Reducing air conditioner impacts: The state of solar cooling
It’s the holy grail of cooling–using the sun to power your cooling system. Mike Dennis from ANU takes us on a tour of the solar cooling market (as at 2015) and where it’s headed.
This article was first published in Issue 130 (January–March 2015) of Renew magazine.
In 2015, most Australian homes are now equipped with some kind of air conditioner, but their rise in popularity over the last decade has put substantial pressure on the electricity transmission and distribution network—and the required investment in ‘poles and wires’ has been blamed for recent spikes in retail electricity bills.
Some electricity retailers charge a premium for grid electricity drawn during afternoon periods when air conditioning may be in use, but offer a paltry sum in return for photovoltaic power supplied to the grid during the same period. One NSW retailer charges over 50c/kWh between 2pm and 8pm on weekdays while offering only 6c/kWh in return for net photovoltaic energy exported to the grid.
Water heating and air conditioning are usually the two main energy sinks in a residence. To some extent, water heating may be time-shifted to avoid exposure to peak tariffs, but air conditioning load offers less flexibility. What can be done about this?
The first consideration, of course, should be to try to reduce or eliminate the need for active air conditioning. A well-designed building with appropriate shading, insulation and thermal mass is a good start. Secondly, householders should explore opportunities for passive air conditioning using prevailing breezes and carefully designed cross and stack ventilation. ReNew readers will no doubt be well aware of this from previous articles (for example, see Alan Pears’s article ‘Guilt-free cooling’ in Renew 122 and ‘Design for a changing climate’ in Renew 130).
As a last resort, a householder may decide to install an electric air conditioning system. These devices are intoxicatingly effective in providing comfort with convenience and immediacy. The shopfloor price may not be as confronting as the first electricity bill, however! So, how can householders sidestep peak electricity charges and be comfortable in their homes at the same time?
Active air conditioning options to minimise environmental impact
The obvious option is to install a solar photovoltaic (PV) system to drive a regular air conditioner (see Figure 1). Several companies offer packages to do this directly, or it can simply mean installing a larger PV system to run the whole house (see ‘Solar cooling options available now’).
However, electrical supply will be required late in the afternoon and into the evening to offer proper service during summer cooling and winter heating periods.
It is worth noting that peak summer cooling loads often occur late in the afternoon, while peak solar is at noon. In winter, peak heating loads are in the evening.
Hence, backup will be required in the form of either a grid connection or a local electricity storage device. Both of these options come at a price. It is likely that future electricity storage prices will make this option viable. No doubt someone will produce a simple Sunulator-type calculator to size the PV collector and battery bank for such purposes. [Ed note: The battery component of the ATA Sunulator was released in November 2015. See Renew 134 (released mid Dec 2015) for details.]
A potential threat to this approach is the impending phase-out of the refrigerant fluids used in conventional air conditioners. Australia has committed to large reductions in the deployment of hydrofluorocarbon (HFC) refrigerants by 2030. Alternative refrigerants have been slow to appear in air conditioners to date, but it is likely they are on their way.
[Ed note: The environmentally benign (low GWP— global-warming potential —of 1) and energy- efficient CO2 natural refrigerant is starting to appear in hot water heat pumps (e.g. in Sanden units), but is yet to appear in domestic space heating/cooling heat pumps, possibly due to the higher pressures required. However, Pioneer has produced air conditioners and retrofit units using hydrocarbons, another natural refrigerant with a lower GWP than most other refrigerants (except CO2) and high efficiency. Keep an eye out for forthcoming developments in this area.]
An alternative is to use an air conditioner driven by solar heat rather than solar electricity. One incarnation of a heat-driven air conditioner is the sorption chiller (Figure 2). Sorption chillers come in two flavours: absorption and adsorption. Both of these technologies use environmentally friendly refrigerants.
Sorption machines take in solar heat and produce cold water that can be used to cool a house. They require a large solar collector, perhaps 20 m2 of a water heating collector, to cool a typical house in summer. Although the sorption chiller is not a reverse-cycle machine, the large solar heat collector is able to provide some degree of winter space heating and a high proportion of domestic hot water.
Absorption chillers require heat in a temperature range of 70–95°C from the solar collector. They are unable to operate above ambient temperatures of 35°C, unless a cooling tower is used. They are not maintenance-free and, moreover, the infrastructure required to support absorption systems renders them too expensive for residential application. Despite several decades of intensive research, the price and performance of absorption chillers in domestic applications remains unattractive.
Although adsorption and absorption cooling kits are available and technically feasible, capital cost and payback periods are expected to remain out of reach for some time to come (at least at a residential scale). Interested readers will find useful content in a presentation by Jacob1.
An alternative heat-driven solar cooling technology that has very low capital cost is based on the ejector principle (Figure 3, above). This technology is not yet commercially available and is being developed at the Australian National University 2 .In a similar manner to the sorption systems, a water heating collector supplies heat to the ejector chiller, which in turn produces cold water for distribution in a home. As well as having low capital cost, the ejector is maintenance-free and overcomes the operational difficulties of the sorption systems. Installation of the heat-driven chillers involves no on-site refrigeration work. Solar-assisted winter heating may be provided in reverse-cycle heat pump mode and solar hot water is a byproduct of the system’s operation. Trials of this technology are expected to run over the 2015/2016 summer.
A wildcard technology for reducing exposure to peak electricity prices is cold storage. By operating an air conditioner of any kind using off-peak electricity and storing the cooling effect in a cold store, the air conditioner may be switched off during periods of peak electricity pricing. A reasonably sized tank of chilled water would not hold sufficient capacity for domestic applications. A phase- change cold store is required. The cheapest and most effective of these would appear to be the freezing of water. A holding tank of 300L would suffice for a family home for four to six hours of cooling. The Australian National University is working on hydrate (warm ice) technology to allow water to be frozen and thawed at 7–12°C. These temperatures are easily reached by conventional air conditioning units, whereas 0°C is not usually achievable, particularly with solar cooling.
Solar desiccant cooling
Another option being explored by CSIRO is solar desiccant cooling. This approach operates by passing air through a desiccant wheel to dry the air and make it suitable for cooling via an evaporative cooling process (thus extending the range of locations where evaporative cooling can be used, as it usually isn’t effective in places with high humidity). Moisture deposited on the desiccant wheel is removed by hot air, which is heated using solar energy. By drying, or regenerating, the desiccant wheel, the dehumidification process can continue. CSIRO’s solar desiccant air conditioning technology uses heat from conventional solar hot water panels to create this hot air for regeneration. The system can provide space cooling, space heating and hot water. It is currently being trialled in Townsville in Queensland on several homes.
Heat pumps with solar assist
Readers may be aware of some imported heat pumps being sold as solar air conditioners. These devices belong to a class known as multi-function heat pumps. They are usually designed to provide solar-assisted winter heating and solar-assisted hot water. However, they do not provide solar-assisted summer cooling and it is misleading to promote these devices as solar air conditioners. Typically, they are sold with a very small solar collector that may or may not be mounted under the eaves in the shade. These devices meet the Minimum Energy Performance Standard (MEPS)3 as unassisted air-conditioners and do provide summer cooling, but not with the savings in electricity bills offered by proper solar cooling. I would like to be proved wrong by independent testing.
MEPS and solar cooling
Assessment for Minimum Energy Performance Standards (MEPS) is based on physical testing of a heat pump under controlled conditions. This standard, based on AS3812, mandates a minimum level of electrical efficiency and provides a Star rating for an air conditioner. Domestic split system or multi-head heat pumps that just comply with MEPS requirements will be awarded one or two Stars. Market-leading residential heat pump air conditioners reach 6 Stars. Solar air conditioners powered by PV panels or heat would rate at 10 to 15 Stars.
Although the current MEPS standard also applies to domestic solar air conditioners, a standard specific to solar air conditioners has recently been drafted. Australian Standard AS5389: ‘Solar cooling and heating systems—Calculation of energy consumption’ uses software modelling to predict annual energy consumption relative to a reference air conditioner. For now, this standard only includes desiccant systems, but could be extended to other solar air conditioning technologies. The standard is seen as a mechanism to support an incentive program in a similar manner to the STC program for solar water heaters.
Solar cooling potential?
Solar air conditioning is technically feasible but, as you have read, there are pros and cons to each of the various options. There are around 1200 systems installed worldwide, but these are mostly commercial-scale demonstration systems and mostly in Europe. There are likely to be a number of installations in China, but it is difficult to find information about them.
Europe looks to Australia to take the lead on solar air conditioning. Coastal Australia has a great solar resource, a mild climate and an affluent lifestyle widely comforted by air conditioners. Alas, we have but nine solar air conditioning systems, mainly commercial- scale sorption demonstration systems.
Local support for solar air conditioning has been provided by the Australian Institute for Refrigeration, Air conditioning and Heating (AIRAH). AIRAH operates a special technical group, and hosts an annual conference, dedicated to solar air conditioning4. Group members are quite open about sharing their experiences. A wealth of publicly available information on solar air conditioning, albeit with a Eurozone flavour, is also available through the International Energy Agency Solar Heating and Cooling Program, Tasks 25, 38, 44 and 48 5–9
Hopefully, as interest in solar cooling grows and R&D continues to explore new options, the gap between what is technically feasible and domestically viable will reduce substantially in the near future.
This article was first published in Issue 130 (January–March 2015) of Renew magazine. Renew 130 has ‘sustainable cooling’ as its focus.