General questions

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A solar air collector uses an absorber to convert solar energy directly into heated air. Solar cells convert solar radiation into electricity, and then into heat – with very modest efficiency…

Fullblack is more than four times more efficient for heated air production compared to sun cells.

Fullblack also adds heat even in “moderate” cloudiness. Certainly less than full sun, but still a significant annual addition to the sunshine hours.

See the answer on efficiency below.

Fullblack has a so-called radiation absorber (much, much more effective compared to a black painted sheet). A black sheet reflects as much as it absorbs, which means that the heat radiates from the front/sunny side of the sheet, and thus works less well in an air collector.

Fullblack absorbs >90% of solar radiation and only emits (re-radiates) <10%! The indoor air passes the back of the radiation absorber, heats up, and blows back into the house.

An “ordinary” black sheet of metal re-radiates as much heat as it absorbs, making it a poor absorber in an air collector. See answer above.

Answer yes. The Fullblack air collector is the most affordable way to add heat. Pays for itself much faster than the best heat pump or solar panels. During the 9 colder months of the year September – May, a Fullblack solar air collector provides a heat boost even on days with lighter clouds and low sun. Thanks to the radiation absorber, additional heat is obtained even at minus 20 degrees – and colder. The winter yield is very impressive thanks to the low sun angle and the snow reflection from the ground in front. The snow reflection is 40-90%, depending on the “snow type”. “The ‘winter dividend’ is very significant.

The Indoor model has almost the same annual yield/savings as the Outdoor model, as the Indoor provides supplements regardless of the outdoor temperature, and even with very modest solar radiation. (This is partly due to the fact that Fullblack Indoor is placed at indoor temperature).

Fullblack can be installed on all types of houses/buildings where you want to reduce heating costs: apartments, townhouses, villas, holiday homes, warehouses, storerooms, etc.

A lot of people also use our Fullblack for commercial buildings, which is an outstanding investment as the cost of the solar air collector can be deducted from the business, and the VAT is reimbursed. The pay off time is then even much, much shorter(!), e.g. farms, workshops, garages, greenhouses, warehouses, offices etc.

The Fullblack air collector is mounted on the outside of the house wall and is of the outdoor type. The air collector can be mounted vertically or horizontally.

Our indoor model is called “Indoor” and is set up directly inside the window. This model requires no assembly.

How much you save depends on how much electricity you use and, of course, the price of electricity.

The annual yield/savings of the large Fullblack, Outdoor, measuring 100x150x3cm is up to 2000 kWh per year. The smaller Outdoor model with dimensions of 100x75x3cm, up to 1000 kWh per year. The annual yield/savings of the indoor model depends on the type of window it is placed inside and the direction of the window. The annual yield is not quite as high as the Outdoor model of the same size, but the efficiency is quite sufficient to make the Indoor model pay for itself several times over within the warranty period.

The repayment period for ex. air source heat pumps are much longer, due in part to the need for recurring costly maintenance, and rooftop solar PV has a real payback period of up to 15 years. (According to Boverket.)

The annual yield is up to 1000 kWh for the smaller one and up to 2000 kWh for the larger one. The yield depends, among other things, on the temperature to be heated. The cooler the heating air, the higher the efficiency. Snow reflection also significantly increases the annual yield. See previous answer.

A sun channel that illuminates a room, warms the floor, the furniture it hits. As they are not good absorbers – they don’t get very warm and also spread/exchange the heat poorly to other parts of the room. The light channel itself – also switches the heat poorly to the shaded part of the room, as air is an excellent insulator. Thus, much of the heat stays in the light channel and leaks back out through the window.

In case of side sun, even from above, the light channel hits the floor near the window wall or the wall next to the window, and is poorly diffused further into the room/space.
With Indoor, directly inside the window, the IR/UV radiation is captured by the absorber and sent perpendicularly far into the room/venue.

Thanks to the Indoor’s 0.2 mm thin radiation absorber, all radiation is directly converted into heat which is much better distributed in the room/space. So no passive thermal radiation as in the case of sunlit floors/furniture.

Solar radiation must be instantly converted into heat in order to be optimally utilized. The absorber cannot be touched by fingers/hands, for example, as it will oxidize. That’s why Indoor has a front glass. The glass has to be extremely thin so as not to prevent IR/UV radiation from reaching the absorber, and this means that the front glass, 0.5 mm (!), may become slightly wavy/bumpy over time due to the heat, but this has no impact on the effect.

As the Indoor circulates the indoor air, the exhaust air can never have a lower temperature than the indoor air. Indoor never exchanges heat from the room/space. It may feel ‘cold’ on the hand, but this is because the strong airflow cools the skin. (Blowing in the palm of your hand gives the impression that the wind is “cold”, but of course it is not. Even a table fan gives the same experience, but likewise it is the same temperature it blows, which is indoors). For correct measurement of the exhaust air temperature from the Indoor, the measurement must be made directly against the exhaust hole. Use e.g. a kitchen thermometer and attach the sensor body to the exhaust hole.

When the sun shines on Indoor, and it blows out up to 40 degrees (depending on the type of window), 50 kBm/hour, a room of, for example, 20 sqm will naturally be warmer during the day compared to the solar radiation from the window alone, without Indoor on the inside.

The benefit of Indoor is, as mentioned, when the sun does not shine straight through the window, with a low sun angle. In this case, the small room is just as warm with or without an Indoor in the window. But in case of side radiation, even from above, sunshine, “moderate” cloudiness – the Indoor uses the IR/UV radiation from the light and provides more heat gain compared to not having it in the window. Since the need for additional heat, depending on where you live, is from August to June, this adds up to many hundreds of kWh over a year. With a 10-year warranty, the Indoor pays for itself several times over during the warranty period – thanks to its “affordable” price. The adapter that powers the fan costs only a few tens of euros a year to operate. The adapter is set in a day/light timer so that the Indoor only runs during the day. (At night it doesn’t do any good).

The walls, ceiling and floor absorb most of the heat blown in by Indoor. If the Indoor blows in over 40-degree air, a small room will not be 40 degrees warm, because the ceiling and walls “saturate” first. This means that when evening comes, the roof, walls and floors are warmer than they would be without an Indoor in the window. The room therefore stays warmer for longer as the ceiling, walls and floor re-radiate the accumulated heat.

Effect example:

For example, if the indoor temperature is 20 degrees, and the exhaust air is 30 degrees, the nominal output is 200 W. A worthwhile and simple heat gain.
Similarly, the entire back side (which also gets hot) acts as an “element” and contributes additional heat, on top of the temperature increase in the return air.

It’s always a good idea to have indoor walking during the day, regardless of the weather, in areas you don’t go to on a daily basis. Even in very bad weather, the temperature increase in the exhaust hole is at least 1 degree, which means 20 W of power. Since the fan only draws 6 W, the heat gain is still 12 W. If there would be a whole day with heavy rain/snowstorm, there will be no difference in the outlet/inlet temperature, but since the adapter/fan only costs one penny in operation/hour, that day’s “loss” will be negligible. When used in a residential environment such as an apartment or house, the Indoor is placed in the window in the morning when you go to work, and taken down when you return in the afternoon. Then you can choose the weather in which it is used.

Similarly, Indoor already benefits from the high airflow that circulates the air in the room/space, preventing mold growth and dehumidifying. So even on a snowstorm day, with no temperature increase, it is worth a penny an hour just to circulate the indoor air.

See the real-time measurement of Indoor here:

Technical issues

The air flow from the air solar collector is about 43 cubic meters of heated air per hour. In comparison, in a room of 18 square meters, there is about 43 cubic meters of volume, that is, the air is “exchanged” every hour.

To avoid cold bridges. Metal conducts both heat and cold much more than plastic, so plastic components ensure that less heat is lost through the structure. When the air collector is warm inside, less heat “leaks” out, and at cold temperatures, less cold enters, thanks to the plastic material.

The solar fan has a built-in thermostat for starting and stopping. The fan only operates when the temperature of the return air blowing into the room is 25 degrees or more. An adjustable thermostat serves no purpose, as the heat output will be the same even if an adjustable thermostat is set at, say, 40 degrees. This means that the fan would have to run at a lower speed, resulting in lower airflow/power. The fixed 25 degree thermostat is also extremely reliable as it is not controlled by electronics. Thus, the air collector never runs at night or in bad weather, and ensures that indoor heat is not exchanged.

Nothing can go wrong.

The fan can be powered by an adjustable 5-12 V adapter or by a 10 W solar panel if electricity is not available. The speed/sound of the fan can be controlled via the voltage on the adapter, with 5V providing the lowest speed/sound and airflow. At a lower set speed, however, the “heating value” remains the same, as the exhaust air becomes warmer with lower airflow. The annual cost of adapter operation is a few tens, and thus negligible compared to the value of the heat gain.

The solar fan is thus powered either by an adapter and fixed thermostat – or by a 10 W solar cell. Not both.

Circulating the indoor air through the solar air collector provides more efficient heating compared to heating the outdoor air. Blowing in outdoor air can also have a number of other disadvantages:

  • Spores and pollen blow in and start germinating in cracks.
  • If the outdoor unit that blows in air is on the outside of the house wall, mold can form in the one that blows in.
  • Blowing in outdoor air with excess pressure creates moisture damage. It is for this reason that all premises have so-called balanced ventilation with neither overpressure nor underpressure.

The efficiency of our Fullblack models is by far the best of all air collectors on the market. The outdoor models have no problem delivering in sub-zero temperatures when the absorber is on the house wall or in moderately cloudy conditions, see the video on the home page:

The efficiency of solar cells is around 15%, meaning that they only convert 15% of the energy in the solar radiation into electricity.

The effect of solar radiation on one square meter of wall is approx. 800 W. 15% of this value corresponds to 120 W, which is what solar cells can produce in electricity/heat per square meter of irradiated area.

Fullblack, on the other hand, provides around 500 W of heat under similar conditions.

The efficiency of a solar air collector is calculated by the difference in temperature between the outgoing and incoming air, multiplied by the air flow rate.

Power (W) = T (outlet-inlet temperature difference) x 1.3(constant) x Flow (liters of air/second)

So even though the exhaust air in an air solar collector may feel warm, the effect may still be insignificant (the “value” of the heat) if the airflow is very low.

As we have seen from the previous answer, the efficiency of an air collector depends, among other things, on the capacity of the fan. It is therefore important that the fan is strong enough without becoming power hungry, which would defeat the whole purpose of the air collector.

In our Fullblack models, a 6W fan delivers 43 cubic meters of air per hour, which is equivalent to 12 liters of air per second. There will be about 3 of them. black 240 liter garbage bags with hot air per minute!

The efficiency of an air solar collector thus depends on the air flow and the temperature increase. see previous answer.

Several manufacturers indicate the air flow rate of an air collector by referring to the capacity of the fan, but this can be directly misleading as the indicated values of axial fans always refer to the “free blow” of the fan, i.e. when it does not need to suck or push.

For example, if an axial fan with a free blow of 100 kBm/h is used to suck air through an air collector, the air flow rate drops dramatically. The actual airflow can then be as low as 15 kBm/h.

When the sun is at its maximum, the output is about 982Wper square meter of irradiated area.

From this value we can extract the maximum temperature difference between the inlet and outlet air that the air collector can create from the following formula

W (Maximum power at 1sqm irradiated area) = T (temperature difference exhaust air – supply air) x 1.3 (a constant) x airflow (liters/second)

Suppose an air collector fan is claimed to have a flow rate of 100 cubic meters of air per hour (28 liters/second) (a common but often exaggerated marketing claim, see previous answer).

T = 982 /(1.3 x 28liter/second) = 27.4 degrees theoretical maximum temperature difference between inlet and outlet air.

An air collector with a specified flow rate of 100 cubic meters of air per hour = 28 liters/second can in practice never provide even close to a temperature increase of 27.4 degrees, per square meter of air collector area. (Important to keep in mind when making comparisons/commitments regarding air collectors on the market).

Of course, more efficient heating is obtained by circulating indoor air through air solar collectors.

The efficiency of heating also depends on how well insulated the premises are, but also on what is meant by heating. Is half a degree of warming ‘warming’? However, annual consumption in kWh is relevant information. The functioning of the air collector in sub-zero temperatures and moderate cloudiness is crucial for the annual yield. Solkungen Energi’s air solar collectors deliver both at sub-zero temperatures and in moderate cloudiness, thus ensuring that energy savings are significant all year round even in non-optimal weather conditions. Stating how much house area an air collector can “heat” is directly misleading, as it depends on the type of premises, insulation, etc.