Revision as of 11:54, 27 August 2006

Solar Drying - Technical Brief

Short Description

Problem:
Idea:
Difficulty:
Price Range:
Material Needeed:
Geographic Area:
Competencies:
How Many people?
How Long does it take?

Introduction

Agricultural and other products have been dried by the sun and wind in the open air for thousands of years. The purpose is either to preserve them for later use, as is the case with food; or as an integral part of the production process, as with timber, tobacco and laundering. In industrialised regions and sectors, open air-drying has now been largely replaced by mechanised dryers, with boilers to heat incoming air, and fans to force it through at a high rate. Mechanised drying is faster than open-air drying, uses much less land and usually gives a better quality product. But the equipment is expensive and requires substantial quantities of fuel or electricity to operate.

'Solar drying' in the context of this technical brief, refers to methods of using the sun's energy for drying, but excludes open air 'sun drying'. The justification for solar dryers is that they may be more effective than sun drying, but have lower operating costs than mechanised dryers. A number of designs are proven technically and while none are yet in widespread use, there is still optimism about their potential.

How solar dryers work

One well-known type of solar dryer is shown in Figure 1. It was designed for the particular requirements of rice but the principles hold for other products and design types, since the basic need to remove water is the same.

Air is drawn through the dryer by natural convection. It is heated as it passes through the collector and then partially cooled as it picks up moisture from the rice. The rice is heated both by the air and directly by the sun.

Warm air can hold more moisture than cold air so the amount required depends on the temperature to which it is heated in the collector as well as the amount held (absolute humidity) when it entered the collector.

Figure 1: Rice solar dryer

The way in which the moisture absorption capability of air is affected by its initial humidity and by the temperature to which it is subsequently heated is shown in Table 1.

Air enters at 20°C and leaves at 80% RH

Initial relative humidity	Moisture absorption capability (grammes of water/m° of air)
Not heated	Heated to 40°C	Heated to 60°C
40%	4.3g/m°	9.2g/m°	16.3g/m°
60%	1.4g/m°	8.2g/m°	15.6g/m°
80%		7.1g/m°	14.9g/m°

Table 1: The drying process

The objective of most drying processes is to reduce the moisture content of the product to a specified value. Moisture content (wet basis) is expressed as the weight of water as a proportion of total weight. The moisture content of rice has typically to be reduced from 24% to 14%. So to dry one tonne of rice, 100kg of water must be removed.

If the heated air has a 'absorption capacity' of 8g/m³ then 100/0.0008 = 12,500/m³ of air are required to dry one tonne of rice.

The heat required to evaporate water is 2.26kJ/kg. Hence, approximately 250MJ (70kWh) of energy are required to vaporise the 100kg water. There is no fixed requirement for solar heat input to the dryer. This is because the incoming ambient air can give up some of its internal energy to vaporise the water (becoming colder in the process). Indeed, if the ambient air is dry enough, no heat input is essential.

Nevertheless, extra heat is useful for two reasons. First, if the air is warmer then less of it is needed. Second, the temperature in the rice grains themselves may be an important factor, especially in the later stages of drying, when moisture has to be 'drawn' from the centres of the grains to their surfaces. This temperature will itself depend mainly on the air temperature but also on the amount of solar radiation received directly by the rice.

In a natural convection system, the flow of air is caused by the fact that the warm air inside the dryer is lighter than the cooler air outside. This difference in density creates a small pressure difference across the bed of grain, which forces the air through it. This effect increases, the greater is the height of the bed above the inlet (h1) and the outlet above the bed (h2). The effect of an increased h2 is less than that of an increased h1 because the air is cooled as it passes through the bed.

Approximate densities for a variety of cases are shown in Table 2.

Air enters at 20 °C and leaves at 80% RH

Initial relative humidity	Density of the air (kg/m³) (Drop in density, in brackets)
Not heated	Heated to
30 °C	40 °C	60 °C
40%	Ambient 1.19	1.19	1.19	1.19
Below bed 1.19 (.00)	1.15 (.04)	1.12 (.07)	1.05 (.14)
Above bed 1.21 (-.02)	1.19 (.00)	1.17 (.02)	1.14 (.05)
60%	Ambient 1.19	1.19	1.19	1.19
Below bed 1.19 (.00)	1.15 (.04)	1.11 (.08)	1.05 (.14)
Above bed 1.20 (-.01)	1.18 (.01)	1.16 (.03)	1.13 (.06)
80%	Ambient 1.18	1.18	1.18	1.18
Below bed 1.18 (.00)	1.14 (.04)	1.11 (.07)	1.04 (.14)
Above bed 1.18 (.00)	1.16 (.02)	1.15 (.03)	1.11 (.07)

Table 2: Air density variation in a natural convection dryer

It can be seen that if the incoming air is heated by only 10-30°C then the presence of a chimney on top of the dryer would make little or no difference, unless it acted efficiently as a solar collector and raised the temperature of the air significantly.

It should be noted that even if the difference in densities is as much as .05kg/m², then the resulting pressure difference is only 0.5 Pa (5 millionths of atmospheric pressure) per metre of chimney. For comparison, forced convection systems commonly operate with pressure differences of 100-500 Pa.

Many products are damaged by excessive temperatures. The most severe constraints are on beans (35°C), rice (45°C), and all grains if they are to be used for seed (45°C).

Other types of dryers and their performance

Forced convection solar dryer

(Figure 2)

By using a fan to create the airflow, drying time can be reduced by a factor of 3. Also, the area of collector required is reduced by up to 50%. Therefore, the area of collector required for a given throughput of product could be reduced by a factor of 5-6. The initial cost of a one tonne per day dryer is in the region of £1500-2000. The fan would consume about 500 watts for 6 hours, and so electricity cost (at 0.07/kWhr) would be about 0.20 per tonne of rice dried

File:P04a.gif
Figure 2: Forced convection solar dryer

Tent dryer

The distinguishing feature of tent, box and cabinet dryers is that the drying chamber and the collector are combined into one, see Figure 3. This allows a lower initial cost. Drying times are however not always much lower than for open-air drying. (Probably, insufficient attention has so far been paid to utilising natural convection.) The main purpose of the dryers may be to provide protection from dust, dirt, rain, wind or predators and they are usually used for fruit, fish, coffee or other products for which wastage is otherwise high. There are numerous other types. Greenhouse dryers are a more sophisticated version of tent dryers. Box dryers may incorporate thermal insulation to achieve higher temperatures. Storage bin dryers combine the functions of drying and long-term storage. Solar timber kilns may include hot water storage to enable the necessary control of drying rate.

[p04b.gif File:P04b.gif]
Figure 3: Tent dryer

Solar drying or open-air drying?

The great advantage of open-air drying is that it has little or no equipment costs. It is labour-intensive but this may not involve much economic cost in rural areas in developing countries. It requires about three times as much land (100m² per tonne of rice) compared to solar drying, but this may not matter too much in many cases.

Firstly, one important advantage of solar drying is that the product is protected from rain, insects, animals and dust that may contain faecal material. Some systems also give protection from direct sunlight. Second, faster drying reduces the likelihood of mould growth. Third, higher drying temperatures mean that more complete drying is possible, and this may allow much longer storage times (but only if rehumidification is prevented in storage). Finally, more complex types of solar dryers allow some control over drying rates.

Solar dryers or fuelled dryers?

The choice between using solar radiation or fuel, to heat the air is mainly one between a higher initial cost and continuing fuel costs which needs to be analysed for each location.

In some circumstances, it may be possible to burn rice husks or other fuel with low opportunity cost. One tonne of rice gives 200kg of husks.

Fuel heating usually allows better control of the drying-rate than solar heating; it also enables drying to be continuous. If either of these is required, a combined system may be appropriate with pre-heating of air by solar energy.

Which solar dryer?

The choice between alternative types of solar dryer will depend on local requirements and in particular on the scale of operation. If intended for peasant farmers then initial capital cost may be the main constraint and plastic-covered tent or box dryers may be appropriate.

There may however be a trend towards more centralised drying to enable more intensive usage of the equipment. The greater initial cost of glass covers may then be affordable, and grid electricity may be available to run fans and obtain much faster throughput for a given collector area.

For intermediate scale and capital cost the natural convection rice dryer is a well proven design.

References and further reading

This Howtopedia entry was derived from the Practical Action Technical Brief Solar Drying.
To look at the original document follow this link: http://www.practicalaction.org/?id=technical_briefs_food_processing

A survey of solar agricultural driers - Brace Research Institute - 1975
Preparing grain for storage - Action/Peace Corps and VITA - 1976
Solar driers - Commonwealth Science Council - 1985
Solar drying - Practical methods of food preservation - ILO 1988
Producing Solar Dried Fruit and Vegetables for Small-scale Enterprise Development - NRI 1996
Try Drying It!: Case studies in the dissemination of tray drying technology - IT Publishing 1991

</div>

Usefull addresses

Practical Action The Schumacher Centre for Technology & Development, Bourton on Dunsmore, RUGBY, CV23 9QZ, United Kingdom.
Tel.: +44 (0) 1926 634400, Fax: +44 (0) 1926 634401 Email: practicalaction@practicalaction.org.uk web:http://www.practicalaction.org

NR International
Central Avenue
Chatham Maritime
Kent
ME4 4TB
United Kingdom
Tel: +44 1634 880088
Fax: +44 1634 880066/77

Website: http://www.nrinternational.co.uk/

Categories:

Difference between pages "How to Use Photovoltaic Energy" and "How to Preserve Food with a Solar Dryer"

Revision as of 11:54, 27 August 2006

Contents

Solar Drying - Technical Brief

Short Description

Introduction

How solar dryers work

Forced convection solar dryer

Solar drying or open-air drying?

Solar dryers or fuelled dryers?

Which solar dryer?

References and further reading

Usefull addresses

Categories:

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@@ Line 1: / Line 1: @@
-=Solar Photovoltaic Energy - Technical Brief=
+=Solar Drying - Technical Brief=
+==Short Description==
-<div class="booktext">
+*'''Problem:'''
+*'''Idea:'''
-<center>'''PRACTICAL ACTION'''<br />'''Technology challenging poverty'''</center>
+*'''Difficulty:'''
+*'''Price Range:'''
-</div>
+*'''Material Needeed:'''
+*'''Geographic Area:'''
+*'''Competencies:'''
+*'''How Many people?'''
+*'''How Long does it take?'''
 ==Introduction==
@@ Line 11: / Line 16: @@
 <div class="booktext">
-Photovoltaic (PV) is a technology that converts sunlight directly into electricity. It was first observed in 1839 by the French scientist Becquerel who detected that when light was directed onto one side of a simple battery cell, the current generated could be increased. In the late 1950s, the space programme provided the impetus for the development of crystalline silicon solar cells; the first commercial production of PV modules for terrestrial applications began in 1953 with the introduction of automated PV production plants.
+Agricultural and other products have been dried by the sun and wind in the open air for thousands of years. The purpose is either to preserve them for later use, as is the case with food; or as an integral part of the production process, as with timber, tobacco and laundering. In industrialised regions and sectors, open air-drying has now been largely replaced by mechanised dryers, with boilers to heat incoming air, and fans to force it through at a high rate. Mechanised drying is faster than open-air drying, uses much less land and usually gives a better quality product. But the equipment is expensive and requires substantial quantities of fuel or electricity to operate.
-Today, PV systems have an important use in areas remote from an electricity grid where they provide power for water pumping, lighting, vaccine refrigeration, electrified livestock fencing, telecommunications and many other applications. With the global demand to reduce carbon dioxide emissions, PV technology is also gaining popularity as a mainstream form of electricity generation. Some tens of thousands of systems are currently in use yet this number is insignificant compared to the vast potential that exists for PV as an energy source.
+'Solar drying' in the context of this technical brief, refers to methods of using the sun's energy for drying, but ''excludes'' open air 'sun drying'. The justification for solar dryers is that they may be more effective than sun drying, but have lower operating costs than mechanised dryers. A number of designs are proven technically and while none are yet in widespread use, there is still optimism about their potential.
-Photovoltaic modules provide an independent, reliable electrical power source at the point of use, making it particularly suited to remote locations. PV systems are technically viable and, with the recent reduction in production costs and increase in conversion efficiencies, can be economically feasible for many applications.
+</div>
-<center>
-[[Image:p01.jpg]]<br /> Figure 1: Array of PV Panels<br />
-</center><blockquote>
-© Smail Khennas/Practical Action
-</blockquote>
-<br /> The use of PV electricity in developing countries
-The majority of the world's developing countries is found within the tropics and hence have ample sources of solar insolation (the total energy per unit area received from the sun). The tropical regions also benefit from having a small seasonal variation of solar insolation, even during the rainy season, which means that, unlike northern industrial countries, solar energy can be harnessed economically throughout the year.
-Currently, there is a fairly high uptake of solar technology in developing countries. The chart below (Figure 2) shows the status of the PV technology worldwide.
-<center>
-[[Image:p02.jpg]]<br /> Figure 2: PV module use by region
-</center></div>
+==How solar dryers work==
-==Technical==
 <div class="booktext">
-The nature and availability of solar radiation
+One well-known type of solar dryer is shown in Figure 1. It was designed for the particular requirements of rice but the principles hold for other products and design types, since the basic need to remove water is the same.
-Solar radiation arrives on the surface of the earth at a maximum power density of approximately 1 kilowatt per metre squared (kWm<sup>-2</sup>). The actual usable radiation component varies depending on geographical location, cloud cover, hours of sunlight each day, etc. In reality, the solar flux density (same as power density) varies between 250 and 2500 kilowatt hours per metre squared per year (kWhm<sup>-2</sup> per year). As might be expected the total solar radiation is highest at the equator, especially in sunny, desert areas.
+Air is drawn through the dryer by natural convection. It is heated as it passes through the collector and then partially cooled as it picks up moisture from the rice. The rice is heated both by the air and directly by the sun.
-Solar radiation arrives at the earth's outer atmosphere in the form of a direct beam. This light is then partially scattered by cloud, smog, dust or other atmospheric phenomenon (see Figure 3 below). We therefore receive solar radiation either as ''direct'' radiation or scattered or ''diffuse'' radiation, the ratio depending on the atmospheric conditions. Both direct and diffuse components of radiation are useful, the only distinction between the two being that diffuse radiation cannot be concentrated for use.
+Warm air can hold more moisture than cold air so the amount required depends on the temperature to which it is heated in the collector as well as the amount held (absolute humidity) when it entered the collector.
 <center>
-[[Image:p03.gif]]<br /> Figure 3: Direct and Diffuse Solar Radiation
+[[Image:Solardrying01.gif]]
+Figure 1: Rice solar dryer
 </center>
-The geometry of earth, sun and collector panel
+The way in which the moisture absorption capability of air is affected by its initial humidity and by the temperature to which it is subsequently heated is shown in Table 1.
-The earth revolves around the sun with its axis tilted at an angle of 23.5 degrees. It is this tilt that gives rise to the seasons. The strength of solar flux density is dependent upon the angle at which it strikes the earth's surface, and so, as this angle changes during the yearly cycle, so the solar insolation changes. Thus, in northern countries, in the depths of winter, the sun will be low in the sky to the south, or may not even be seen at all in arctic regions. The radiation strikes the earth's surface obliquely and solar gain (solar yield) is low. If we are using a solar photovoltaic panel to capture the sun's energy then the orientation of this panel is also critical to the amount of energy we will capture. The relationship is complex and only with sophisticated tracking systems can the maximum energy be extracted for any given location.
+<center>Air enters at 20°C and leaves at 80% RH</center>
-The PV cell, modules and arrays
+<div align="left">
-When light falls on the active surface, the electrons in a solar cell become energised, in proportion to the intensity and spectral distribution (wavelength distribution) of the light. When their energy level exceeds a certain point a potential difference is established across the cell. This is then capable of driving a current through an external load.
+{| border="1" cellpadding="5"
+|- valign="top"
+| valign="top" |
+Initial relative humidity
+| colspan="3" valign="top" |
+Moisture absorption capability (grammes of water/m° of air)
+|- valign="top"
+| valign="top" |
+Not heated
+| valign="top" |
+Heated to 40°C
+| valign="top" |
+Heated to 60°C
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+.3g/m°
+| valign="top" |
+.2g/m°
+| valign="top" |
+.3g/m°
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+.4g/m°
+| valign="top" |
+.2g/m°
+| valign="top" |
+.6g/m°
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+| valign="top" |
+.1g/m°
+| valign="top" |
+.9g/m°
+|}
-All modern, commercial PV devices use silicon as the base material, mainly as monocrystalline or multi-crystalline cells, but more recently also in amorphous form. Other materials such as copper indium diselenide and cadmium telluride are being developed with the aim of reducing costs and improving efficiencies. A mono-crystalline silicon cell is made from a thin wafer of a high purity silicon crystal, doped with a minute quantity of boron. Phosphorus is diffused into the active surface of the wafer. At the front electrical contact is made by a metallic grid; at the back contact usually covers the whole surface. An antireflective coating is applied to the front surface. Typical cell size is about 15 cms diameter. The process of producing efficient solar cells is costly due to the use of expensive pure silicon and the energy consumed, but as materials technology improves costs are slowly dropping making PV technology more attractive.
+</div>
-<center>
-[p04.jpg [[Image:p04.jpg]]]<br /> Figure 4: PV price trends
-</center>
-Cost has been the major barrier to the widespread uptake of PV technology. Cost of PV modules is usually given in terms of Peak Watt (Wp), which is the power rating of the panel at peak conditions - that is at 1kWm<sup>-2</sup> irradiance at 25°C.
-Solar cells are interconnected in series and in parallel to achieve the desired operating voltage and current. They are then protected by encapsulation between glass and a tough resin back. This is held together by a stainless steel or aluminium frame to form a ''module''. These modules, usually comprised of about 30 PV cells, form the basic building block of a ''solar array''. Modules may be connected in series or parallel to increase the voltage and current, and thus achieve the required solar array characteristics that will match the load. Typical module size is 50Wp and produces direct current electricity at 12V (for battery charging for example).
-Commercially available modules fall into three types based on the solar cells used.<br />
-<blockquote>
-• ''Mono-crystalline cell modules.'' The highest cell efficiencies of around 15% are obtained with these modules. The cells are cut from a mono-crystalline silicon crystal.
-• ''Multi-crystalline cell modules''. The cell manufacturing process is lower in cost but cell efficiencies of only around 12% are achieved. A multi-crystalline cell is cut from a cast ingot of multi-crystalline silicon and is generally square in shape.
-• ''Amorphous silicon modules.'' These are made from thin films of amorphous silicon where efficiency is much lower (6-9%) but the process uses less material. The potential for cost reduction is greatest for this type and much work has been carried out in recent years to develop amorphous silicon technology. Unlike monocrystalline and multi-crystalline cells, with amorphous silicon there is some degradation of power over time.
-</blockquote>
-<br /> An array can vary from one or two modules with an output of 10W or less, to a vast bank of several kilowatts or even megawatts.<br />
-<blockquote>
-• ''Flat plate arrays'', which are held fixed at a tilted angle and face towards the equator, are most common. The angle of tilt should be approximately equal to the angle of latitude for the site. A steeper angle increases the output in winter; a shallower angle - more output in summer. It should be at least 10 degrees to allow for rain runoff.
-• ''Tracking arrays'' follow the path of the sun during the day and thus theoretically capture more sun. However, the increased complexity and cost of the equipment rarely makes it worthwhile.
-• ''Mobile (portable) arrays'' can be of use if the equipment being operated is required in different locations such as with some lighting systems or small irrigation pumping systems.
-</blockquote></div>
-==Solar PV systems==
-<div class="booktext">
-PV systems are most commonly used for stand-alone applications. They can be used to drive a load directly; water pumping is a good example - water is pumped during the hours of sunlight and stored for use; or a battery can be used to store power for use for lighting during the evening. If a battery charging system is used then electronic control apparatus will be needed to monitor the system. All the components other than the PV module are referred to as the balance-of-system (BOS) components. Below, in Figure 5, three possible configurations of stand-alone PV system are shown. Such systems can often be bought as kits and installed by semi-skilled labour. (Source: The Power Guide, ITDG Publishing 1994)
-For correct sizing of PV systems, the user needs to estimate the demand on the system, as well as acquiring information about the solar insolation in the area (approximations can be made if no data is readily available). It is normally assumed that for each Wp of rated power the module should provide 0.85watt hours of energy for each kWhm<sup>-2</sup> per day of insolation (Hulscher 1994). Therefore if we consider a module rated at 200 Wp and the insolation for our site is 5 kWhm<sup>-2</sup> per day (typical value for tropical regions), then our system will produce 850Wh per day (that is 200 x 0.85 x 5 = 850).
-Some systems use lenses or mirrors to concentrate direct solar radiation onto smaller areas of solar cells. As the power output is directly proportional to the solar power directed onto the PV cell, this method is useful for reducing the area required for collection. The cost of the concentrators, however, often offset the cost of savings made on reducing the size of the module.
-<center>
-[[Image:p06.gif]]<br /> Figure 5: Common configurations of PV systems
-</center>
-Some benefits of photovoltaics<br />
-<blockquote>
-• No fuel requirements - In remote areas diesel or kerosene fuel supplies are erratic and often very expensive. The recurrent costs of operating and maintaining PV systems are small.
-• Modular design - A solar array comprises individual PV modules, which can be connected to meet a particular demand.
-• Reliability of PV modules - This has been shown to be significantly higher than that of diesel generators.
-• Easy to maintain - Operation and routine maintenance requirements are simple.
-• Long life - With no moving parts and all delicate surfaces protected, modules can be expected to provide power for 15 years or more.
-• National economic benefits - Reliance on imported fuels such as coal and oil is reduced.
-• Environmentally benign - There is no pollution through the use of a PV system - nor is there any heat or noise generated which could cause local discomfort. PV systems bring great improvements in the domestic environment when they replace other forms of lighting - kerosene lamps, for example.
-</blockquote>
-<br /> PV applications in lesser developed countries
-''1. Rural electrification''<br />
-<blockquote>
-• lighting and power supplies for remote building (mosques, churches, temples etc farms, schools, mountain refuge huts) - low wattage fluorescent lighting is recommended
-• power supplies for remote villages
-• street lighting
-• individual house systems
-• battery charging
-• mini grids
-</blockquote><center>
-[[Image:p07.jpg]]<br /> Figure 6: PV can be used to power water pumping systems<br />
-</center><blockquote>
-© Practical Action
-</blockquote>
-<br />''2. Water pumping and treatment systems''<br />
-<blockquote>
-• pumping for drinking water<br /> • pumping for irrigation<br /> • dewatering and drainage<br /> • ice production<br /> • saltwater desalination systems<br /> • water purification
-</blockquote>
-<br />''3. Health care systems''<br />
-<blockquote>
-• lighting in rural clinics<br /> • UHF transceivers between health centres<br /> • vaccine refrigeration<br /> • ice pack freezing for vaccine carriers<br /> • sterilises<br /> • blood storage refrigerators
-</blockquote><center>
-[p08.gif [[Image:p08.gif]]]<br /> Figure 7: PV is frequently used to power vaccine refrigeration in remote health centres
-</center>
-''4. Communications''<br />
-<blockquote>
-• radio repeaters<br /> • remote TV and radio receivers<br /> • remote weather measuring<br /> • mobile radios<br /> • rural telephone kiosks<br /> • data acquisition and transmission (for example, river levels and seismographs)
-</blockquote>
-<br />''5. Transport aids''<br />
-<blockquote>
-• road sign lighting<br /> • railway crossings and signals<br /> • hazard and warning lights<br /> • navigation buoys<br /> • road markers
-</blockquote>
-<br />''6. Security systems''<br />
-<blockquote>
-• security lighting<br /> • remote alarm system<br /> • electric fences
-</blockquote>
+Table 1: The drying process
-<br />''7. Miscellaneous''<br />
+The objective of most drying processes is to reduce the moisture content of the product to a specified value. Moisture content (wet basis) is expressed as the weight of water as a proportion of total weight. The moisture content of rice has typically to be reduced from 24% to 14%. So to dry one tonne of rice, 100kg of water must be removed.
-<blockquote>
+If the heated air has a 'absorption capacity' of 8g/m<sup>3</sup> then 100/0.0008 = 12,500/m<sup>3</sup> of air are required to dry one tonne of rice.
-• ventilation systems<br /> • calculators<br /> • pumping and automated feeding systems on fish farms<br /> • solar water heater circulation pumps<br /> • boat/ship power<br /> • vehicle battery trickle chargers<br /> • earthquake monitoring systems<br /> • emergency power for disaster relief
+The heat required to evaporate water is 2.26kJ/kg. Hence, approximately 250MJ (70kWh) of energy are required to vaporise the 100kg water. There is no fixed requirement for solar heat input to the dryer. This is because the incoming ambient air can give up some of its internal energy to vaporise the water (becoming colder in the process). Indeed, if the ambient air is dry enough, no heat input is essential.
-</blockquote></div>
+Nevertheless, extra heat is useful for two reasons. First, if the air is warmer then less of it is needed. Second, the temperature in the rice grains themselves may be an important factor, especially in the later stages of drying, when moisture has to be 'drawn' from the centres of the grains to their surfaces. This temperature will itself depend mainly on the air temperature but also on the amount of solar radiation received directly by the rice.
-==Other issues==
+In a natural convection system, the flow of air is caused by the fact that the warm air inside the dryer is lighter than the cooler air outside. This difference in density creates a small pressure difference across the bed of grain, which forces the air through it. This effect increases, the greater is the height of the bed above the inlet (h1) and the outlet above the bed (h2). The effect of an increased h2 is less than that of an increased h1 because the air is cooled as it passes through the bed.
-<div class="booktext">
+Approximate densities for a variety of cases are shown in Table 2.
-Manufacture in developing countries
+<center>Air enters at 20 °C and leaves at 80% RH</center>
-PV technology is sophisticated and the manufacturing plant is expensive. There is little scope for local manufacture in rural areas of developing countries, although some BOS components such as frameworks for mounting PV modules can be made in small workshops and will save on expensive transportation costs. There are however, large-scale manufacturers of PV modules working in developing countries. In India, for example, Central Electronics of Ghaziabad is not only the nation's largest PV producer, but are the fifth largest producer of monocrystalline silicon solar cells in the world (D.V. Gupta cited in Garg et al, 1997). There are over 60 companies in India alone producing solar cells, modules and systems.
-Dissemination
 <div align="left">
@@ Line 230: / Line 109: @@
 |- valign="top"
 | valign="top" |
-There is a vast scope for and potential for the use of PV technology in India. There are still over 90,000 villages in the country to be electrified. Recognising the importance of PV technology in the Indian context, the Government has been implementing a comprehensive programme covering R & D, demonstration, commercialisation and utilisation for more than 15 years.
+Initial relative humidity
+| colspan="4" valign="top" |
-Among the elements of the action plan are the following aims:<br />
+Density of the air (kg/m<sup>3</sup>) (Drop in density, in brackets)
+|- valign="top"
-<blockquote>
+| valign="top" |
+Not heated
-• deployment of 400,000 solar lanterns as a substitute for kerosene lanterns<br /> • rural electrification through PV systems covering 400 villages/hamlets<br /> • a special programme on water pumping systems<br /> • intensified R & D on technologies which can lead to a reduction in cost<br /> • commercialisation of PV systems for various applications by giving a market orientation to the programme and promoting manufacturing and related activities
+| valign="top" |
+Heated to
-</blockquote>
+|- valign="top"
+| valign="top" |
-<br /> As a result of these measures India is among the leading countries in the world in the development and use of PV technology.
+°C
+| valign="top" |
+°C
+| valign="top" |
+°C
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+Ambient 1.19
+| valign="top" |
+.19
+| valign="top" |
+.19
+| valign="top" |
+.19
+|- valign="top"
+| valign="top" |
+Below bed 1.19 (.00)
+| valign="top" |
+.15 (.04)
+| valign="top" |
+.12 (.07)
+| valign="top" |
+.05 (.14)
+|- valign="top"
+| valign="top" |
+Above bed 1.21 (-.02)
+| valign="top" |
+.19 (.00)
+| valign="top" |
+.17 (.02)
+| valign="top" |
+.14 (.05)
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+Ambient 1.19
+| valign="top" |
+.19
+| valign="top" |
+.19
+| valign="top" |
+.19
+|- valign="top"
+| valign="top" |
+Below bed 1.19 (.00)
+| valign="top" |
+.15 (.04)
+| valign="top" |
+.11 (.08)
+| valign="top" |
+.05 (.14)
+|- valign="top"
+| valign="top" |
+Above bed 1.20 (-.01)
+| valign="top" |
+.18 (.01)
+| valign="top" |
+.16 (.03)
+| valign="top" |
+.13 (.06)
+|- valign="top"
+| valign="top" |
+%
+| valign="top" |
+Ambient 1.18
+| valign="top" |
+.18
+| valign="top" |
+.18
+| valign="top" |
+.18
+|- valign="top"
+| valign="top" |
+Below bed 1.18 (.00)
+| valign="top" |
+.14 (.04)
+| valign="top" |
+.11 (.07)
+| valign="top" |
+.04 (.14)
+|- valign="top"
+| valign="top" |
+Above bed 1.18 (.00)
+| valign="top" |
+.16 (.02)
+| valign="top" |
+.15 (.03)
+| valign="top" |
+.11 (.07)
 |}
-</div><blockquote>
+</div>
-Source: E.V.R. Sastry, cited in Garg et al, 1997.
-</blockquote>
-<br /> Hybrid systems
-Solar PV systems can be used in conjunction with other energy technologies to provide an integrated, flexible system for remote power generation. These systems are referred to as hybrid systems. Common configurations of hybrid systems could include a solar PV array, wind generator and diesel generator set which would allow generation in all weather conditions. Such systems need careful planning.
+Table 2: Air density variation in a natural convection dryer
-Solar lanterns
+It can be seen that if the incoming air is heated by only 10-30°C then the presence of a chimney on top of the dryer would make little or no difference, unless it acted efficiently as a solar collector and raised the temperature of the air significantly.
-A recent innovation in solar technology is the solar lantern. Originally designed for the outdoor leisure market in western countries, this simple lantern with a small PV module (5-10 watts) is extremely appropriate to use in rural areas of developing countries for replacing kerosene lamps. Cost is still a barrier, as is the potential for local manufacture, but there is enormous scope for widespread dissemination of a simple, robust solar lantern.
+It should be noted that even if the difference in densities is as much as .05kg/m<sup>2</sup>, then the resulting pressure difference is only 0.5 Pa (5 millionths of atmospheric pressure) per metre of chimney. For comparison, forced convection systems commonly operate with pressure differences of 100-500 Pa.
-Micro grids or stand-alone
+Many products are damaged by excessive temperatures. The most severe constraints are on beans (35°C), rice (45°C), and all grains if they are to be used for seed (45°C).
-Solar PV technology is presently best suited to stand-alone applications but can also be used for providing power for small grid systems, with centralised power generation. As the cost of PV cell production drops, their use for medium scale electricity production is being adopted more widely. There is also scope for large-scale electricity production for such applications as peak power provision.
+Other types of dryers and their performance
 </div>
-==Resources and references==
+==Forced convection solar dryer==
 <div class="booktext">
-H.P. Garg, D. Gouri, and R. Gupta: Renewable Energy Technologies. Indian Institute of technology and the British High Commission, 1997.
+(Figure 2)
-S. Karekezi and T. Ranja: Renewable Energy Technologies in Africa. AFREPREN/SEI/Zed Books, 1997.
+By using a fan to create the airflow, drying time can be reduced by a factor of 3. Also, the area of collector required is reduced by up to 50%. Therefore, the area of collector required for a given throughput of product could be reduced by a factor of 5-6. The initial cost of a one tonne per day dryer is in the region of £1500-2000. The fan would consume about 500 watts for 6 hours, and so electricity cost (at 0.07/kWhr) would be about 0.20 per tonne of rice dried
-T.B. Johansson, H. Kelly, A.K.N. Reddy and R.H. Williams: Renewable Energy - Sources for fuels and electricity. Island Press, 1993.
+<center>
-J. Twidell and T. Weir: Renewable Energy Resources. E & F.N. Spon, 1990.
+[[Image:p04a.gif]]<br /> Figure 2: Forced convection solar dryer
-W. Hulscher and P. Fraenkel: The Power Guide - An international catalogue of small-scale energy equipment. ITDG Publishing, 1994.
+</center>
-A. Derrick, C. Francis and V. Bokalders: Solar Photovoltaic Products - A guide for development workers. IT Publications and IT Power, 1991.
+'''Tent dryer'''
-J.P. Louineau, M. Dicko, P. Fraenkel, R. Barlow and V. Bokalders: Rural Lighting - A guide for development workers. IT Publications and The Stockholm Environment Institute, 1994.
+The distinguishing feature of tent, box and cabinet dryers is that the drying chamber and the collector are combined into one, see Figure 3. This allows a lower initial cost. Drying times are however not always much lower than for open-air drying. (Probably, insufficient attention has so far been paid to utilising natural convection.) The main purpose of the dryers may be to provide protection from dust, dirt, rain, wind or predators and they are usually used for fruit, fish, coffee or other products for which wastage is otherwise high. There are numerous other types. Greenhouse dryers are a more sophisticated version of tent dryers. Box dryers may incorporate thermal insulation to achieve higher temperatures. Storage bin dryers combine the functions of drying and long-term storage. Solar timber kilns may include hot water storage to enable the necessary control of drying rate.
-S. Roberts: Solar Electricity - A practical guide to designing and installing photovoltaic systems. Prentice Hall, 1991.
+<center>
-G. Foley: Photovoltaic Applications in Rural Areas of the Developing World. World Bank, 1995.
+[p04b.gif [[Image:p04b.gif]]]<br /> Figure 3: Tent dryer
-A. Cabraal, M. Cosgrave-Davies and L. Schaeffer: Best Practices for Photovoltaic Household Electrification Programs. World Bank, 1996.
+</center></div>
-</div>
+==Solar drying or open-air drying?==
-==Internet addresses==
 <div class="booktext">
-Solarbuzz Inc.<br />http://www.solarbuzz.com
+The great advantage of open-air drying is that it has little or no equipment costs. It is labour-intensive but this may not involve much economic cost in rural areas in developing countries. It requires about three times as much land (100m<sup>2</sup> per tonne of rice) compared to solar drying, but this may not matter too much in many cases.
-British Photovoltaic<br /> Association http://www.pv-uk.org.uk
+Firstly, one important advantage of solar drying is that the product is protected from rain, insects, animals and dust that may contain faecal material. Some systems also give protection from direct sunlight. Second, faster drying reduces the likelihood of mould growth. Third, higher drying temperatures mean that more complete drying is possible, and this may allow much longer storage times (but only if rehumidification is prevented in storage). Finally, more complex types of solar dryers allow some control over drying rates.
-International Centre for Application of Solar Energy<br />http://www.case.gov.au
+</div>
-International Energy Agency Photovoltaic Power Systems Programme<br />http://www.iea-pvps.org/
+==Solar dryers or fuelled dryers?==
-Ekomation Solar Energy Consultants, Netherlands<br />http://www.pvportal.com
+<div class="booktext">
-Home Power Magazine<br />http://www.homepower.com/
-U.S. National Centre for Photovoltaics<br />http://www.nrel.gov/ncpv
+The choice between using solar radiation or fuel, to heat the air is mainly one between a higher initial cost and continuing fuel costs which needs to be analysed for each location.
-Centre for Renewable Energy and Sustainable Technology<br />http://www.solstice.crest.org
+In some circumstances, it may be possible to burn rice husks or other fuel with low opportunity cost. One tonne of rice gives 200kg of husks.
-Independent site operated by ECOFYS BV, Utrecht, Netherlands<br />http://www.mysolar.com
+Fuel heating usually allows better control of the drying-rate than solar heating; it also enables drying to be continuous. If either of these is required, a combined system may be appropriate with pre-heating of air by solar energy.
-ISES's World-wide Information System for Renewable Energy<br />http://www.wire.ises.org
 </div>
-==Manufacturers/Suppliers of photovoltaic products==
+==Which solar dryer?==
 <div class="booktext">
-Note: This is a selective list of suppliers and does not imply endorsement by Practical Action.
+The choice between alternative types of solar dryer will depend on local requirements and in particular on the scale of operation. If intended for peasant farmers then initial capital cost may be the main constraint and plastic-covered tent or box dryers may be appropriate.
-Solar systems
+There may however be a trend towards more centralised drying to enable more intensive usage of the equipment. The greater initial cost of glass covers may then be affordable, and grid electricity may be available to run fans and obtain much faster throughput for a given collector area.
-Siemens Bangladesh,<br /> Jiban Bian Tower, 12<sup>th</sup> Floor, 10 Dikusha<br /> Commercial Area, Dhaka 1000, Bangladesh<br /> Tel: +880 2 956 3734<br /> Fax: +880 2 956 3740<br /> Email: [mailto:sblpower@bangla.net sblpower@bangla.net]
+For intermediate scale and capital cost the natural convection rice dryer is a well proven design.
-Animatics Ltd.,<br /> Haile Selassie Avenue, P.O. Box 72011, Nairobi, Kenya<br /> Tel: +254 2 210 300<br /> Fax: +254 2 210 315
+</div>
-Lotus Energy Pvt. Ltd.,<br /> Bhatbhateni Dhunge Dhara, P.O. Box 9219, Kathmandu, Nepal<br /> Tel: +977 1 418 203<br /> Fax: +977 1 412 924<br /> Email: [mailto:info@lotusenergy.com info@lotusenergy.com]<br /> Web: http://www.lotusenergy.com
+=='''References and further reading'''==
-CIME Commercial S.A.,<br /> Av. Libertadores # 757, San Isidro, Lima 27, Peru<br /> Tel: +511 222 6083<br /> Fax: +511 222 6330
+'''This Howtopedia entry was derived from the Practical Action Technical Brief ''Solar Drying''.  <br />To look at the original document follow this link:
+http://www.practicalaction.org/?id=technical_briefs_food_processing
-Link Intertrade (Private) Ltd.,<br /> 385C Old Kotte Road, Rajagiriya, Sri Lanka<br /> Tel: +94 1 873 211-2<br /> Fax: +94 1 867 952<br /> Email: [mailto:intertrade@link.lk intertrade@link.lk]
-Solarman Co.,<br /> P.O. Box 11545, Khartoum, Sudan<br /> Tel: +249 11 472 337<br /> Fax: +249 11 473 138<br /> Email: [mailto:solarman29@hotmail.com solarman29@hotmail.com]
+'''A survey of solar agricultural driers''' - Brace Research Institute - 1975<br />'''Preparing grain for storage''' - Action/Peace Corps and VITA - 1976<br />'''Solar driers''' - Commonwealth Science Council - 1985<br />'''Solar drying''' - Practical methods of food preservation - ILO 1988<br />''Producing Solar Dried Fruit and Vegetables for Small-scale Enterprise Development'' - NRI 1996<br />''Try Drying It!: Case studies in the dissemination of tray drying technology'' - IT Publishing 1991
-Alternative Technologies Pvt. Ltd.,<br /> 3 Canald Road, Graniteside, Harare, Mash Cent, Zimbabwe<br /> Tel: +263 4 781 972-7<br /> Fax: +263 4 775 264<br /> Email: [mailto:snakes@zambezi.net snakes@zambezi.net]
+</div>
-Kenital Solar Energy,<br /> Ngong Road, P.O. Box 19764, Nairobi, Kenya<br /> Tel: +254 2 715 960<br /> Fax: +254 2 718 959<br /> Email: [mailto:info@kenital.com info@kenital.com]<br /> Web: http://www.kenital.com
-Wisdom Light Group (P) Ltd.,<br /> Gha/2, 178 Siphal-Kalopul, GPO Box 6921, Kathmandu, Nepal<br /> Tel: +977 1 483 154/5<br /> Fax: +977 1 482 154<br /> Email: [mailto:info@wisdomlight.com.np info@wisdomlight.com.np]<br /> Web: http://www.wisdomlight.com.np
-Ferreyros,<br /> Av. Industrial 675-Lima, Peru<br /> Tel: +511 336 7070<br /> Fax: +511 336 8331<br /> Web: http://www.ferreyros.com.pe
-Solar Power & Light Co. Ltd.,<br /> 10 Havelock Place, Colombo 5, Sri Lanka<br /> Tel: +94 1 688 730<br /> Fax: +94 1 686 307
-U.T.E. Group of Companies,<br /> P.O. Box 2074, Khartoum, Sudan<br /> Tel: +249 11 70147<br /> Fax: +249 11 70147
+==Usefull addresses==
+'''Practical Action'''
+The Schumacher Centre for Technology & Development, Bourton on Dunsmore, RUGBY, CV23 9QZ, United Kingdom.<br />
+'''Tel.:''' +44 (0) 1926 634400, '''Fax:''' +44 (0) 1926 634401
+Email: [mailto:practicalaction@practicalaction.org.uk practicalaction@practicalaction.org.uk] '''web:'''http://www.practicalaction.org
+<center>[[Image:Pa-logo-200x103.gif]]</center>
-Solamatics,<br /> 31 Edison Crescent, Graniteside, P.O. Box 2851, Harare, Zimbabwe<br /> Tel: +263 4 749 930<br /> Fax: +263 4 771 212<br /> Email: [mailto:mikem@mcdiarmid.co.zw mikem@mcdiarmid.co.zw]
+NR International<br /> Central Avenue<br /> Chatham Maritime<br /> Kent<br /> ME4 4TB<br /> United Kingdom<br /> Tel: +44 1634 880088<br /> Fax: +44 1634 880066/77<br /> <br /> Website: http://www.nrinternational.co.uk/
-Solar modules
+=='''Categories:'''==
-Shell Solar<br /> Postbus 3049, 5700 JC, Helmond, Netherlands<br /> Tel: +31 492 50 86 08<br /> Fax: +31 492 50 86 00<br /> Email: [mailto:info@shellsolar.nl info@shellsolar.nl]<br /> Web: http://www.shellsolar.com
+[[Category:Medium]]
+[[Category:Less than 50 US$]]
-Sharp Photovoltaics Div<br /> 282-1 Hajikami, Shinjo-cho, Kita-Katsuragigun,<br /> Nara Prefecture 639-2198, Japan<br /> Tel: +81 745 63 3579<br /> Fax: +81 745 62 8253<br /> Email: [mailto:webmaster@sharp.co.jp webmaster@sharp.co.jp]<br /> Web: http://www.sharp.co.jp
+[[Category:Up to 5 Persons]]
+ [[Category:Global Technology]]
-BP Solar Headquarters<br /> 989 Corporate Drive, Linthicum, Maryland, MD 21090, USA<br /> Tel: +1 410 981 0240<br /> Email:<br /> Web: http://www.bpsolar.com
+ [[Category:Energy]] [[Category:Agriculture]]  [[Category:Food Processing]] [[Category:Health]] [[Category:Ideas]] [[Category:Small Business]] [[Category:Products]]
-Bharat Heavy Electricals Ltd. (BHEL)<br /> Jeevan Tara Building, 5 Sansad Marg, New<br /> Delhi, 110001, India<br /> Tel: +91 11 334 7331<br /> Fax: +91 11 334 2769<br /> Email: [mailto:bhelsanchar@vsnl.com bhelsanchar@vsnl.com]<br /> Web: http://www.bhel.com
-Angewandte Solar Energie (ASE) GmbH (now RWE Solutions)<br /> Industriestrasse 13, Alzenau, Germany D63755<br /> Tel: +49 (0)6023 91 1712<br /> Fax: +49 (0)6023 91 1700<br /> Email: [mailto:asesales@ase.tessag.com asesales@ase.tessag.com]<br /> Web: http://www.ase-international.com
-Photowatt International SA (Part of Matrix Solar Technologies Inc.)<br /> 33 rue St. Honore, Z.I. Champfleuri,<br /> 38300 Bourgoin Jallieu, France<br /> Tel: +33 (0)474 93 80 20<br /> Fax: +33 (0)474 93 80 40<br /> Email: [mailto:marketing@photowatt.com marketing@photowatt.com]<br /> Web: http://www.matrixsolar.com
-Central Electronics Ltd. (CEL)<br /> 4 Industrial Area, Sahibabad, Uttar Pradesh 201010, India.<br /> Tel: +91 120 4771941, 4771910<br /> Fax: +91 120 4771843<br /> Email: [mailto:cel@celsolar.com cel@celsolar.com]<br /> Web: http://www.celsolar.com
-'''Practical Action, The Schumacher Centre for Technology & Development'''<br />'''Bourton Hall, Bourton-on-Dunsmore, Rugby, Warwickshire CV23 9QZ, UK'''<br />'''Tel: +44 (0)1926 634400 Fax: +44(0)1926 634401'''<br />'''E-mail: [mailto:Infoserv@practicalaction.org.uk Infoserv@practicalaction.org.uk] Web: http://www.practicalaction.org'''
-'''Intermediate Technology Development Group Ltd Patron HRH -'''<br />'''The Prince of Wales, KG, KT, GCB'''<br />'''Company Reg. No 871954, England Reg. Charity No 247257 VAT No 241 5154 92'''<br />
-</div>
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