Thermal Solar Experiment – Solar Air Heater (Solar Furnace)

With the experimental photovoltaic (PV) solar panels in place and operating, I started to wonder about the feasibility of getting useful benefits from thermal solar panels.

PV solar is not particularly efficient, converting only around 12-14% of the 'insolation' into electricity for polycrystalline cells, and maybe up to 18% for monocrystalline cells. 

Insolation is just a word abbreviated from 'incident solar radiation', i.e. the amount of energy from the sun received at a particular location, and is usually expressed in terms of watts per square metre.  As a rough guide the insolation is generally around 1,000 W/m2 at the earth's surface, although it does vary by location.   The 1,000 W/m2 value is also the basis on which the rated Watt-peak outputs of commercial PV solar panels are determined.

Thermal solar panels can be much more efficient, when the intention is to convert the sun's energy to heat and not directly to electricity.

I watched a few YouTube videos on making solar air heaters (or solar furnaces, as they're called across the pond), and reasonable results seemed achievable from home-made versions mostly using aluminium drinks cans.

However, the techniques used all seemed a little fiddly and time-consuming, in that the cans required cutting of both their ends and then sticking together to form a stack, although undoubtedly it's a cheap way to try it.

Instead, I decided to use 80mm flexible aluminium HVAC ducting.  This 'slinky' is reasonably cheap, comes in one continuous length and seemed ideal for the job.  It has an irregular 'crinkly' surface which has two distinct advantages :-

• a greater external surface area per unit length than a smooth tube or drinks can, to collect more of the sun's energy

• a rougher internal surface than a drinks can, which causes the air to be more turbulent within the slinky and pick up more heat.

So, I bought a 10 metre length of flexible ducting from eBay and an 8'x4' sheet of insulation board from Wickes.  This 'Celotex' insulation material is what's used in the cavity walls in new build projects, and comprises a lightweight closed-cell insulation material backed on both sides with aluminium foil.  I bought the thinnest and cheapest available at Wickes, i.e. 25 mm thick.

I dug out some timber from the shed, and initially made an outer frame which fitted inside one of my ex-caravan window frames – see my earlier PV solar post.

I then made an inner box lining from the Celotex, sized to fit inside the outer wooden frame, and fitted the HVAC ducting coil in place, making openings in the insulation and frame for both the inlet and outlet.   The slinky was doubled back on itself several times to form a single-tube single-pass heat exchanger.  At each return point, I used a piece of garden wire to secure the duct in place.

inner Celotex box with coil in place

garden wire used at the returns

The slinky was then sprayed with matt black high-temperature stove paint.

spraying the coil

The finished lined box and outer frame were then fitted into the back of the window frame, and secured with a few screws.

I'd previously bought a brand-new 3" bilge blower from eBay to push the air through the heater panel.  This is the sort of thing used in petrol-fuelled boats to evacuate the bilge space before cranking the engine, to get rid of potentially explosive gases.

The 12V DC blower is an inline version, and therefore just pushes directly into the slinky ducting, and I secured it by screws to the wooden outer frame.  A couple of cheap LCD thermometers completed the installation, with a probe placed in the airflow at both the inlet and outlet. 

blower fitted - note the inlet temperature probe

To power the blower, and make the installation a totally standalone solar installation, I hooked up my rough-and-ready 36 cell solar PV panel directly to the blower motor terminals.   The blower specification says that it pulls around 2A, and putting the voltmeter across it when operating showed that the PV panel pushed out almost exactly 12V when under load.

This direct coupling of the PV panel to the blower terminals also make the installation self-controlling to a degree – when the sun's bright, the blower hits its rated performance and pushes more air through the slinky at the time when the thermal panel is receiving the most energy from the sun.  When it's partially cloudy, the blower just ticks over and the airflow through the slinky is reduced, allowing more time for it to pick up heat from the reduced sunlight conditions.

The initial testing was carried out in direct sunlight one morning in late July, and the results were spectacularly good.     The panel was first left in the sun for around 45 minutes until steady-state conditions were reached.  I recorded the temperatures and also measured the airflow speed at the outlet using my hand-held anemometer.  

on test - thermal and PV panels together

steady-state inlet and outlet temperatures

anemometer reading at the outlet

As an engineer, I'm naturally very conservative when it comes to analysing test data, and I'm very sceptical of performance claims made for equipment by commercial organisations that often don't even come close to real-life conditions. 

However, even using my lowest mean air speed values, the panel produced way over 500W.  With a panel surface area of 0.81m2, I calculated that it was converting around two-thirds of the sun's available energy into useable heat.  The power produced is the product of the heat capacity, the mass flow and the temperature differential.

heater calculations.......

Now it's October and winter's fast approaching, I intend to repeat the testing on colder sunny days.   

I've already done the basic design of a 3.5 kW roof-mounted version of such a heater which also incorporates the PV cells within the same frame, to drive several blowers, but I first need to look at the results from much more testing on our experimental panel.  

I'd obviously expect the inlet and outlet temperatures to be considerably lower in the winter, but the temperature differential to be around the same as in July.   It's the differential that's important in terms of power output.

However, on a freezing cold day (zero degrees ambient), the maximum outlet air temperature is expected to be only around 20 degrees, and that's not quite enough to blow it directly into the house for heating purposes given that there'll be further heat losses in the panel insulation and delivery ducting.

The potential is definitely worth further investigation though, when I can get around to it.

For information, I reckon this 500W or so experimental panel cost around £80 in total to make, but that excludes the window frame and the 36 cell PV panel....