Solar air complementing Australian’s photovoltaic boom

A black SolarWall transpired solar air heating system at Toronto's Pearson International Airport, image courtesy Conserval Engineering Inc
A black SolarWall transpired solar air heating system at Toronto's Pearson International Airport, image courtesy Conserval Engineering Inc

By Paul Grad, engineering writer

Australian energy consumers may have their sites set on photovoltaics, however, experts are presenting compelling reasons why solar air technology should be brought into the mix.

Solar heating is one of the most cost-effective ways for consumers to mitigate CO2 emissions and to heat homes.

But, Australia is missing out on the advantages of solar air heating, according to visiting solar expert Wes Johnston, vice president of the Canadian Solar Industries Association (CANSIA).

While photovoltaics (PV) have been a keen area of interest of the Australian energy sector, solar air heating has not been considered, or developed, to the extent it has been in North America, Europe, India and China.

Speaking at the All-Energy Australia 2013 conference in October, Mr Johnston was quick to point out, his role is not to convert Aussies to solar air technology. Rather it’s to inform industry and consumers and present compelling reasons why solar PV used in conjunction with solar air is a better mix than solar PV alone.

So what is solar air technology? Solar air heating uses solar energy directly to heat or condition air for buildings or process heat. It addresses the largest use of building energy in countries where heating is required, and can therefore drastically reduce the building’s electricity bills. In agricultural buildings, solar air technology can be used to dry crops, to eliminate humidity in poultry barns and maintain high temperature.

Many companies around the world are marketing solar air heating systems. Among the most widely used technologies are the unglazed transpired solar collector, the glazed solar collector and the “pop can” collector.

The unglazed transpired solar collector consists mainly of a metal plate of dark colour, with small holes spaced uniformly across the entire plate. The plate is mounted next to a wall of the building to be heated. The sun warms the metal plate and the plate heats the air in close contact with it.

Solar heat conducts from the surface to the thermal boundary layer of air about 1mm thick next to the plate. Ventilation fans at the top of the space between the plate and the building wall create a negative pressure, and the warmed air is sucked through the plate’s holes. The boundary layer of air is drawn into a nearby hole before the heat can escape by convection, eliminating heat loss off the surface of the plate. A thermostat in the building space to be heated controls the speed of the ventilation fan(s) and the warm air is then ducted through the building.

Unglazed systems are all-metal and are typically used to heat high volumes of ventilation or makeup air up to 38°C above ambient temperature.

The unglazed transpired collector was invented by John Hollick, chief executive officer and a founding partner of Conserval Engineering of Toronto, Canada. It was developed and refined in co-operation with the US’ National Renewable Energy Laboratory (NREL), of Colorado. The company is marketing the system as SolarWall.

Glazed systems have some type of glass or polycarbonate over the metal solar collector. This allows the air to be heated to a higher temperature up to 55°C above ambient. Glazed systems protect the collector from losses due to wind, but also entail disadvantages such as higher cost and some heat loss due to radiation.

It is also possible to install a second stage glazed section over the unglazed plates to allow the air to be heated twice by perforated collectors, improving the solar efficiency. After the air passes through a standard transpired collector (first stage), it rises to a narrow cavity between a glazing and a second perforated collector where it is heated again. After the second heating stage, the air flows to the inlet of a fan or HVAC unit as with a single stage transpired solar heater.

In case the plates are roof mounted, winds – which are stronger on the roof than on walls – may reduce the performance of the unglazed design. In a two-stage system the air passes twice through perforated plates, which doubles its flow rate, minimising wind effects and increasing the heat gain.

There is also increasing use of hybrid solar photovoltaic/thermal technology, a combination that generates up to four times the energy from the same surface area. The SolarWall PV/T hybrid system is said by Conserval Engineering to provide up to 300 per cent more energy (solar electricity plus solar heat) than a conventional solar PV system. The PV electrical output is 100W/sq m, the SolarWall thermal output is 200 to 300W/sq m and the hybrid system’s output is 300 to 400W/sq m.

One version of the pop can collector uses columns of aluminium soda pop cans with the ends cut out. The sun heats the cans and air flowing through the inside of the columns is heated and delivers the heat to a room.

According to the International Energy Agency, more than 1.7 million sq m of solar air collectors have been installed worldwide for both residential and commercial applications, with Switzerland, Japan and Canada leading the total installation capacity.

In all cases, it is crucial to take careful consideration of the way the sun moves, for example, the seasonal variations in the sun’s path throughout the day, which is unique for any given latitude. A fairly constant amount of solar radiation strikes the atmosphere, about 1350W/sq m. A lot of that radiation is lost in the atmosphere by absorption and reflection.

In latitudes farther than 23.5 degrees from the equator, the sun will reach its highest point toward the south in the northern hemisphere and toward the north in the southern hemisphere.

In the northern hemisphere, as winter solstice approaches, the angle at which the sun rises and sets moves further toward the south and the daylight hours become shorter. In summer the sun rises and sets further toward the north and the daylight hours become longer. The converse happens in the southern hemisphere.

In both hemispheres, however, the sun rises to the east and sets toward the west. Therefore, a solar air heating system should usually be installed in the south wall of a building in the northern hemisphere, and in the north wall in the southern hemisphere.

Whichever technology is used, solar air heating targets the largest usage of energy in the building sector – and corresponding CO2 emissions – at the lowest capital cost of any renewable energy option, according to the Solar Air Heating World Industries Association (SAHWIA), with offices in Canada and Spain.

The vice president operations with Conserval Engineering, Victoria Hollick, said a study by the C D Howe Institute – a Canadian not-for-profit think-tank that fosters economically sound public policies – has identified the lowest-cost and highest-value renewable energy programs include solar air heating, solar water heating, solar electricity, wind and biomass. According to the study, the cost to the government (in Canadian dollars) of displacing one ton of CO2 ranges from $4/ton for solar air heating, to $30/ton for biomass. The most expensive programs are for liquid fuels. It costs the government $295 to $430/ton of CO2 displaced from ethanol and $122 to $175/ton of CO2 displaced from biodiesel.

The study clearly indicates the most cost-effective form of CO2 abatement is from unglazed solar air heating technologies.

Proper installation and application of solar air heating systems requires considerable expertise, however, if proper advantage is to be taken from those systems.

A software system called RETScreen – a clean energy decision-making software – is provided free-of-charge by the government of Canada, aiming to reduce the costs associated with identifying and assessing potential energy projects. RETScreen software has nearly 400,000 users in more than 200 countries.

In the international Solar Heating and Cooling Conference in Freiburg, Germany, on September 24, the president of the European Technology Platform on Renewable Heating & Cooling, and Coordinator Smart Energy Cities of the Fraunhofer Institute for Solar Energy Systems ISE, Gerhard Stryi-Hipp delivered a talk on the future of solar thermal energy in buildings. He said the European Technology Platform envisages that by 2030, about 50 per cent of the low temperature needs (up to 250°C) will be provided by solar thermal. According to European policy, buildings will be nearly zero-energy buildings by 2020.

For solar thermal energy to compete successfully with other forms of renewable energy, Mr Stryi-Hipp said it will be necessary to reduce the installation costs, reduce system complexity and exercise transparency on energy yield. Simplification will help reduce installation failures and costs and increase reliability. Transparency – informing the customer about the expected solar energy yield and the energy saved is another essential factor.

In Australia, the Solar Thermal Air Association (STA), incorporated in Victoria, was formed in 2008 with the objective of promoting the benefits of utilising solar energy to assist space heating and cooling in residential and commercial dwellings.

STA member companies, offering solar thermal systems include: Smart Roof Australia (Smartbreeze), of Melbourne; Sola-Mate, of Melbourne; Solectair, of Perth; Ventis HQ, of Sydney; and Global Eco and Environmental Solutions (Ges), of Melbourne, also trading as Heatwithsolar.

The Australian utilities have mixed feelings about the “disruptive technologies” that have been introduced by the increasing use of renewable energy. The chief executive of Energy Networks Association John Bradley said those technologies create difficult issues in planning and management for the utilities. He said the utilities have to do a difficult balancing act involving stability and reliability of supply and fairness to the customers.

The utilities’ ambivalence regarding renewable energy has been highlighted by a recent report from Greenpeace, titled Strangling renewables: Orgin Energy’s campaign against renewable energy. The report says Origin Energy, retailer to 4.3 million Australian customers, has invested massively in gas extraction, generation, and export.

However, the report says, “Once a great supporter of the transition to a renewable energy powered future, Origin has changed its tune and has been one of the companies doing the most to undermine renewable energy investment in Australia.” The report says this is because Origin saw its investments threatened by the emergence of cheaper, cleaner wind and solar.

The Small-scale Renewable Energy Scheme of the Clean Energy Regulator creates a financial incentive for owners to install eligible small-scale installations such as solar water heaters, heat pumps, small-scale wind systems, or small-scale hydro systems. Solar air heating is not currently an eligible installation type, under the scheme, however.

While Australia has been a leading force in the technology and application of renewable energy sources such as photovoltaics, solar thermal, wind, ocean wave and geothermal, it has neglected solar air heating, which has been an increasingly significant part of the energy mix in much of the rest of the world. It will be necessary for the various stakeholders – the users, the utilities and related businesses, to find enough common ground to foster the use of solar air heating in Australia.