Silica aerogel obtained from MS51 transparent with slightly blue coloration as shown in Fig. 6. Density and thermal conductivity of the silica aerogel obtained from MS51 were 0.16 g/ cm3 and 0.016 W/Km, respectively. And density and thermal conductivity of TMOS were 0.18 g/ cm3 and 0.018 W/Km, respectively. The shrinkage ratio

W ave length (nm)

Fig. 5 UV-VIS spectrum of selectively solar-absorbing coatings.

of silica aerogels obtained from MS 51 and TMOS, after supercritical drying, was 4.1 and 1.4%, respectively.

Fig. 7 shows transmittance of the silica aerogels obtained from MS 51 and TMOS. Transmittance of both them was about 80 % at 550 nm. Silica aerogel has pores of several nm, visible light was not scattered in the silica aerogel. Therefore, the transparency of the silica aerogel is good and its thermal conductivity is lower than general insulation. Employing silica aerogel as a transparent insulator for the selectively solar-absorbing coatings may control.

Two ways are possible to optimise the micro climate in solar collectors, as discussed above. Changing the ventilation rate either by increasing or decreasing the size of the ventilation holes yields only small changes in the wetness behaviour, as shown in figure 11a while a change of the insulation materials as a much bigger impact (figure 11b). The optimisation of different collectors displayed in figure 9a by changing the ventilation rate resulted in the wetness behaviour shown in figure 9b. The unchanged reference collector (No. 7) serves as reference for the eye.

Attention has to be paid to the position of the ventilation holes. Usually diagonal ventilation (see figure 2) is more effective when dispersed ventilation holes along the edges. The effect is clearly diminished if insulation materials cover the ventilation holes (see figure 12).

Figure 11 : Impact of changes on the micro-climate on the humidity response function of collectors

2. Conclusion

The micro climate in ventilated flat plate collectors is dominated by the moisture adsorption of the insulation material or other parts of the frame and by the ventilation rate. Reliable measurement procedures for the ventilation rate and for the micro climate have been developed. They are suitable tools for optimisation of the micro climate by selecting the appropriate insulation materials and ventilation rates.

Further work is necessary for quantifying the properties of the insulation material, it’s degradation behaviour and the geometrical dimensions of the collector.

In operation LHPs realize the same basic physical processes as conventional heat pipes. These are, first of all, absorption and emission of considerable latent heat during the evaporation and condensation of a working fluid, heat transfer in the vapor phase and the motion of a working fluid at the expense of capillary forces created in the wick. However, in LHPs these processes are organized in a different way, which corresponds to the distinctive features of the design of this device. Among these are:

— the shape and local disposition of the wick only in the evaporator;

— the wick structure;

— special organization of the evaporation zone on the principle of "inverted menisci”;

— separate pipe-lines for the motion of vapor and liquid.

These peculiarities in the LHP design make it possible to achieve a much higher heat — transfer capacity owing to a relatively low hydraulic resistance in all its sections, the high capillary pressure created by the wick and an efficient organization of heat exchange in the evaporation zone.

To describe the LHP operation, it is convenient to use its scheme shown in Fig. 2 and the diagram of the working cycle with respect to the saturation line of the working fluid in "pressure — temperature” coordination in Fig. 3.

Fig. 3. Diagram of the LHP working cycle

If an LHP is situated vertically, and the evaporator is above the condenser, the free level of the liquid L-L is located in the liquid line and the evaporator as in communicating vessels. The wick in this case is fully saturated. When a heat load is supplied to the evaporator, the liquid begins to evaporate from the wick, absorbing in doing so the latent heat of vaporization. The main evaporation takes place at the evaporator wall, where the vapor temperature is T1. At the exit of the evaporator vapor may be slightly superheated and have a temperature T2 > T1. Since the wick has a certain thermal resistance, the vapor temperature T7 in the inner space of the wick and in the compensation chamber is lower than the temperatures T1 and T2. This results in a difference of vapor pressures ДР=Р1-Р7, which displaces the liquid from the vapor line and the condenser into the compensation chamber and the inner space of the wick. From here the liquid soaks into the wick closing the LHP working cycle.

From analysis of the diagram of the working cycle it follows that there are two main conditions of LHP serviceability. The first condition may be written as the balance of the capillary pressure created in the wick and the pressure losses in all the sections of circulation of the working fluid:

=ZAP, (1)

i=1 …n

where ДРс is capillary pressure determined by the effective pore radius of the wick, the coefficient of surface tension at the liquid-vapor interface and the angle of wetting of the wick with the liquid, APi presents the pressure losses in the i-th section of an LHP.

The second condition relates the value of the pressure drop between the evaporating surface of the wick and the compensation chamber:

ДР1-7 ^ ATi_7 , (2)

dT

SP

where — is the derivative characterizing the slope of the saturation line at a point

dT

between T1 and T7.

Since the difference between the values of the temperatures T1 and T7 required for creating the necessary pressure drop ДР1-7 is quite insignificant, the condition (2) is sufficiently correct for some average value ЗРЛ9Т in the range between T1 and T7. Then an additional condition following from the main conditions (1) and (2) is the relation:

ДР7—8 =APC — AP1-7 , (3)

which shows that the pressure drop between the zone of the evaporator and the compensation chamber should not exceed the value of the capillary pressure created by the wick. Together with the condition (2) it also means that in LHPs the wick serves not only as "a capillary pump”, but also as "a thermal lock”, which makes it possible to create in the evaporator a temperature drop and the corresponding pressure drop required for displacing the working fluid into the compensation chamber.

A program was written in Visual Basic for Applications (VBA) to generate the PV module I­V characteristic for a given irradiance and module temperature and then solve for the speed of the motor and flow rate of air in a given duct system. The code was initially applied to measurements of irradiance and temperature to calculate values of voltage and speed which were then compared to measurements. Furthermore, in order to compare the performance of both fans, the speed of each was simulated as a function of time of day for an imaginary irradiation profile. Extraterrestrial irradiation is calculated for May 17 in Edinburgh and the irradiation profile for that day is calculated as 80 % of extraterrestrial radiation. In the program, the length and other properties of duct are fixed and so the system is defined through the specifications of the PV module, the DC motor and fan characteristics. The program uses the hourly irradiance and temperature data to determine the PV I-V characteristic, which is then solved simultaneously with the motor equations to determine speed. The H-Q characteristic is then determined at the calculated speed.

I. Rodriguez, R. Consul, A. Oliva, C. Orozco

Centre TecnOlogic de Transferbncia de Calor (CTTC)
Lab. de Termotecnia i Energetica
Universitat Politecnica de Catalunya (UPC)
labtie@labtie. mmt. upc. es, www. cttc. upc. edu

The aim of this work is to present the numerical tools developed by our Center, to be employed in the design and optimization of hybrid latent/sensible storage devices. Two main kind of codes are distinguished: i) a general purpose Computational Fluid Dynamic (CFD) code used in the detailed multidimensional simulation of these equip­ment; ii) and a group of subroutines, based on global or one-dimensional mass and energy analysis, used to determine the store thermal performance, and to be em­ployed in long term thermal solar systems simulation codes. Illustrative results of the application of these numerical tools are presented.

Introduction

In thermal solar energy systems, heat storage devices help to match the delay between energy production and consumption. Among the different alternatives of thermal energy storage, sensible heat stores of liquid water are the most extended ones.

A promising alternative is the use of phase change materials (PCMs), with melting tem­peratures within the range of working temperatures, as a medium to store thermal energy by latent heat. The advantage of latent heat over sensible heat storage is its high thermal storage capacity at relative low temperatures. This property allows to store energy in a com­pact manner if appropriated heat transfer mechanisms between PCM and the fluid flow in the system loops are designed.

There are several proposals in the literature to take benefit of the attractive thermal prop­erties of the PCMs, among them, can be found what are defined as hybrid latent/sensible storage devices [9]. These stores consist in the classical storage tank that incorporate some PCM modules.

On the design and optimisation of these hybrid equipment, detailed numerical simula­tions of heat transfer and fluid flow using Computational Fluid Dynamics (CFD) codes, can be used as a powerful tool. CFD simulations can provide the designer with a way to test im­proved designs virtually. Mathematical models employed may be appropriate to reproduce heat transfer, fluid flow and solid/liquid phase change. Experimental validation plays here an important role.

Complementing detailed numerical simulations, simplified numerical models, based on global or one-dimensional mass and energy analysis, can help to predict the global thermal and fluid-dynamic behaviour of the hybrid devices. This kind of models can be used both to determine the store thermal performance (i. e. store parameters identification, ENV 12977­3), and to be included in long term thermal solar systems simulation codes.

The aim of this work is to present illustrative results of the employment of this two main kind of numerical tools in the design and optimisation of hybrid latent/sensible storage tanks. Main features of the numerical tools employed in these studies are described.

A. SREEKUMAR BAIJU JOHN C SUDHA KARTHA and K. P.VIJAYAKUMAR Dept. of Physics, Cochin University of Science and Technology, Kochi-682 022

&

GEORGE PETER PITTAPPILLIL Mithradham, Renewable Energy Centre, Aluva, Kochi

1. INTRODUCTION

Agriculture is the backbone of India. The heart of India lies in its ploughing fields. Having the largest human resource base of around 700 million with a contributing share of 26 percent of GDP, the Indian agricultural sector has assumed greater importance in the development of India in rural perspectives. Agriculture development is a key to eradicate poverty, ensure food security, generate demand for industrial goods and promote overall development. Hence, it is important to develop post-harvesting technology for agricultural products for its value addition.

Drying is one of the most practical methods for preserving the quality of agricultural products. Solar drying as a means of food preservation has been considered as one of the most promising areas for the utilization of solar energy. The customary technique adopted by the farmers is open sun drying and it is associated with so many disadvantages like dust contamination, insect infestation and spoilage due to rains. The product dried in this way is not sufficiently of high quality/standard for the market (local or international) and cannot therefore be sold at competitive prices. As a result, new drying methods with conventional heat sources have been widely developed and used in order to solve these problems. Conventional drying technique utilizes large scale, fossil fired air dries. In many cases the hot combustion gases are passed directly through the product, which is often contaminated by unburned fuel, fumes and soot. Electrical heating of air for drying is preferred, but it is very expensive and not feasible in rural areas of developing countries. Because of energy crisis and intensive energy consumption in the drying process, solar drying has been studied widely in many countries in order to reduce cost and substitute conventional energy. More over our country receives a high degree of solar radiation throughout the year and it is free for taking and non-polluting.

Chilli is the universal spice of India. As per the latest statistics, India produced 8,00,100 tonnes of dry chilli from an area of 9,30,000 hectare. No country in the world has so much area and production of chilli as in India. This paper analyzes the design details and performance analysis conducted in 165 m2 solar air heating system for drying one tonne fresh chilli per batch. The prime objective of the project is the production of hygienically processed chilli to meet market needs particularly for exports.

Christoph Trinkl*, Wilfried Zorner*, Vic Hanby**

*Centre of Excellence for Solar Engineering at
Ingolstadt University of Applied Sciences (D)

Esplanade 10, D-85049 Ingolstadt, Tel +49 841 9348-372 Fax +49 841 9348-99372
E-Mail trinkl@fh-ingolstadt. de Internet www. solartechnik-ingolstadt. de

**Institute of Energy and Sustainable Development, De Montfort University Leicester (UK)

Given the finite nature of fossil fuel resources, the increasing pollutant emissions (particu­larly CO2) caused by their combustion and the change of the earth’s climate, innovative technologies for sustaining ecological heat and power generation, especially in the field of solar heating, are gaining more and more importance. Supported by the increase in costs for fossil energy over the last few years, solar heating systems and the use of environ­mental thermal energy have become viable technologies for heating applications. Starting with technologically simple domestic hot water supply, solar space heating with larger col­lector areas and more complex storage units for enlarged solar fraction have become in­creasingly popular. State-of-the-art solar heating systems, however, suffer from the neces­sity for an additional heat source, in most cases based on fossil fuel. The problem derives from the gap between solar radiation availability and thermal energy requirement (day/night and summer/winter shift).

In the course of research in regenerative energy systems for family houses in addition to solar systems, heat pump heating systems have been a focus of researchers for several years. As heat pumps use mechanical energy to transfer ambient energy from a source at a low temperature (o…+15°C) to a sink at a higher temperature (+35…+50°C) they con­sume less primary energy than oil or gas fired systems. For domestic heating systems the sources for a heat pump are mainly ambient air or the ground. The compressor is fre­quently driven by electrical energy. Today this “thermodynamic heating” can use up to 75% of thermal energy for domestic heating from ambient sources, while 25% are electri­cal energy. The coefficient of performance (COP: ratio of heat delivered by the heat pump and the electricity supplied to the compressor) of a heat pump and therewith the fraction of ambient energy depends on the temperature difference between the source and the sink. Apart from the saving of primary energy, heat pumps have the advantage of independence from fossil fuels. The heat pump market has grown steadily in Central Europe for many years.

As both solar heating and heat pumps are sustainable and innovative technologies in the field of domestic heating their combination has often been subject to research in many countries. These so-called “solar heat pump systems” were examined theoretically as well as experimentally in several research laboratories and experimental houses. There are three basic configurations of solar thermal collector and heat pump incorporation for do­mestic heating purposes, as shown in Figure 1. This paper concentrates on the series so­lar heat pump and on the parallel / series combination possibilities as the more innovative and promising options concerning an enhancement of solar fraction for domestic heating and hot water preparation. The basic advantage of the series and combination configura­tions is that the efficiency of the solar collector is enhanced, as it works with low inlet tem­perature which reduces the heat losses to the ambient. This effect was for example shown by Freeman et al. [1] and O’Dell [2] in simulation studies. Apart from that, the heat pump allows collector operation even during low-temperature and low-radiation periods and this extends its utilisation time considerably.

For grading the product series, concentration factor C has been chosen as a step basis. Then every type will be fully defined by a pair of two parameters — acceptance angle ©A and number of mirrors n. If two different geometries of concentrator will have comparable concentration capabilities, the one with higher acceptance angle will be preferred.

Assumptions:

a) Number of mirrors on one side of symmetrical concentrator will not exceed 5

b) Acceptance angle will not exceed 20°

c) Minimal acceptable concentration factor is limited to 2

Ad a)

More mirrors lead to more complicated technology and production difficulties because height of concentrator rapidly increases and contribution of higher parts of mirrors to the overall concentration is not significant. Taking into account the linear dependance of incremental concentration factor by linear (flat) character of reflecting surface, we cannot expect positive utilisation of mirrors for concentrator with n > 5 even at truncation. Concentrator should have number of mirrors also not less than 3 because below this limit the concentration level cannot reach desired values.

Ad b)

The series will recognize types with different acceptance angles with step 5°. Concentrator with ©A = 5 will for example capture sunlight from 5° spatial cut-off around the optical axis and this type will be with lowest acceptance angle, highest concentration factor, for concentrating primarily the direct radiation. As such concentrator has a large height, possibly it need to be truncated. Acceptance angles higher than 20° on the other side will not significantly concentrate the sunlight, concentration factor will step under 2 what is not in accordance with point c).

Ad c)

Concentration below C = 2 will not advancing sufficiently the flat-plate collector systems.

The thermal performance of the collector described in Table 1 was measured in an outdoor test facility where the inlet temperature, the outlet temperature and the volume flow rate was measured. The temperatures were measured with copper — constantan thermocouples (Type TT) and the volume flow rate was measured with a HGQ1 flow meter. A 31% glycol/water mixture was used in the solar collector loop. Further, the global radiation and the diffuse radiation on horizontal were measured with two Kipp&Zonen CM5 pyranometers.

The collector performance was measured for

two different tilts: 45° and 90° (both facing south). A period of 11 days (17/5-28/5 2003) has been selected for validating the Trnsys model for the collector at 45° and a period of 7 days (12/8-19/8 2003) has been selected for validating the Trnsys model for the collector at 90°.

The necessary data for describing the collector are shown in Table 1. The heat loss coefficient, k0, was determined from efficiency measurements (Shah, L. J. & Furbo, S. (2004)) and split into two parts for the evacuated tubes and the manifold pipes respectively. F’ was calculated from theory (Duffie J. A. and Beckman W. A. (1991)), (Incropera F. P. and de Witt D. P. (1990)) and (ra)e and a were calculated with a simulation program for determining optical properties (Svendsen S. and Jensen F. F. (1994)).

In Fig. 6 the measured and calculated collector outlet temperatures are compared. It can be seen that there is a good degree of similarity between the measured and calculated temperatures. Further Fig. 5 shows the measured and calculated collector performance for the two periods. The difference between the measured and calculated performance lies

Dr.-Ing. Ulrich Leibfried

Consolar Energiespeicher — und Regelungssysteme GmbH
StrubbergstraBe 70, D-60489 Frankfurt/M * Gewerbestr. 7, D-79539 Lorrach
Tel.: (0049 -69) 61 99 11-30 Fax: -28 * Tel.: (0049 -7621) 42 22 8-30, Fax: -31:

E-Mail: info@consolar. de
Internet: http://www. consolar. de

1. Introduction

It has been known for more than 10 years that low-flow solar systems increase the energy yield by up to 10 % and that additional savings lie in the installation process of such technology. Nonetheless there are only few suppliers in the German Market offering stratification-type domestic hot water heater tanks suitable for low-flow solar applications. The last few years showed even a trend towards cheaper hot wa­ter storage tanks, although often at the cost of reduced quality.

In 2003 Consolar entered the market against this trend with a new compact storage tank for solar applications. Inside this storage tank a highly efficient stratifying heat exchanger, for which a patent is pending, provides for very good cooling of the so­lar circuit even in low-flow operating mode. The development of this heat exchanger was further advanced at the end of 2003.