GB2158930A - Drying tea with solar heat - Google Patents

Abstract

Heat-sensitive materials, such as tea, are dried with two or more separate streams of hot air generated from solar collectors, with some heat input from electrical heaters, steam-heated heat exchangers or solar-heated heat reservoirs. The air streams may be applied in succession, at different temperatures. Two solar heater may supplied with the same air stream in succession, and air exhausted from the dryer may be used to heat a reservoir, the heat from which is used to pre-heat air fed to a solar collector.

Description

SPECIFICATION Improvements in the application of solar energy for dehydration Various methods of drying are known, and while many materials can be dried in a satisfactory manner using a wide range of temperatures and time scales, there are a large number of heat sensitive materials which need to be dried under closely controlled conditions in order to avoid damage or degradation. Indeed in some instances some of the quality characteristics of the produce are a direct result of the conditions of dehydration. Many materials of vegetable or animal origin and certain pharmaceutical and chemical products fall into this category.

Many materials exhibit several distinct stages of drying. The first is usually a more rapid loss of superficial or interstitial moisture at or near the surface of the particle being dried; the later stages of drying are often slower as the moisture level drops and the slow diffusion of the more closely bound water molecules becomes the rate governing factor. In the first stage of drying lower temperatures are usually adequate when warm air is used to remove readily available moisture. In the later stages higher temperatures may be required to counter the increased difficulty of water removal and the limitations imposed by relative humidity equilibria.

Air cooled Solar collectors have advantages in cases where conventional fuel for heating hot air is either scarce or expensive, but the output of hot air from the collectors can vary over short time scales owing to rapid changes in insolation levels.

Such collectors particularly those of the concentrating type are capable of providing temperatures adequate for many such drying operations, but it is well known that the efficiency of energy collection by such collectors is markedly dependent on the air flow rate passing through the collecting system, and on the air temperature being demanded.

This invention relates to a process for the operation and control of systems for dehydrating heat sensitive moist materials using two or more separate streams of hot air at different temperatures generated by means of solar collectors. The various air streams may be controlled within specific temperature ranges according to the requirements of the specific dehydration process.

The process overcomes some of the problems associated with the drying of heat sensitive materials by carrying out the dehydration step in several stages. In this way the initial moisture removal is carried out by air cooled solar collectors operating at a high level of efficiency at lower air temperatures, while the later drying stages are carried out at higher temperatures using collectors which are either of a different design, better suited to produce high temperatures, or are operated at a lower efficiency.

The process may be controlled by employing heat storage facilities in combination with back up heat available from conventional sources and utilising established methods for controlling a solar drying system such as those described in British Patent Application No. 8117394.

A specific embodiment of the process utilises two air streams, heated by separate solar collectors to feed a two stage drying process. In this situation the lower of the two temperatures will typically lie within the range 50-90"C and the higher temperature would typically lie within the range 75-110"C.

The invention will now be described by reference to the following drawings. Fig. 1 illustrates the main features of the process as applied to a continuous drying system. Fig. 2 shows a simpler batch system in which it is possible to use either two separate solar collectors for the two periods of drying, or utilise 1 collector to operate at a lower temperature for the first part of the batch drying cycle, and then raise the temperature by, for example reducing the air flow rate, for the latter part of the drying cycle.

While the description refers specifically to a two stage process employing one or two solar collectors it should be understood that the process may also utilise more than two stages and more than two separate air streams derived from a multiplicity of solar collectors.

Referring to Fig. 1, solar energy is collected by solar collectors 1 and 2 through which air is drawn by fans 3 and 4 respectively. It should be understood that collectors 1 and 2 may be of the same or different designs but will be operated in such a manner that the air stream from fan 4 will be at a substantially higher temperature than that from fan 3. The outputs from fans 3 and 4 are directed to separate drying sections of the continuous drier 9. Small heat storage units, 5 and 6, for example of the rock type, and back up heater units 7 and 8 are placed in the respective air flows. The heater units 7 and 8 may be steam heated radiators or other conventional heaters.

Surplus heat from collectors 1 and 2 may be stored by diverting part of the air streams to the larger heat stores 10 and 11. These may be of the water or rock type or any other suitable storage systems. Heat may then be recovered from these stores during periods of poorer insolation, or at night. This is achieved by resetting of dampers at A,B,C,D,E,F,G and H.

Further heat can be recovered by passing the exhaust from drier 9 through a heat exchanger or store, 12, from which the heat can be withdrawn for use, for example, in pre-heating incoming fresh air.

It may be advantageous under certain conditions to use a pre-heated air flow to collector 2 in order to assist in achieving a higher output temperature. This may come from heat store 12, or from collector 1, via fan 3. This is depicted by the dotted routes J and K respectively.

During periods of relatively short reductions in insolation levels, heat stores 5 and 6 will assist in preventing the temperature falling too sharply and back-up heaters 7 and 8 may or may not be needed. For longer periods of low insolation levels, for example in the late part of the day or after dark, the air flows through 10 and 11 are reversed, and it may then become necessary to supply heat via conventional heaters 7 and 8. Simple and well established methods of air flow and temperature control are available. These are not shown in Fig. 1 although naturally they would be incorporated in the practical applications of the process.

Fig. 2 illustrates the batch drying version of the invention. Solar collectors 1 and 2 which may be of the same or different design, collect heat energy and fans 3 and 4 are used to draw air through 1 and 2 respectively, but the air flows are combined in a common duct containing a small heat store 5 and back up heater 7 before entering drier 9. The two solar collectors may not be needed simultaneously and while one is providing heat to the drier, the other can be storing heat in the main stores, 10 and 11. For example, during the first stage of drying air is drawn through collector 1 by fan 3 and ducted through 5 and 7 to drier 9 by suitable positioning of dampers, A,B,C and D, while at the same time air is drawn through collector 2 by fan 4 and fed to store 11 by suitable positioning of dampers E,F,G and H.During the second stage of drying the air from collector 2 which is operated in such a manner that the temperature of the air flowing from it is substantially higher than that flowing from collector 1, is passed to the drier by appropriate positioning of the dampers. The air from collector 1 may be routed either to heat store, 10 or used to supply the air feed to collector 2, via route K.

As with the continuous version of the process a further heat store 12, may be used to collect heat from the drier exhaust and the dotted lines J,L depict the route whereby heat may be withdrawn from the heat store 1 2 for the purpose of preheating the air fed to collectors 1 and 2 respectively.

The batch process can be simplified by eliminating one of the main heat stores 10 or 11, and arranging the ducting and dampers to that each collector alternately feeds the drier and the same heat store in turn.

A further simplification involves eliminating one of the solar collectors and its ancilliary fans and ducts. In order then to carry out the drying effectively it becomes desirable to adjust the operation of the solar collector, for example by reducing the air flow rate, in order to increase the temperature of the air during the later stages of drying.

This solution sacrifices efficiency in the interests of a saving of capital expenditure.

While it should be understood that the solar collectors referred to herein may be of any type, simple flat plate collectors will commonly be the most appropriate for supplying warm air in the lower temperature ranges, while more sophisticated collectors, e.g. those involving concentration and focussing of the sun's rays on a suitable absorber will prove more effective for generating the higher temperatures.

A specific embodiment of the process can be illustrated by the drying of moist fermented tea.

Tea is an example of a heat sensitive material which exhibits the two distinct drying rates as described above, combined with the need to control precisely the drying times and temperatures for reasons of product quality.

Example The apparatus consisted of a pair of solar collectors, of 2m2 surface area each, connected by ducting and a fan to a drying chamber. This took the form of a box containing a tray made of a perforated wire mesh. The hot air entered the foot of the box and passed upwards through the wire mesh on which sat the layer of moist tea. The system was operated in batch fashion. The moist tea was submitted to a drying cycle consisting of lower and higher temperature periods of approximately 10 minutes each. An electrical heater placed in the duct carrying the hot air from the solar collectors to the drying chamber, provided additional heat to cover variations in the level of solar energy available.

The moist tea being dried was taken from an adjacent tea factory operating under normal conditions of tea manufacture, and samples of tea made simultaneously in this factory and in the experimental apparatus were taken for comparison purposes. These samples were then submitted to professional tasters for their expert opinion.

Results Obtained Weight of moist tea sample 450g Weight of dried tea product 240g Zage heat energy supplied by solar panels 89% %age heat energy from back up heater 11% Final product moisture 3.5% Time of drying cycle 21 mins Efficiency of solar collection (average) 61% 2 Solar insolation levels 820-850 w/m Weather - slight haze The air temperature was kept below 75"C for the first seven minutes then raised slowly over the next few minutes of 90"C at which temperature the run was completed.

This sample together with the corresponding control sample from the factory was assessed by a reputable firm of London Tea Brokers who reported that the samples were very similar, but a very slight preference was stated for the solar dried sample.

Claims (7)

1. Process for the operation and control of systems of dehydrating heat sensitive materials using a multiplicity of heated air streams, all or part of the heat energy of said air streams being supplied by solar collectors and the temperature of each of such air streams being controlled within different limits according to the requirements of the specific dehydration process.

2. Process according to Claim 1 using two air streams one of which operates in the range 50-90"C and the other within the range 75-110"C.

3. Process according to Claim 1 where the heat energy of the multiple air streams is provided by different designs of solar collectors according to the temperature required.

4. Process according to Claim 1 in which the solar collectors were used in conjunction with heat stores to minimise fluctuations in temperature or to allow operation after sunset.

5. Process according to Claim 2 in which heat from the lower temperature stream is used to preheat air for the higher temperature stream.

6. Process according to Claim 1 in which the material to be dehydrated is tea.

7. Products dehydrated by the process according to any of the preceeding claims.