WO2014097307A1

WO2014097307A1 - Solar dryer

Info

Publication number: WO2014097307A1
Application number: PCT/IN2012/000843
Authority: WO
Inventors: Vaibhav TIDKE; Bhaskar THORAT; Swapnil KOKATE
Original assignee: Tidke Vaibhav
Priority date: 2012-12-21
Filing date: 2012-12-21
Publication date: 2014-06-26

Abstract

A solar dryer for drying organic or inorganic material with controlled radiation. The dryer comprises a radiation absorbing heat conducting surface (22), and a convection channel (24) formed by a radiation controlling cover (26) over the conducting surface. The present invention uses the heat transfer modes of conduction, convection and radiation in the solar dyer.

Description

SOLAR DRYER FIELD OF INVENTION:

The present invention relates to the field of solar drying technology. BACKGROUND OF INVENTION:

Solar dryers developed so far are generally based on convection and radiation as modes of heat transfer, which is an inefficient way of drying. On the other hand, conduction drying despite being a more efficient way of drying requires greater heat transfer surface area. Large surface areas are usually required for heating the conducting surface through solar energy, since solar radiations are a dilute source of energy. Several types of solar dryers are known, which combine the principles of convection and radiation for heat transfer. For example, US 6922908 claims a dryer wherein the thermal conductor which converts solar energy into heat energy, with a heat transfer system and a housing that includes a drying chamber. The heat transfer system is in thermal communication with both the thermal conductor and the drying chamber.

US 5065528 claims a drying room surrounded by a solar-heat transmitting/absorbing wall. In the drying room, the matter is deposited on an elongated drying cage and is exposed to dry hot air produced by the solar heat.

All convectional solar dryers rely on air for supply of heat to the material to be dried.

This air requirement-^{^}creates two problems:

i 1. Need of high air flow rate and good air distribution. This may not be possible with natural circulation alone and one may need energy input to attain the desired air flow rate. If this flow rate is not achieved, efficiency drops and uniform drying is not achieved.

2. The high air flow rate draws away the heat, reducing efficiency of dryer.

The present invention uses the heat transfer modes of conduction, convection and radiation in the solar dyer which are not envisaged in the prior art.

SUMMARY:

In its main aspect, the present invention discloses a solar dryer for drying organic or inorganic material with controlled radiation. The dryer comprises a radiation absorbing heat conducting surface, and a convection channel formed by a radiation controlling cover over the conducting surface. In another aspect of the invention, heat conducting surface of the solar dryer is metallic.

In yet another aspect of the invention, the heat conducting surface is either planar or undulating or has one or more troughs on it.

In a further aspect of the invention, the heat conducting surface of the solar dryer is angularly in relation to the horizontal plane. The angle between the conducting surface and the horizontal plane could be varied.

In a further aspect of this invention, the radiation controlling cover extends parallelly over the heat conducting surface of the solar dryer or extends over the heat conducting surface partly parallelly and partly perpendicularly.

In a preferred aspect of the invention, a fan drives the convection channel formed between the radiation controlling cover and the conducting surface. Figures

FIGURE 1: Side view of the dryer

FIGURE 2: Angularly placed heat conducting surface DESCRIPTION:

The present invention discloses a solar dryer (20) for drying organic or inorganic material with controlled radiation. The dryer comprises a radiation absorbing heat conducting surface (22), and a convection channel (24) formed by a radiation controlling cover (26) over the conducting surface. This invention is illustrated in the accompanying drawings, through out which like reference letters indicate corresponding parts in the various figures.

The heat conducting surface is preferably dark metallic even though any good conducting material would suffice. The material to be dried is kept in contact with the heat conducting surface in such a way, that the surface can absorb radiations, convert them to heat, and transfer heat to the material to be dried. The heat conducting surface, could be planar or with undulations or could feature troughs as shown in

FIG 1 along its length. The nature of the heai conducting surface is largely dictated by the nature of the material to be dried on it. In a special embodiment of the invention, the heat conducting surface is placed angularly with respect to the horizontal plane as seen in FIG 2. This ensures a variable airflow rate in the convection channel along with optimized reception of solar radiation. The cover over the radiation absorbing -heat conducting could be translucent or transparent, and could comprise of materials that exclude specific rays of the ambient solar radiation spectrum, ultimately making the cover into a radiation controlling cover. The choice of cover material and the planar characteristics of the heat conducting surface would depend upon the material to be dried. Heat transfer from heat conducting surface to material to be dried occurs by uniform conduction. Direct radiation heat transfer to the material to be dried is incidental and may depend upon the surface character of the heat conducting surface. The material to be dried is dried from its bottom, which is in contact with the heat conducting radiation absorbing surface, additionally the material gets dried from the top through convection, within the convection channel since hot air is present in the channel between the heat conducting surface and its cover. The cover additionally reduces heat transfer losses by convection and thus increases efficiency of drying.

Depending upon the choice of cover and heat conducting surface, there could be direct exposure of the material to be dried to radiation through the transparent cover, and thus this system increases drying efficiency by using all three means of heat transfer, namely conduction, convection and radiation.

Moisture removed from the material is drawn out by air convection in the channel between the heat conducting- radiation absorbing surface and the cover. The radiation controlling cover extends parallelly over the heat conducting surface of the solar dryer. In a preferred embodiment, the radiation controlling cover creating a convection channel extends over the heat conducting surface partly parallelly and partly perpendicularly to form a chimney (28). The warm air within the channel is lighter than the cooler air outside, this creates a small pressure difference between the channel and the outside, forcing air carrying the moisture to escape from the chimney. Optionally, vacuum may be created in the system when colour retention in the material to be dried such as tomato is desired. In case air is present, it provides heat to the material to be dried by convection and at the same time, it draws moisture away. The air flow rate is sufficient for the removal of the moisture by natural convection. The air flow rate may be varied by keeping the heat conducting- radiation absorbing surface tilted at an angle to receive maximum radiation. The tilted path for air increases the air flow rate, resulting in higher drying efficiency. Alternatively the air flow can be enhanced by fan (30) in the convection channel as per requirement of the material to be dried.

The solar dryer disclosed in this invention is suitable for drying material such as vegetables, fruits, grains, fish or chemicals, sludges or fertilizers with the heat absorbing conducting surface being appropriately modified.

Claims

CLAIMS:

1. A solar dryer for drying organic or inorganic material comprising:

a radiation absorbing heat conducting surface (22), and a convection channel (24) formed by a radiation controlling cover (26) over the conducting surface.

2. The solar dryer as claimed in claim 1, wherein the heat conducting surface is metallic.

3. The solar dryer as claimed in claim 1 and 2, wherein the heat conducting surface is planar.

4. The solar dryer as claimed in claim 1 and 2, wherein the heat conducting surface is with undulations.

5. The solar dryer as claimed in claim 1 and 2, wherein the heat conducting surface has one or more troughs.

6. The solar dryer as claimed in claims 3,4 and 5, wherein the heat conducting surface is angularly placed from a horizontal plane wherein the angle between the conducting surface and the horizontal plane varies between 0° and 60°.

7. The solar dryer as claimed in claims 1 to 6, wherein the radiation controlling cover extends parallelly over the heat conducting surface.

8. The solar dryer as claimed in claims 1 to 6, wherein the radiation controlling cover extends over the heat conducting surface partly parallelly and partly perpendicularly.

9. The solar dryer as claimed in claims 7 and 8, wherein a fan (30) drives the convection channel.

10. The solar dryer as claimed in claims 1 to 9, wherein the material dried is vegetables, fruit, fish, sludge, chemicals coatings or fertilizers.