Solar attic fan. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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The easiest way to cool your home is to install an air conditioner. However, it is expensive and inefficient. It is much cheaper to use an inexpensive ventilation system that primarily prevents overheating of the air in the room and an increase in humidity.

The ventilation system must be installed in such a way as to remove air from the attic. Why from the attic? Because he is the source of all problems.

It all starts early in the morning, as soon as the sun begins to shine on the roof. I don't know if you know it or not, but roof tiles absorb solar radiation quite effectively. Bitumen-coated roofs attract and retain the heat of the sun especially well.

The heat from the roof is then transferred to the air that fills the attic. As the day progresses, more and more heat enters the attic air space. Now another mechanism comes into play inside the attic. It is well known that warm air rises and cold air sinks. Since the air in the attic does not mix, a temperature distribution is created in the house, shown in Fig. 1. Layered temperature distribution causes heat accumulation. We have a huge reservoir of heat that needs to be used.

In many homes, it becomes too hot due to heat intrusion from the attic. When you turn on the air conditioner, you are trying to remove heat from living spaces to make the conditions more comfortable. However, at the same time, the attic continues to heat the house. Such confrontation is costly and does not lead to the desired results.

The only way to stop this influx of heat from the attic into the living space is to insulate the house from the attic. Thermal insulation with glass wool is very effective. A layer of glass wool with a thickness of not more than 15 cm, covering the ceiling, significantly affects the amount of heat penetrating down.

Cooling mechanisms

However, no amount of insulation can completely isolate the lower rooms from the penetration of heat from the attic. Heat will penetrate into living spaces through heat transfer and radiation.

To illustrate this, consider the following example. Suppose that the attic of your house has dimensions of 9x12 m (area 108 m2). If the temperature in the attic averages 55°C, and you want the temperature in the living room not to exceed 27°C, then the best you can hope for is to achieve a heat transfer of no more than 2000 J/h. And this is in the case of a perfect isolation system. For a typical house with single-layer glass wool ceiling insulation, the heat penetration is about 4500 J/h.

Ris.1

It has been experimentally established that in order to neutralize 9000 J of heat, the air conditioner must pump 1 ton of air. Thus, to eliminate the effect of attic heating, we need to pump an extra 0,5 tons of air with an air conditioner!

However, the actual amount of heat penetrating down depends on the temperature difference between the attic and the house. A temperature difference of 5°C corresponds to thousands of joules. Therefore, the colder it is in the attic, the less the air conditioner works.

Attic ventilation

How can you cool your attic? You just need to ventilate it! There are very rare cases when the outside air temperature is higher than the air temperature in the attic, where it is usually hot, like in a stove; You can cool the attic by replacing the hot, stagnant air in it with colder outside air.

This is relatively easy to do by cutting a vent in the roof near its crest and installing an exhaust fan in it. A fan blows cold air through a projecting roof eaves and draws warm, stale air out of the attic through a vent.

This circulation of air inside the attic causes the mixing of hot and cold air and eliminates temperature differences (Fig. 2). It should be noted how it affected the temperature inside the attic. Now the temperature is more evenly distributed, and the average temperature has dropped.

Ris.2

I want to note that a very large fan is not required to ventilate the attic. The goal will be achieved if the air exchange in the attic is carried out approximately every 3 minutes.

The size of the fan is determined by the size of the attic. The attic of standard dimensions (9x12 m2) has a volume of approximately 135 m3. To exchange this volume of air every 4 minutes, a fan is required that will pump out 34 m3 / min.

If the attic is smaller, a smaller fan will be required. The ratio here is simple: the attic volume in m3 is divided by the desired air change time (in minutes) and the fan performance is obtained. For example 135 m3 / 4 min = 34 m3/min.

The main elements of the fan

The fan is driven by a small DC motor, which is usually linear: the more power supplied to it, the faster it rotates. It is known that power depends on two quantities: voltage and current. Changing any of these values ​​will cause a change in power.

For example, a motor with a voltage of 12 V at a current of 6000A can rotate at a speed of 6 rpm. If we reduce the electrical energy supplied to the motor by lowering the voltage to 2 V, then the rotation speed will decrease by 3000 times and become equal to XNUMX rpm.

On the other hand, if in the same motor at 12 V at 3 A, rotating at the same speed of 6000 rpm, we reduce the current by 2 times, keeping the voltage at the same level (12 V at 1,5 A), we get the same result: the motor speed will be 3000 rpm. Given the principle of operation of photovoltaic converters, understanding the reason for changing the speed of rotation of the motor with a change in the consumed current is especially important.

The volume of air that the fan blades will distill is directly proportional to the speed of rotation. This indicates that the air flow can be controlled by simply changing the speed of the motor.

Solar battery

Undoubtedly, photoelectric converters can be used to power the exhaust fan. This is the preferred choice. At the same time, it should be noted that when a photovoltaic source is connected to a fan electric motor, an interesting relationship arises.

Photovoltaic solar cells can usually be considered as current sources. In low light, the solar panel generates a small current, although the voltage remains normal. As a result, the fan (if it is rotating) rotates slowly and therefore only pumps a small amount of air.

This circumstance just meets the task of ventilating the attic. In the morning, the roof is practically not heated, and at this time of the day there is no need for ventilation or only a little ventilation is needed.

During the day, with increasing solar radiation, more and more power is supplied to the fan motor from photovoltaic converters, and the fan speed increases. With the increase in solar insolation, an increasing amount of heat enters the attic. It should be noted that an increase in the fan speed (air exchange) is observed exactly when it is needed.

Toward evening, the intensity of solar radiation decreases again, the roof absorbs less heat and the need for ventilation decreases. This is consistent with the change in output power of photovoltaic converters, which rotate the fan at a lower speed.

As a result, we have developed a self-regulating attic ventilation system that keeps the attic temperature at a relatively constant level. Usually, the fan control, depending on the heating of the attic, is carried out by a mechanical thermal switch.

Solar battery design

For the purposes mentioned, two commercially available commercially available fans designed specifically for these applications have been selected. Let's place our photovoltaic sources near the fans. Remember, however, that you can use any combination of motor and fan that suits you.

The first fan is an exhaust fan from Solarex Corp.

Said fan is driven by a 12V DC motor. However, Solarex recommends running the motor at 6V for longevity. m6/min.

It will not be difficult to develop a 7 W battery that satisfies the mentioned requirements. First you need to imagine the required maximum current strength. As mentioned above, it corresponds to 1,2 A.

It is common knowledge that a 7,5 cm round solar cell produces a current of 1,2 A. In fact, you can find fairly cheap substandard 7,5 cm cells that develop "only" 1 A. These cells are suitable for the purposes mentioned.

To achieve a power of 7 W at maximum solar radiation intensity, 12 elements are required. The elements can be soldered in series, placing them in 3 rows of 4 elements each. If substandard elements of 1 A are selected for use in the design, then to compensate for their defectiveness, it is necessary to increase the number of elements in the battery by 2 and bring their number to 14.

The second fan we'll look at comes from Wm. Lamb. Its diameter is 35 cm; It is equipped with a linear electric motor with ball bearings. Pressed ball bearings extend motor life. The motor is powered by any voltage: 6-48 V. For our purposes, the manufacturer recommends using a voltage of 12V.

A 30 W solar generator will spin the fan at a speed sufficient to exchange air at about 30 m3/min, while a 7 W battery will provide enough energy to exchange air at a rate of 14 m3/min. On fig. 3 shows the dependence of the air exchange rate on the power of the photoelectric converter.

Ris.3

Installation of the structure on the roof

In accordance with one of the options for installing a ventilation device, it will be necessary to make holes in the roof. Since any work on the roof is associated with the risk of possible water leaks, accuracy is the key to a successful job.

First, a round hole in the roof is sawn with a hacksaw. Both fans are supplied fixed in metal casings and the opening in the roof must exactly match the diameter of the casing. Make sure that the location for the hole is chosen between the roof rafters!

Then a fan is installed in the hole. Now a metal reflector is placed around the device, and all possible gaps are abundantly filled with tar to prevent leaks. To prevent rain from entering through the hole made, the fan is covered with a cone-shaped or U-shaped cap.

If there is no desire to make a hole in the roof, there is another option. The fan can be mounted above one of the vents located under the roof eaves. The best way to do this is to mount the fan at a 45° angle to the attic floor. It is recommended to make a frame from a pair of frames with an aspect ratio of 2:1 (Fig. 4), and then attach the fan to one of them (Fig. 5). After that, you can place the frame over the vent. Make sure that the opening is large enough so that all the exchanged air passes through it, otherwise the fan will not work efficiently enough.

Ris.4

Fig. 5

The solar panel is attached to the south-facing section of the roof and attached to the fan. It is better to run the wires down to the edge of the roof and pass them through the vent in the eaves than to drill a special hole for them in the roof: there is less chance of breaking the roof.

Ris.6

When connecting a solar battery to a fan, attention is drawn to the direction of rotation of the electric motor. In one direction of rotation, the air will be drawn out, in the other direction it will be drawn into the room. If the fan does not rotate in the correct direction, the supply wires must be reversed.

Author: Byers T.

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