Using the inputs available, namely the characteristics of drying product, characteristics of drying air and location, each component was first analysed individually to obtain a co-relation of various process parameters. A method for sizing of solar water heating system was established and then the load behaviour was matched with the supply of energy available from the solar system. This thesis finally looks at a method to compare the economics of soalr energy system and conventional fuel system.
INTRODUCTION
1.1 Context
Food is a perishable commodity. A global supply chain allows food grown in one part of the world to be consumed elsewhere. This has led to an increasing need for superior preservation techniques. Preservation for longer shelf life is important not just for ease of transportation but also to reduce wastage in case of excess production in a good season. Farmers are now more concerned about getting better returns from their produce. (Jangam 2010) reports that (Rahman 1999) suggests during harvest and processing, food products are prone to various damages like mechanical, physical, chemical and microbial damage. Out of these, drying mainly reduces or helps overcome potential microbial damage by reducing moisture content below a certain level. If the product has undergone appropriate pre treatment before drying, then chemical damage could also be overcome to certain extent.
Industrial drying requires hot air which is usually provided by conventional fossil fuel based heat sources. Since drying is an 'essential' operation in a variety of industries like pharmaceutical, ceramic, paper, food, mineral and wood processing although the purpose of drying may vary depending on the industry. In some industries drying may be carried out for ease of transportation as in timber drying or to remove solvents from the finishing layers of the product. It could also be done for washing and varnishing (Schnitzer, Brunner et al. 2007). In case of food products it usually involves elimination of moisture. Hot air is the medium mostly used for this. Food products, especially fruits and vegetables can be safely dried using hot air at temperatures of 40-70 ͦC. Drying under controlled conditions of temperature and humidity allows agricultural food products to dry to a safe moisture content whilst retaining their quality (Sharma, Colangelo et al. 1995). The process is energy intensive since latent heat of evaporation is required to remove the vapour. (Jangam 2010) reports various studies have concluded that national energy consumption for industrial drying operations ranges from 10-15% for USA, Canada, France, and UK to 20-25% for Denmark and Germany.
Agricultural areas, mostly rural areas in tropical countries may not have easy access to grid electricity. Renewable energy systems are probably unaffordable or inaccessible to many of the farmers. The most convenient option in such cases remains fuel based heating systems or electrically powered fans, heating coils. Solar powered systems might be a more sustainable alternative in such cases. It could become feasible to many if designed and installed correctly with adequate financial support mechanisms.
1.2 AIMS & OBJECTIVES
This thesis is concerned with incorporating renewable energy sources in food drying. Considerable research has been carried out by (Torres-Reyes, Navarrete-Gonzalez et al. 2002),(Aboul-Enein, El-Sebaii et al. 2000) and many others to evaluate performance of solar air collectors and its integration with cabinet dryers. These systems are usable for drying only when sufficient solar radiation is available since they do not have any thermal storage devices. Performance of solar water heating systems for the same purpose have been explored and analysed to a lesser extent as far as I am aware. This thesis is an attempt to understand the use of solar energy systems for operating shelf type cabinet dryers having conventional fuel based heating systems. Intermittent supply is a well known drawback of solar energy resource. Limited availability of sunlight is a hindrance to making solar hot water the primary heat source in any application. On the other hand, conventional fuel based heating systems are less sustainable and in many locations like rural agricultural areas in tropical countries, conventional fuel could possibly be a costly option being used due to lack of alternative options. Also, drying of food products often requires continuous supply of hot air. Unreliable supply of hot air could increase drying time or deteriorate the quality of product being dried. Hence, a thermal storage set up to store heat to be used in non sunlight hours and solar photo voltaic array with a battery backup has also been proposed in system design in this thesis.
The objectives of this thesis are:
To study the mechanism of drying of various food products and understand the common parameters that affect drying rate and drying characteristics/determine dryer design and design of auxiliary systems like solar or biomass
To understand methods to calculate useful solar radiation available to any solar energy system (PV or thermal) at a given location anywhere in the world on any given day in the year.
To establish a method to match heat load demand from shelf type cabinet dryer while drying of any food product and solar energy supply characteristics on an hourly basis on any day at any location.
To design a simulation tool on MS Excel to be able to perform the above tasks and subsequently determine the size of solar energy system (thermal and photovoltaic) required to optimise the use of renewable energy source.
To analyse economic factors of incorporating solar energy systems along with conventional fuel based heat sources in dryers.
Literature Review
2.1 Industrial Drying
Drying commonly describes the process of thermally removing volatile substances (moisture) to yield a solid product (Majumdar 2007). Convection is the most common heat transfer mechanism in dryers used in the food industry. Further, in case of most products, the moisture on the surface of the food product is eliminated in the drying process rather than the moisture within the product. Hence, flow rates of convection currents play an important role. (Jangam 2010) claim that over 85% of industrial dryers are of the convective type. Freeze dryers, vacuum dryers and superheated steam dryers remain a notable exception to this.
Concerns in the drying industry
(Majumdar 2007) has listed some of the unique features of industrial drying:
Particle sizes vary vastly. Each may require customised drying set up and solution.
Various drying media may be required to account for different porosity.
Time of drying may vary from 0.25 secs (tissue paper) to 5 months (hardwood)
Production capacities ranging from 0.10 kg/h to 100 tons/h.
Drying temperatures vary to a large extent. Food stuff ususally require 40-80ͦC
Operating pressure may range from fraction of mb to 25 atm.
Rate of heat transfer required could be continous or intermittent.
2.1.1 Classification & Selection of Dryers
(Keey 1978) suggests three principal factors that could be used for classifying dryers:
The manner in which heat is supplied to the material.
Temperature and pressure of operation.
Manner in which material is handled within the dryer.
Classification based on Heating Methods
Conduction based dryers - Heat for evaporation is supplied through heated metal surface placed within the dryer. Product is in contact with a metal. Hence it will attain the temperature of metal. The evaporated moisture is then carried away by air stream. Temperature of gas does not influence thermal efficiency.
Convection based dryers - Heat for evaporation is supplied by heated air/gas flowing over the product. Air, Inert gases, superheated steam or direct combustion gases could be used for such dryers. The rate of drying remains constant initially when the surface moisture is being removed. The temperature of the solid approaches wet bulb temperature. Once the surface moisture is lost, it approached dry bulb temperature. Thermal efficiency increases with increase in inlet temperature of drying gas. Product generally adopts adiabatic saturation temperature at that pressure when superheated vapour is used.
While heat supplied solely by radiation is rare and restricted only to infra-red dryers, radiation does gain significance in conduction and convection based dryers.
The convection medium is generally introduced on top of the cabinet in most cases. This leads us to the challenge of ensuring flow of hot air in a manner that heat transfer is optimised, and the product is dried evenly. In cabinet dryers the products remain stationary during the drying process. Hence it may be necessary, in case of some products, to dry them more than once in different positions. But this then leads to loss of time and energy.
2.2 Food Drying
Food processing can refer to many technologies and methods to preserve and/or transform the product from the site of agricultural production to the consumer (Hui 2008). The dehydration of foods for preservation is probably the oldest technique of them all.
2.2.1 Mechanism of drying in solids
Drying fundamentally is a process of heat and mass transfer between a wet solid and a dry/less moist air. During drying of a wet product, simultaneous heat and mass transfer occurs both inside the solid and in the boundary layer of the drying agent (Strumillo and Kudra 1986). The temperature of the wet solid changes due to heat transfer and vapour gets transported from inside the wet solid to its surface. Then the vapour evaporates from surface to the drying medium. Evaporation and vaporisation can remove the unbound vapour from the surface of the product. Vapour pressure can be increased by either raising the temperature of moisture to boiling point or lowering the boiling point itself. One of the following mass transfer mechanisms is generally responsible for the "transport of moisture"(Majumdar 2007):
Liquid diffusion - when wet solid is at temperature below boiling point of the liquid.
Vapour diffusion - if the liquid vaporizes within the material
Knudsen diffusion - when drying takes place at very low temperatures and pressures.
Hydrostatic pressure differences when internal vaporisation rates exceed the rate of vapour transport through the solid to the surroundings. The mechanism of moisture transport also changes with changes to the product during the drying process itself.
All modes except dielectric drying supply heat at the boundaries of the drying object. So heat has to diffuse into the solid mostly by conduction. The product temperature rises when the moisture at the surface is depleted. This indicates the start of the falling rate period. Energy absorbed by the product through vaporisation of moisture at the surface decreases. More energy is absorbed by the material as sensible heat (Hui 2008). This continues till the temperature of product and drying medium equalise. Many attributes of product quality depend highly on the temperature. Hence it becomes important to understand the effect of temperature on drying.
Food products are transformed into stable product ususally due to alteration in the chemical constituents of the food product. This has a major impact on the quality of the food after drying. This also gives rise to some other common concerns like:
Retaining nutritive value in dried food products.
Minimize change in colour, flavour due to thermal degradation.
Development of a simulation model provides a valuable tool for design, optimization and control of process for each food product. It is observed that since each product differs in chemical composition, the simulation model also changes with respect to each product. Once the drying charcteristics are understood, a suitable dryer can then be selected for the concerned product. The selection of suitable dryer follows from the drying characteristics
Factors affecting drying
Figure 2.1 Main factors affecting drying
2.2.2 Product properties
Water Activity
The volatile content of most food products is moisture. This describes the role of moisture in the drying of a food product. The chemical activity of moisture in food drying is described in terms of water activity. This quantity for moist food is analogous to relative humidity for moist air moisture content Water activity is temperature dependent and this effect is different for each food product.
It is related to the moisture content using the Guggenheim-Anderson-de Boer (GAB) equation(Majumdar 2007).
Where C, K, Ms are constants obtained from experimental values and a - water activity.
Drying of food products require careful hygienic attention because growth of micro organisms and chemical reaction in the product become an important issue.
While temperature, pH and several other factors can influence whether organisms will grow in a product and how fast they might grow, (Jangam 2010) suggests water activity is probably the most important factor in controlling spoilage. If water activity is reduced without adequate control over the drying rate then could degrade. It predicts stability with respect to physical properties, rate of deterioration reaction and microbial growth.
Equilibrium Moisture Content & Sorption Isotherm
A food product is in equilibrium with its surroundings if the internal vapour pressure due to moisture particles in its pores is equal to the vapour pressure of the external surroundings of the product. At this stage the moisture content in the product is the Equilibrium Moisture Content EMC. This quantity varies with temperature since the saturation vapour pressure also changes.
Moisture content of a wet solid in equilibrium with air of given humidity and temperature is Equilibrium Moisture Content. A plot of EMC vs Relative Humidity is called the sorption isotherm. Such an isotherm obtained by exposing solid to air of decreasing humidity is desorption isotherm. Desorption isotherm is the more important one for food drying.
Fig2.2: Graph showing types of moisture content(Strumillo and Kudra 1986)
Fig2.3: Water activity versus moisture content plot for different types of food products(Strumillo and Kudra 1986)
2.2.3 Kinetics of Drying
Agricultural products are hygroscopic. Hence, it becomes important to determine the drying characteristics, of the wet solid. Drying kinetics is connected with the changes of average material moisture content and average temperature with time. Drying rate is determined by
temperature, moisture content of the wet solid (product) and
the temperature, relative humidity and moisture content of drying air.
Difference in partial pressure of water vapour in the air and pressure of moisture in the product.
The drying rate curve is unique to each product and is obtained experimentally by a series of tests to get the rate of drying using different air temperatures and air velocities, assuming relative humidity of ambient air is constant. These tests then indicate an optimum combination of air temperature and flow to obtain the desired dry product in the minimum time. A graph of rate of moisture loss versus time of drying is plotted as shown in figure. It shows that rate of drying varies during the course of drying. For products with wet surfaces three distinct stages are observed in the drying curve:
Figure 2.4: Drying curve showing drying rate w.r.t drying time(Keey 1978)
Phase 1: The material's surface is saturated with vapour and evaporation takes place continuously. The rate of drying is constant with respect to time. Vaporization takes place from the surface of the product containing moisture. Some shrinkage is possible. Diffusion of water vapour across air moisture interface is the rate controlling step in this stage of drying. Towards the end of this stage moisture is transported from inside the solid to the surface by capillary forces and drying rate may still be constant.(Majumdar 2007)
It is worth noting here that the drying rate is computed with respect to overall solid surface area.
Phase 2: The surface is not vapour saturated. This is the second stage of drying - the falling curve. Surface keeps getting depleted with water and diffusion is controlled by internal liquid movement. This continues till the surface layer has completely evaporated.
Phase 3: The moisture content further reduces until equilibrium is achieved and drying stops. Now the rate controlling step is the rate at which moisture moves through the solid as a result of concentration gradient between deeper parts and surface. Heat is transferred through conduction and this influences the drying rate.
The rate if internal movement of moisture decreases as the moisture concentration decreases until the equilibrium value. The drying time of each period differs with changes in nature of product, drying conditions and amount of drying desired.
A plot of rate of water evaporation N versus X the dry basis moisture content is the drying rate curve. Whilst this curve is generated under constant drying conditions, the drying product is exposed to varying drying conditions(Jangam 2010). The drying air velocity, temperature and humidity is generally varied during the course of drying process. Hence there is a need to establish a method to obtain drying rate data over a wider range of operating conditions.
Fig 2.5: Drying rate curve under constant drying conditions
Using the drying rate curve, the total drying time to reduce the moisture content of a solid from X1 to X2 can be obtained as the area under the curve
Moisture diffusivity in solids is a function of temperature, moisture content and geometries. For materials that shrink substantially, the mathematical model used to define DL must also account for changes in diffusion path. The temperature dependence of diffusivity is adequately described by the Arrhenius equation
Where DL is diffusivity, Ea is activation energy. (Xiong, Narsimhan et al. 1992) have compiled extensively the values of DL and Ea for various food products and (Zogzas, Maroulis et al. 1996) provide methods for moisture diffusivity measurement.
(Keey 1978) has provided analytical expressions for liquid diffusion and capillarity models of falling rate drying. Diffusivity is a strong function of Xf and temperature and has to be determined experimentally. Hence liquid diffusion model must be regarded purely as an empirical representation drying in the falling rate period.
One simple approach for interpolating a given falling rate curve over a narrow range of operating conditions was first proposed by van Meel in 1958. It was found that plot of normalised drying rate vs normalised free MC was nearly independent of drying conditions. This plot is the characteristic drying rate curve.
Fig 2.6: Solution to Fick's second law for some simple geometries(Jangam 2010)
The expressions in the above table show the solutions of the 1 dimensional transient PDE for Cartesian, cylindrical and spherical co ordinate systems. These results can be used to estimate the diffusivity from falling rate drying data or drying rate & time using diffusivity.
The purpose of falling rate drying models is to allow reliable extrapolation of drying kinetic data over various operating conditions and product geometries.
In conclusion, drying kinetics allow us to calculate amount of moisture evaporated, drying time, energy consumption etc. of the drying process.
2.2.4 Process conditions
Psychrometry
Dry air and water vapour exert pressure upon each other. In cabinet dryers, ambient air of low relative humidity RH enters the dryer and mixes with outgoing moist air which is at relatively higher temperature and RH.
Figure 2.7: Diagram of drying process with enthalpy diagram
Hence psychrometric relations become an important consideration. Temperature and relative humidity RH of air are the two most easily measureable parameters of initial air. Whilst it is common practice to use psychrometric charts to obtain the other parameters and work out the various states of the moist air, in this case the typical psychrometric equations are used to facilitate iterative calculations for various points in the dryer.
Saturation vapour pressure Psat can be calculated using Temperature T. Using definition of Relative humidity H, partial vapour pressure Pv can be obtained and thus humidity ratio H and specific volume of moist air can be calculated.
The specific heat of food products is often calculated from the specific heat of its constituents using the general equation(Majumdar 2007)
Where Xc, Xp, Xf, Xa and Xm are mass fractions of carbohydrates, proteins, fat, ash and moisture content respectively.
2.2.5 Energy Efficiency in food drying
Thermal drying of food products is an energy intensive process because of latent heat of vapourisation of water which is the most common solvent in foods. Also, food products cannot be exposed to very high temperature just to speed up the drying process since this could result in deterioration of quality of the dried products (colour, texture, rehydration properties, loss of nutrients etc) as reported by (Jangam 2011)
Efforts to improve energy efficiency could lead two ways:
Product design improvement : The design of dryer could be improved to consume lesser hot air for drying the same amount of food product as before. (Majumdar 2007)indicates some energy saving measures with respect to conditioning of the feed, dryer design and heat recovery from the exhaust stream including the use of heat pumps.
Process efficiency improvement : The overall drying process could be analysed for improvement in energy efficiency. This would imply using energy efficient auxillary heating systems to heat the air, reducing heat loss in the process etc.
In many convective dryers, the drying medium (mostly air) is discarded directly into the atmosphere resulting in loss of both sensible and latent heat of the moisture present in the exiting air. (Kemp 2007) claims "Convective dryers are generally known to have low thermal efficiency often below 50%". Higher process efficiency has become a priority for users with an aim to reduce cost of drying operation and lower fuel overheads. The efficiency of dryers has gained importance in the current energy debate in the process industry. This is the point where it is best to think about the scope of renewable energy sources leading to energy savings in the dryer system external to the dryer component.
2.3 Renewable energy in food drying
While there exists, a large scope for energy efficiency improvement in the drying process, having an efficient source of hot air that is used in the dryer could also contribute to benefits. (Kudra 2009) mention many simple yet effective ways to minimize energy consumption of dryers like improving control of dryers, dryer insulation, use of indirect heating wherever possible, recovery and recycling the thermal energy from dryer exhaust (e.g. using heat pump system) Meanwhile most dryers still get their heat from fossil fuel or direct electricity. Rising costs of conventional fuel is one of the major driving factors for users of such systems to switch to renewable sources of fuel.
(Ekechukwu and Norton 1999)had classified the solar drying systems primarily according to their heating modes and how the solar heat is utilized, i.e. passive (natural-circulation) or active or forced-convection (hybrid solar dryer). Most of the solar dryers investigated and reported in the literature are for agricultural products notably fruit, vegetable or crop drying, for e.g. (Doymaz 2007) (Torres-Reyes, Navarrete-Gonzalez et al. 2002)
Determination of temperature dependent drying parameters for potato cylinders and slices during solar drying While few others have analysed solar water heating systems for drying (Kalogirou 2004)
Figure 2.8: Classification of solar dryers (Jangam 2011)
According to the drying process, e.g. direct or indirect, solar dryers may be classified as passive and active ones:
Passive dryers are heated directly from the sun's radiation with or without natural air circulation.
Active (or forced convection) solar dryers, where hot drying air circulates by means of a ventilator.
(Majumdar 2007) has classified solar assisted dryers based on their energy source in the following manner
Solar natural dryers using ambient energy source only.
Semi-artificial solar dryer with fan driven by electric motor to maintain a continous air flow through the drying space
Solar assisted artificial dryers able to operate using conventional (auxillary) energy source when required
2.3.1 Solar dryers with greenhouse collectors - Hohenheim Type
These types of solar dryers use long transparent plastic tunnels as air heaters. They belong to the mixed mode solar heaters combining passive air heating with forced air circulation. This technology has been pioneered in Germany and consists of a long tunnel type plastic collector. Bottom of the tunnel is covered with black absorbing plastic sheet. A ventilator circulates the heated air up to the dryer. It is mainly used for drying fruits, vegetables and aromatic herbs. The figure below shows tunnel dryers of another type(Bellesiotis V. 2011).
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Figure 2.9: Tunnel dryer
Forced mode of drying reduces the relative humidity inside the greenhouse and increases the vapour pressure difference, resulting in a faster rate of moisture removal suggest (Jain and Tiwari 2004) (Iskandanov Z.S. 1999) presents a calculation procedure for thermal efficiency of solar plants with distributed solar type air collectors. (Othman, Yatim et al. 1996)have developed and analysed four advanced solar assisted, forced convective solar dryers. They use improved V groove solar absorbers to improve thermal efficiency and a 10kW auxiliary heat source for continuous operation.
However, the radiation entering a process chamber can directly
(1) Cause different rates of heating due to internal radiant temperature asymmetry and
(2) Shorten the durability of internal components due to ultraviolet exposure and overheating. For some fruits, exposure to sun reduces the vitamin content considerably. Colour retention in some highly pigmented commodities can also be affected adversely by direct exposure to sunlight, although solar tunnel dryers have been used to dry, and retain colour in chillies(Hossain and Bala 2007). Greenhouse dryers have employed photovoltaic-powered ventilation(Janjai, Lamlert et al. 2009) and incorporated photovoltaic/thermal collectors(Barnwal and Tiwari 2008). A photovoltaic array has also been employed to power the fan and control systems in dryers with separate air heating solar collectors (Fargali, Nafeh et al. 2008). Dynamic control is essential if a crop dryer is to achieve the desired process conditions under varying radiation.
2.3.2 SHELF TYPE CABINET DRYERS
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Fig 2.10: Shelf Type Cabinet dryer (Ing 2012)
The shelf type cabinet dryer is a forced convection system in which ambient air enters the cabinet. It mixes with the recirculating hot moist unsaturated air. This leads to decrease in the moisture content of the mixed air and this passes through the air-air heat exchanger. This drying air gets heated from temperature T1 to T2 on passing through the heat exchanger although the humidity ratio of this air remains constant. This air now passes through the fan, generally a DC fan. The velocity of air increases and this air comes in contact with the product that is kept on the trays for drying. The arrangement of trays has to be such that products at the extreme ends of the stack of trays should also recieve the same heat from the drying air. The trays are arranges in a slant to ensure this. This is an important consideration failing which the products at the ends of the top trays will not recieve sufficient heat and so the drying will not be uniform. This could lead to excess energy consumption in the drying process if the product has to be overturned or dried once again.
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Fig 2.11 Inside of a shelf type cabinet dryer (Ing 2012)
2.3.3 Flat Plate Collectors
Flat plate collectors are found to be the best suited to supplement dryers. In case of drying applications, cost of solar collector system is a major concern. These dryers are being used by farmers and in many cases the user is likely to buy the cheaper available product. While ETC may be more efficient , maintaining the glass tubes in outdoor conditions may be a concern in some locations where such cabinet dryers are being used.
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Figure 2.12 Graph showing temperature vs efficiency for various collectors
Solar air collectors have developed to a lesser extent owing to two disadvantages:
Low thermal efficiency of air
Low absorber - air heat transfer ratio.
Description: air flat-plate collector
Figure 2.13: Section of an flat plate air collector
The use of flat plate air collectors is very common in solar drying. This could be probably due to the ease with which it can be integrated with the dryer system. (Karim M.A. 2004) presented an experimental study of three types of solar air-collectors (flat-plate, finned and V-corrugated) for drying applications. They concluded that V-groove collectors have higher efficiency than the other types. They further went on to obtain results of a performance study on V-groove solar air collector for drying applications.
Collector type
Flow rate (kg/m2s)
Outlet Temperature
Efficiency (%)
V groove
0.031
53
68.5
Finned
0.029
50
65.0
Flat Plate
0.030
48
62.0
Table2.1: Optimum conditions of three collectors for solar drying.
Energy storage provides a buffer between the two time dependent functions of solar heat supply and process load (Duffie and Beckman 1991). Some form of conventional heat energy supply is mostly accessible in such processes. In this case the air in the dryer is being heated by fuel based boiler or electrically heated
Renewable energy sources are known to be intermittent. The most common way to address this mismatch between supply and demand is to have a storage device. Apart from addressing reliability issues, this also makes the energy source independent of constantly changing weather conditions.
(Bal, Satya et al. 2010) report that (Butler J.L., Troeger J.M., 1980) have experimentally evaluated a solar collector-cum-rock bed storage system for peanut drying. The drying time ranged from 22 to 25 h to reduce the moisture content from 20% to the safe storage moisture level with an air flow rate of 4.9m3/s.
2.3.4 Economics of Solar assisted food drying
Solar drying can become economical if overall cost of the drying system is low and the gain from solar energy utilization is high. Two common reasons for selecting solar energy for drying may be:
Unavailability of conventional energy in remote areas in developing countries. High cost of transporting fuel into places which have abundant sunlight.
Energy gains in using sun's heat (natural) over other options.
Large active drying systems require substantial capital and investments compared to conventional systems.
Thus the existing literature on drying mechanisms of various food products, dryer designs, various types of solar dryers etc has been reviewed with an aim to understand the existing research done in this field.
The supply of solar energy from a colector is intermittent and time dependent. On the other hand it heat loads also vary continously. In the context of this study, the heat load (drying air heat requirement) is time dependent.
2.4 SIMULATIONS
Simulations are the best way to compliment physical experiments. Simulations, like other calculations, are only as good as the models that are the basis of the programs and the skill with which they are used.(Duffie and Beckman 1991). The common method noticed in most system scale solar process simulations is to solve a set of differential and algebraic equations to represent physical behaviour of solar energy system. A literature search for simulation methods for solar collectors reveal that (Janjai, Esper et al. 2000) (Kim, Thu et al. 2012) have performed advanced simulations on solar thermal collectors using superior analytical techniques and generalised parametric relations. While TRNSYS is commonly used in the industry for simulations similar to these, this simulation is an attempt to understand the relation between solar radiation incident and collector sizing, understand load sharing behaviour between solar and conventional fuel while catering to a continuously variable process load in the temperature range of 30 - 80 degree C. Collector area is the primary design variable.
2.4.1 Simulation of shelf type cabinet dryer
The objective of the process performance calculations that follow are to
To calculate hourly useful solar energy patterns on a given day at any location.
To compare this with hourly process load (dryer air) heat requirement and check how much auxillary energy and when it must be supplied.
To determine hourly supply of solar and auxillary energy to meet the load
To understand how much of long term (annual) load is met by solar energy system and thus savings in conventinal fuel consumption and costs.
The process load here contain no relation to the process dynamics. It has been deliberately kept independent so tht the tool can be used for drying of any product.
Figure 2.14: Schematic of solar assisted shelf dryer setup
In the schematic diagram of the process to be analysed, the cabinet dryer has an air inlet-outlet at its top. The wet product is placed on trays. Ambient air enters from the top and mixes with a pre determined quantity of outgoing warm air. This recirculation of process air ensures major savings in energy consumption of the drying process. This air passes through an air-air heat exchanger which is placed at the bottom corner. Here, the process air inside the cabinet get heated to desired temperature by externally pre heated gas/air supplied from a boiler operated on conventional fuel. There is also a provision for electric heating instead of the air-air heat exchanger. The heated process air now gains velocity due to the fan placed below the heat exchanger.
This air of known moisture content, velocity and temperature now flows over the wet product placed on each tray. The air absorbs moisture from the wet product and flows towards the exit where a fraction of it is recirculated.
The energy required at the heat exchager to attain desired air temperature for drying is calculated by performing heat and mass balance of the air and product inside the dryer.
Using this heat requirement, the temperature of hot gas/air coming in from boiler and the desired temperature of hot water at the water-air heat exchanger is calculated.
On the other hand, using latitude of the location and day of the year, we can calculate the solar irradiation available to the flat plate collectors if installed at that location. Keeping in mind the quantity of water required to be stored in the buffer tank to provide heat in non sunlight hours, the sizing of solar water heating system can be obtained. Once the specifications of the solar water heating system are in place, we can check the share of energy between solar hot water and conventional heat source and also decide the optimum combination to be used for that location in the given weather conditions.
The direct loads to be catered to by the solar water heating system are:
Sensible heat requirements of the water -
Losses in the distribution system - heat lost in piping, to buffer tank and buffer tank to the water-air heat exchanger.
Heat loss from buffer tank - The estimation from loss co-efficient requires knowledge of thickness and thermal counductivity of insulation. The effects of piping joints, supports and other leakages can account for two to four times the loss calculated from the above relation.
2.4.2 Auxillary electric heating into buffer tank.
The temperature of water in buffer tanks is generally controlled using a thermostat. In many solar hot water tanks, electric heating is provided to ensure the water in it is maintained above a certain minimum temperature. With respect to the above system configuration, if electric heating/fuel heating back up is provided into the buffer tank then the the collector inlet temperature and in turn the useful solar gain would decrease since the and on the contrary to the requirement here, energy consumption would increase. Our aim in this process, the priority is to heat the gas going into the air-air heat exchanger in the dryer. Hence the buffer tank in this system is meant to provide maximum possible hot water to the water-air heat exchanger for maximum possible duration of the day, thus increasing the solar fraction of the system.
The commonly accepted thumb rule for best estimate of buffer tank for solar water heating system is that made by Lof and Tybout (1973) as reported by (Duffie and Beckman 1991). They indicate that the tank capacity must be in the range of 50 -75 litres/m2 of collector area. In this case we aim to store solar energy to be used in non sunlight hours as well. Hence a slightly higher capacity of buffer tank may be proposed.
2.4.3 Quantifying solar radiation
Performance prediction using simplified methods.
(Duffie and Beckman 1991) mention that the design methods used to design and analyse solar thermal processes are of three categories
Systems in which the collector operating temperature is known or estimated. The critical radiation levels are also established for such systems.
Utilizability method also denoted as the ∅method- for active systems where heat load could have any desired temperature. This method requires knowledge of collector inlet fluid temperature
Heat table method of Morse
Methods which contain corelations of the results of large number of detailed simulations. f-chart method is used in building heating processes where the minimum temperature for energy delivery is approximately 20deg C
Methods based on short cut simulations. In these methods simulation s are done using meteorological and results are related to longer term performance. E.g. SOLCOST
2.4.4 Calculation of Utilizability
Utilizability is the fraction of the incident solar radiation that can be converted to useful heat. The reasons for determining utilizability on an hourly basis could be many. In certain process industries the solar water heating system could be a part of the primary loop that provides the process heat. In such cases, the temperature of water at inlet to the collector would vary in very short intervals of few minutes or hours and hence this method. In the current study, an attempt has been made to co-relate time variant solar radiation for any location on any day of the year along with changes in ambient air temperature and weather conditions throughout the day. Food drying is a process that takes few hours from start to end. The heat load requirements of dryers may not vary instantaneously but will vary over few hours interval as the drying progresses from initial to intermediate to final stages. Since hourly data is easily available and in the process of drying the heat requirements can be assumed to remain constant within one hour intervals, the ∅method is used in this case.
(Duffie and Beckman 1991) reports that Whillier (1953) and Liu & Jordan (1963) have shown that in a particular location for a 1month period, ∅is same for all 1 hour periods. They then found that these curves do not vary much with respect to ground reflectance or the view factors from the collectors to the sky and ground. The effect of tilt was taken into account by using the parameter Rb defined as
The generalized ∅curves for four values of KT are shown in appendix
On the other hand Clark et al. (1983) developed a simple algorithm to represent the generalized ∅functions.
2.4.5 Solar Fraction
Long term system analyses is used to determine the contribution of solar energy or the financial benefit of installing a solar energy system along with an existing conventional heat source. In such cases, the contribution of the solar energy system to the total load is better expressed in terms of reduction in energy purchased or supplied from conventional system. In situations like these, the process load demand is independent of solar energy system size. Here a quantity solar fraction is generally used
Where : energy supplied/purchased if only conventionl fuel system exists.
: part of energy supplied/purchased from fuel system when solar also contributes to the total demand.
2.5 Conclusions from Literature review
Thus the existing literature on drying mechanisms of various food products, dryer designs, various types of solar dryers etc has been reviewed with an aim to understand the existing research done in this field. The methods to measure the useful solar radiation on a real time basis was understood to be implemented further into the excel tool. The problem statement was now reviewed as that involving a supply, a demand and a tool required to understand the behaviour of each of these.
METHODOLOGY
3.1 Solar useful gain calculation (resource)
The following methodology has been adopted in the following section:
Obtain utilizability, hourly useful gain and therefore obtain usable solar heat energy available per m2. Approximate collector area can be found using the useful gain.
Calculate energy required at hourly intervals at dryer end.
Calculate desired mass flow rate of solar hot water in the buffer tank - water air heat exchanger loop
Obtain average quantity of hot water required and hence approximate sizing of flat plate collectors depending on load.
Once the available supply (solar gain) per m2 of collector area and load demand characteristics are known, this becomes a problem of matching the characteristics of demand load with supply and eventually sizing an adequate system under the constraints of cost (fund available to incorporate solar energy system) and area available for installing the collectors. Two ways to proceed with further analysis of such a problem are:
Thermodynamic analysis - Using parametric relations from heat balance equations and heat exchanger analysis. Although this problem can be solved for any specific case if data regarding the size, characteristics, performance is available or output of the conventional fuel source for that system is known.
A more general method to tackle this issue would be techno-commercial analysis using the P1, P2 method as described by (Duffie and Beckman 1991). This approach is independent of the detailed set up of the process. It is an analytical solution in which the supply (solar and conventional) and demand (process load) values or trends are used to obtain a suitable size of system taking into consideration the economic constraints and life cycle of the system. This requires setting up an analytical equaton and solving that using MATLAB to obtain a parametric curve of collector area against solar fraction. The maxima of the curve generated will indicate the optimum area of collector.
In order to perform this analysis, more details of the economics of the solar collectors at that location are required as input.
It should be borne in mind while sizing the solar collectors that the system size has be sufficient to generate hot water on least sunny day but at the same time not be extremely oversized and result in an oversized buffer tank. Also it is desired in this case that the solar collectors be sized to heat water for night time along with supplying hot water during the day for the process. This is done to ensure:
Maximum possible share of heat supply comes from the solar system and the conventional fuel system is used only as back up.
It is important to supply uninterrupted heat to the dryer system. Food drying operations typically last 4-15 hours for the kind of system being discussed here.
It is desired that the drying operation be possible irrespective of external weather conditions on any given day. This system is placed indoor or outdoor and has no relation to availability of sunlight. Only the solar water heating system and PV system require sunlight for maximum possible duration. This is the reason for using thermal and electrical storage systems.
Input parameters required are
Latitude of location and
Day of the year is input as per the Julian calendar
Inclination angle of collectors at that location. Generally considered equal to latitude for locations upto 50 degree latitude.
The hour interval is entered in the table along with time difference from solar noon (for every hour the mid-point value). For 8-9am we would enter the value as -4.5 since the mid-point of that hour 8.30am is 4.5 hours before solar noon. The values before noon are negative and those after noon are positive.
Tin: Temperature of water entering the collectors. The temperature of water inside the buffer tank is generally monitored by a thermostat. Hence it should be possible to obtain hourly temperature readings of the water coming to the inlet of the collectors.
Tamb: Ambient temperature obtained from NASA weather data
The remaining parameters are calculated as follows:
Day length N, declination angle d and hour angle ω is calculated.
The monthly average hourly radiation incident on the collector is
In the above relation
KT is the monthly average clearness index obtained from standard NASA data
H0 (J/m2 )is calculated using the relation
rt is the ratio of hourly total to daily total radiation. It is a function of day length and hour in question
Where nd
rd is is the ratio of hourly diffuse to daily diffuse radiation
Hd/H can be obtained from its graphical co relation with KT. For the purpose of calculations the following relation is used:
For and
For and
Rb is the geometric factor. The ratio of beam radiation on the tilted surface to that on a horizontal surface at any time. This value is used as an average of the whole month. Obtained from Appendix
Ïg is the albedo at that location. For many locations albedo changes according to season. For normal surfaces Ïg is taken = 0.2. It is highest for snow = 0.7
Θb = beam radiation. For collectors that are installed facing due south in the northern hemisphere the azimuth angle becomes = 0. Hence angle of incidence of beam radiation = hour angle.
Figure 2.15: flat plate collector efficiency curve
FR(ta)n indicates how energy is absorbed
FRUL indicates how energy is lost from the collector
b0 = is a constant called incidence angle modifier co-efficient with values of:
- 0.10 for single glass cover flat plate collectors.
- 0.17 for double glass collectors.
Kta is the incidence angle modifier.
This quantity Kta is calculated for the mean day of the month and corresponding hour angle.
Xc: the dimensionless critical radiation level is the value of incident radiation that is just sufficient for absorbed radiation equals the losses from the collector. Only when the incident radiation exceeds critical radiation level, the collector will absorb energy and there will be useful gain. In terms of collector parameters Xc is calculated using the following expression:
kT: is the monthly average hourly clearness index. This parameter takes into account the fact that on any given sunny day, clouds will cover the sun several times. Therefore, sunlight of exact same intensity will not be incident on the collector at all instances.
Xm is calculated and then compared to value of Xc. The solar insolation of that hour will be useable only if Xm> Xc
With these values the utilizability ∅is calculated as follows:
3.2 Thermodynamic analysis of dryer (load)
The wet products are weighed initially before being loaded into the dryer. The mass of wet product and initial weight of moisture in the product is known from this measurement. The final weight of product and moisture is also similarly known.
The temperature, relative humidity RH of the air being used for drying is generally known.
As per data obtained from Innotech,
17kg of carrots were dried for 10 hours in the shelf dryer model HT4d consisting of 17 stacks.
Air at 75 degree C was supplied for the first 2 hours of drying. Then the drying air was maintained at an average of 55-60 degree C for the next 8 hours of drying. It was noted that 1 kWh/kg of energy was required for this process.
Using this data, an iterative calculation was performed on Excel as shown in the sheet titled 'demand load'. This was done to eventually find the heat required by the drying air at each hourly interval when a drying process takes place. The intermediate moisture percentages and weight of product were assumed constantly decreasing. Subsequently, the moisture content of air is assumed increasing.
Hence taking these values as input, mass flow rate of drying air Md within the dryer is calculated as follows:
Product parameters:
M2 kg, X2 (kg/kg) : Mass and specific moisture content of wet product at state 2 (start of each drying cycle)
M3 kg, X3 (kg/kg) : Mass and specific moisture content of dry product at state 3 (end of each drying cycle)
Air properties
Md : mass of air inside dryer at point 1
Y2, Y3 : moisture content of air, obtained from Relative humidity of air at each state
Y3 - Y2: change on moisture content of drying air. Y3>Y2 since the air has picked up moisture from the wet product during the cycle.
Total moisture inside the dryer at the start of drying cycle
M2X2: moisture in wet material
MdY2: moisture in ambient air
Total moisture inside dryer at the end of drying cycle
M3X3: Moisture in dry material
MdY3: Moisture in exhaust/recirculating air
In steady state, the total moisture content inside the dryer has to be equal. Hence,
Therefore we can determine
Since we know total drying time t, the specific air consumption of dryer is
The temperature of incoming gas is considered nearly equal to the temperature of the drying air at the end of the process (T1 = T3) because a huge % of outgoing air is recirculated back instead of let out of the cabinet. This is done to save consumption of air and more importantly the air does not become saturated at the end of one single cycle. T2 is the desired temperature of the drying air at the start of each drying cycle. Hence it follows that the air-air heat exchanger within the dryer has to raise the temperature of air from T1=T3 to T2.
Using the definition of enthalpy Qext = Md Cp (T2 - T3) where all quantities on the right hand side of the equation are known.
This Qext is the demand load at hourly intervals for the duration of the process(drying).
Analysis of supply and demand characteristics
Now there are two heat exchangers connecting this load to the supply (solar water heating system)
Assuming an enthalpy transfer efficiency 80% for both the air -air heat exchanger in the dryer and the water-air heat exchanger. This implies that the solar energy system must be capable of supplying a much larger Q at every hour. Hence concluding
The hot water in the buffer tank has to be able to supply Qsol Watts of heat every instant to ensure that the drying air attains the desired temperature. Assuming that the hot water loses 30 degree C of temp in the water air heat exchanger, then we have
The instantaneous values of volume of water that will be required to pass through the water air heat exchanger are calculated in the excel sheet. We observe that approximately 7 litres of water are required per second. This implies about 7 * 3600 seconds = 25200 litres in one hour.
It is common practice to use solar air collectors for drying purposes. Solar water heating collectors at low temperatures of 30-70 degree C are generally used for only in domestic heating appplications. Also, the thermal energy output per unit collector area varies from location to location. Hence the co-relation between quantity of hot water and suggested collector area is not available readily with respect to a process load. An exhaustive search for a relation between the stored/ output water temperature, volumetric capacity of solar flat plate collectors and suggested size of collector absorber area with respect to useful solar radiation incident at any location revealed the following standard measures followed in the solar water heating industry:
1-1.5 m2 of collector area for consumption of 50 litres of hot water per person at temperatures of upto 45degree C with a storage tank capacity of 40-70 litres per m2 of collector installed.(ENVIRONMENTAL and ORGANISATIONS)
(Christiane Egger) suggests 2 m2 collector area per 50 litre requirement with respect to Upper Austria.
ASHRAE suggests a storage tank capacity of 20.4 - 28.6 Litres per m2 and 40.8 Litres per m2 of collector area for daily space heating load requirements.
(Combi+) suggests a storage capacity of 60 Litres per m2 collector area for small systems
It should be noted that all the above data relates to a domestic hot water requirement which is an application that occurs once or twice in a day. For process load applications it may be reasonable to suggest that the same rules be applied for hourly capacity. Also these estimates are meant for water at 45 degree C applications. In our case, the water will be allowed to heat to much higher temperatures of 70-80 degree C on a clear sunny day.
For an approximate requirement of 25,200 litres, a collector area of 25,200/50 = 504 m2 would be required along with a storage tank capacity of 504 m2 * 25 litres = 12,600 litre storage tank capacity for our application.
Assuming around 40% of the total solar energy incident on the ground is absorbed by the collector, we can estimate that this collector area would absorb solar radiation of
504 m2 * 3.8665 kWh/ m2 * 0.4 = 779.5 kWh.
Hence a relation has been established between the heat load required at the dryer end to the area of collector estimated to cater to that load under the specified weather conditions.
This co relation has been obtained for one day at one location. This relation has to be further to be generalised in order to be applicable universally.
3.3 Economics of solar heating against fuel heating
Annual cost system is adapted here because we want to compare the operation of both systems (solar and conventional fuel) that too having different life spans.
Objective: Calculate annual Cost of operating a solar water heating system in any location under any weather condition and compare this to the annual cost of operating a conventional fuel based heating system. The economic feasibility with the optimum lifespan of both the systems is also determined.
In order to find the optimum life span, the operation time N is maintained as a variable parameter.
Where
and
The cost of one unit of energy is given as
Considering AMC proportional to operation time for both systems, we then have
AMC = aN
a is taken as 0.2 for solar water heating system and 0.4 for the conventional fuel system.
Annual Fuel Cost
In case of solar water system the fuel cost is only the cost of electricity consumed by the pumps used in the hot water line.
For fuel heating system
Salvage value of system
The salvage (Scrap) value varies with time N. Assuming a linear depreciation D of the system with time, the salvage value is given as
Where DC = IC/ Nmax
Using the above simplifications we have the relations for Annual cost AC in terms of known parameters. So for different values of N, we can obtain a comparison between solar heating systems against fuel heating system.
Values are input for a solar water heating system diesel operated system in India. The cost of both the systems were obtained from market research and the parameters were input to observe the difference in annual costs between the two types of systems.
RESULTS & CONCLUSIONS
From the graphs, we can conclude that there is always a mismatch between availability of solar energy and demand load . It is difficult to match the supply and demand characteristics instantaneously without an energy storage device such as buffer tank or battery system.
Figure 4.1: Demand vs supply trend
Also, having a storage device allows the drying process to continue irrespective of sunlight hours. In the above simulation, the sunlight hours and drying time were deliberately mismatched to emulate real working situations where the process may not necessarily be carried out during the day time.
The graph shows comparison of unit cost of energy from conventional fuel and solar water heating system. This method was adopted to establish that whilst the initial capital cost of renewable system might be higher than the conventional fuel system, in the lifespan, the solar energy source would be more economical.
Methods to optimise the balance between the use of conventional fuel and solar heat for the same demand load have been mentioned in this thesis.
The various steps involved in making a simulation software were encountered during the course of this thesis
CONCLUSION
This thesis began as an attempt to identify energy saving measures in the food drying process. The aim was to develop a tool that could be used irrespective of the type of food dryer. It was then decided to use the shelf type cabinet dryer as the focus of the research. The drying mechanism of various products in such system was studied. It was learnt that there is no universal characteristic equation to represent the drying of different food products. Although, few researchers have also successfully carried out pioneering work on genreal drying behaviour most others have conducted exhaustive experiments and studied each product separately. This was because of the fact that each product was unique in composition and behaviour towards drying air. Hence the process parameters were taken into account to develop a relation which could lead to a general expression for energy consumed in any drying process.
A general tool to understand how solar heating systems could be incorporated into the existing set up of conventional fuel/electricity based dryers was developed. Initial results from each component of the complete drying system are mentioned in the results section. The behaviour patterns of process heat load, method to determine solar gain at any location on any given day were analysed and incorporated in the excel tool. Apart from these, a generalised thermodynamic approach to such problems is shown in this thesis.
Future work
This thesis has looked at solar resource, process load (demand) and Solar energy (supply) independently. The interactions between each of these systems is the next stage of the study. In the attempt to do so, it was learnt that conventional mathematical techniques were not sufficient to co relate supply and load characteristics in a transient manner.
Although hourly data can be used to understand the trends, this process will have to be performed repeatedly for each day. In view of this, it is suggested that advanced mathematical tools be used to define characteristic relations between the various components in this system. Such a method could be more beneficial to create a more responsive, useful real time simulation software which could take in minimum inputs to give detailed trends between supply, demand and resources.
A method that can incorporate more analysis of the various drying processes could be looked into.
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