Solar air heating: is it worth it?

By Richard Keech

2018-08-14

Do solar air heaters or coolers live up to their promise?

At face value the idea of having a device which uses sunlight directly to heat air in winter seems like a good one.  It’s the idea behind a number of products on the market that promise really high-efficiency heating for low cost. We can lump these together in the category ‘solar air heating’.  Solar air heaters sometimes claim thermal efficiencies greatly in excess of that possible with a solar PV panel.

Examples of solar air heating systems are:

These systems have in common the use of a simple flat-panel glazed roof-mounted solar-thermal collector which warms incoming air for distribution in the home.  Some of these are displacement systems (displacement systems are explained here), others allow recirculation, and some can be configured for either displacement or recirculation.

Operation of solar-thermal collectors

Glazed flat-panel solar-thermal collectors are a versatile and well-understood technology.  They can be used to heat air or a liquid and have a long history in hot-water systems. The key to understanding their operation is the idea that their efficiency decreases when ambient temperature drops (all other things being equal).  Efficiency is a function of the difference between the inlet temperature (Ti, the temperature of the fluid entering the collector), and ambient air temperature (Ta).  This difference, Ti-Ta, is obviously positive if ambient air temperature is less than inlet temperature.

Flagstaff-4 - lo res
These solar-thermal collectors heat water but operate on the same principal as solar air heaters

 

The chart below shows three difference scenarios, A, B and C.  At ‘A’ the input air and the ambient air are the same temperature.  The efficiency here is sometimes cited as the reference panel efficiency, in this hypothetical example 85%.  Whereas in case ‘C’ the inlet temperature is 20 degrees higher than ambient, and efficiency is much lower – in this example only 45%.

solar-thermal-efficiency
Efficiency drops when it’s cold outside

Solar air heating scenarios

Case A – displacement

As applied to solar air heating, Case ‘A’ on the chart above might correspond to drawing air from outside (ie Ti = Ta) on a cold winter’s morning, and heating it before passing to inside. This scenario represents displacement operation (rather than recirculation).  This gives higher notional efficiency. However the air being heated is starting from a low temperature, so the result isn’t necessarily very hot air.

The other implication of this scenario is that inside air has to be displaced to make way for the air warmed by the collector.  So if it’s colder outside than inside, then that displacement is a non-trivial loss of heat energy.

Result. If a solar air heater is operating such that it displaces inside air, then the direct benefit of the heated air is diminished by the loss of inside air that is warmer than outside air.

Case C – recirculation

So imagine we don’t want the heat losses associated with Case A, and we have a slightly more complex configuration of our solar air heater such that it draws in air from within the home.  This is Case ‘C’, which has the air going into the collector being recirculated from inside and is (for arguments sake) 20C warmer than ambient on our very cold winter morning.  So Ti-Ta = 20.

Result. In this case we avoid the displacement heat losses, but the collector efficiency is much less because of the losses within the collector itself.

Temperature vs thermal power

High temperatures don’t necessarily mean high thermal power. One common justification for solar air heaters is the observation that the air in your ceiling warms up quickly when its sunny (or some similar observation of solar heating). The problem with this is that situations which can give large rises in temperature do not necessarily correspond to the capacity to deliver lots of heat energy.

All practical energy systems experience ‘load regulation’, which means the reduction in potential as the load is increased.  A simple example is an electric battery that may have an initial (unloaded) potential of, say, 12 volts. However, when the battery is called upon to provide power, the potential (ie voltage) will drop.  The drop is usually in direct proportion with the power delivered. Good systems will experience lesser drop in potential for a given delivered power.

In our solar air heating case, the ‘potential’ is the temperature, and the flow rate of air is analogous to electrical current in the battery example.  A hypothetical very good solar thermal system in ideal conditions will experience only a small drop in delivered temperature as the air flow rate increases.

In practice, solar air heaters usually experience significant drop in delivered air temperature as the air flow rate is increased.  So the delivered thermal power is often not what the no-load temperature might suggest.

Other limitations and issues

Duct losses. Over and above the issues apparent in these two scenarios A and C above, practical solar air heating systems use ducting between the collector and the conditioned space.  Ducting is another non-trivial source of heat loss.  The heat loss associated with the ducts is proportional to the temperature difference between the air in the duct and the air space the duct is in (either in the ceiling space, or outside). These losses can be considerable.  If the system is recirculating, then there will be two lots of ducts – each with its own heat losses.  Ducting will usually be insulated. However, this only reduces, not eliminate, losses in the the ducting.

Mixing. Solar air heaters tend to have a relatively low rate of air flow, and have vents at ceiling level. Low-velocity warm air entering at ceiling level doesn’t mix very well. This can lead to stratification, ie a distinct layer of warm air up high and cold air down low. To avoid stratification you need the return air to be drawn from floor level, which tends to be more difficult to achieve in a retrofitted system.

stratification_dillon_kyneton
Stratification is a problem when warm air doesn’t mix properly.

Draughts. Practical systems are rarely air-tight. So the installation of a solar air heater might have the unintended consequence of draughts. These draught pathways will usually be present whether or not the system is running.

Secondary heating only. In its very nature, a solar air heater requires sunlight. In any winter heating season, the majority of heating hours are likely to occur at night or in conditions unsuited to useful heat generation.  So a solar air heater can only ever be a secondary heat source.    Looked at another way,  when you need heating the most, a solar air heater is usually going to be useless, and you’ll be dependent on some other heat source.

Opportunity cost compared with PV

A solar air heater, to be useful, needs to occupy a sunny location on your roof.   Imagine if that same sunny location were taken, instead, with one extra solar PV module on a larger PV array.

Efficiency improves with cold. Unlike solar thermal collectors, a solar PV module’s efficiency actually improves the colder it gets (all other things being equal).

Other losses reduced. Unlike solar thermal ducting, the energy lost in the electrical wires and electronics is very low – typically less than 5%.

Excess solar heat is useless. In a scenario where a solar thermal collector is able to collect heat, and that heat is not needed, then there are generally no practical ways of making use of that excess.  So all the months of the year when heating isn’t required, a solar air heater is excess baggage on your roof.

Electricity is more versatile. Generating electricity rather than heat on your roof is all-round a better prospect. Electrical energy is intrinsically more versatile and valuable because it is useful for many other things within or beyond a home.

Electricity can drive a heat pump. Electricity from a solar panel can drive a heat pump which can extract heat energy efficiently from the outside air in cold weather.  The functional efficiency of a heat pump might be typically 3x to 4x, ie one unit of electrical energy can deliver 3-4 units of heat energy.   This benefit from heat pumps cancels out any theoretical efficiency benefit a solar thermal collector may have relative to a PV panel[*1].

Efficient homes will have PV anyway. It’s highly likely that any new efficient home will put on solar PV for the reasons above. So where a solar air heater takes up space of one PV module, does it make more sense to have, say, an installation of six PV modules – or five PV modules, and a second installation of a solar air heater?  The marginal cost of one more solar PV module is always going to be much cheaper than a solar air heater installation.

Conclusion

Solar air heater systems may appeal to some. However, in a world where solar PV is cheap and ubiquitous, it’s hard to see any role for add-on solar-air-heating systems in homes.  I think the purported heating capacity of solar air heaters is real, but greatly exaggerated.  In almost every case you’ll be much better off with rooftop solar PV driving some sort of heat pump such as a split system. This removes the need for multiple heating systems.

The best solar air heater is a room with great passive solar design to capture the winter sun.  And leave the roof for solar PV.

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The best solar air heater is great passive solar design (courtesy: Warren Reed Coast Magazine)

[1] To clarify what I meant by by saying that a heat pump gives 3-4 units of energy for one unit of energy cancels out the benefit of solar air heating – a solar-air heater reference efficiency might be 85%, whereas PV is at best only about 22%. My point is that if you multiply the PV efficiency by the functional efficiency of a heat pump then you get a number in the ball park of the theoretical efficiency of the solar air collector.  So even working at their very best a solar air heater is only about as good as PV plus a heat pump.

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