioning system delivered 10 to 12kW of cooling when operating in the desiccant cooling mode and 5 to 6kW of cooling when operating in the indirect evaporative cooling mode.
Due to the intermittent nature of the solar resource, paying careful attention to the design of the solar heat delivery system is important.
If there is no sun, and no heat to drive the desiccant process, air can still be put through the indirect evaporative cooler (IEC). “You may not get full capacity out of the system but you can still get some cooling,” White says. “It works at least in partial air-conditioning mode 24 hours a day, 365 days a year. You mitigate some of the issues around the intermittentcy of the sun just by switching into this IEC mode. It actually operates more efficiently in that IEC mode; you just don’t get as much cooling because you haven’t got dry air coming in, you’ve just got normal ambient air.”
The CSIRO says the system is also suitable for industries that have waste heat or steam available for desiccant regeneration. Systems in the range of 50-100kW of cooling capacity would be likely contenders.
The demonstration prototype at Stockland Wendouree can be scaled to higher capacities, the agency says, and representatives from Echuca regional health visited the Stockland site in March to understand the operation of the technology.
The CSIRO says it is open to work with commercial partners towards scaling up the prototype and working with a commercial partner regarding licensing and other technology transfer options.
The Australian Renewable Energy Agency provided $520,000 to support the $1.2 million project and the CSIRO will continue to assess and monitor the technology for the next 12 months to establish long-term commercial operations and further investment.
“CSIRO’s energy research is driving down costs of renewable technologies, accelerating the transition to a lower-emissions future,” says CSIRO energy director Peter Mayfield. “We are pioneering new technologies and this project is a world-first demonstration.”