Category Archives: Particle Image Velocimetry (PIV)

Problem description

The fluid dynamic and thermal behaviour of water inside a rectangular storage element forming part of an Integrated Collector Storage (ICS) is analysed. A schematic of the prob­lem under study is shown in figure 1a.

Heat losses at the absorbing (front) and at the insulated surfaces are determined by means of global heat transfer coefficients, (Ut and l/j), and of the ambient temperature. To model the incident solar radiation, a heat flux Gt is applied over the front surface of the reservoir. One part of it, Gtrc/ is supposed to be reflected by the cover, resulting into a net heat flux over the front surface (absorbing surface) of the store of Gt — Gtref.

Mathematical model

The fluid flow and heat transfer phenomena inside the storage element is assumed to be governed by the Navier-Stokes and energy equations in their bi-dimensional form. The following hypothesis are made: the physical properties are constant in accordance with the Boussinesq approximation (density variations are only relevant in the buoyancy terms of the momentum equations), the fluid behaviour is Newtonian, the viscous dissipation and the influence of pressure in temperature is negligible, and radiation effects are not considered. Thus, the corresponding governing equations can be written as follows:

du dv dx + dy

(2)

(3)

дТ дТ дТ А (д-Т

PW + pUdx+pt’dy=7p{^ +

where (ж, у) are the coordinates in the Cartesian-coordinate system ж-у indicated in Fig. 1; T is the temperature; T0 the reference temperature; the dynamic pressure; (u, v) and

the velocity and the gravitational acceleration vector expressed in the reference system — y, and the physical properties of the water р. ц, j3. A and ер are respectively: the density, the dynamic viscosity, the thermal expansion coefficient, the thermal conductivity and the specific heat at constant pressure, which are assumed constant and evaluated at 50zC from the data provided in [1].

COMPUTING MODEL FOR THE THERMAL ANALYSIS OF THE MODULES

To evaluate cell temperature, to use in the Evans equation, it was necessary to develop a finite difference computing model suitable to carry out the transitory thermal analysis of the layers of which the photovoltaic module is made up: the glass covering, a layer of EVA, the silicon layer, a layer of EVA, a layer of Tedlar, layer of PET and a layer of Tedlar (Green, 1992, Duffie et al., 1991, Clark et al. 1984).

In the model, as boundary conditions, the experimental values of the external surface temperatures of the module were considered, acquired by means of two thermal resistors as well as the value of the solar irradiance on the plane of the modules acquired by means of a pyranometer.

A thermal analysis by means of finite element softwares PRO-E and PRO-Thermal was also carried out as a comparison.

The temperature profiles obtained with the two models proved to be perfectly overlapping with small differences only in the silicon layer.

Fig. 5 (a, b, c, d, e, f, g,) shows the temperature profiles for the seven layers composing the module, obtained with the finite differences model, for July 2003.

As the figure shows, the temperature of the silicon layer (cell) is always greater than the reference one and reaches temperatures even greater than 80 °C.

SHAPE * MERGEFORMAT

Analysis of the model versus experiments

The CFD numerical simulation needs the radiative flux distributions on the inner surface of the cavity calorimeter, the surface of the copper cone that receives the concentrated solar radiation. Four different distributions were obtained using the CIRCE2 code [4], three of them corresponding to each of the three set of DEFRAC mirrors (A, B and c) and the

SHAPE * MERGEFORMAT

Figure 8 shows the wall temperature distributions of the inner cone obtained with the CFD simulation of CAVICAL1 calorimeter. It is possible to observe the influence of the heat flux distribution and incident radiative power on the temperature distributions. The maximum temperature value is function of the total incident radiative heat flux; the figure shows that for the tests using A, B and C set of mirrors with a total flux up to 300 W, the maximum temperature are near 60°C, while for the case T when the total incident flux is 876 W, the test shows a maximum temperature value of 90 °C. Also it is clear that there is a shape correspondence between the temperature distributions and the radiative heat flux profiles.

Figure 8. CFD Simulate Temperature profiles in the incident heat flux calorimeter wall. Test A, B, C and D.

Table 1 shows the energy balance for the four tests presented. As the table shows the convection losses in the calorimeter is only 0.22% and increase when the incident radiation increase. The Heat transfer estimated is near 51 W/m2K.

Table 1.

Parameter

Test A

Test B

Test C

Test T

Incident Energy, Qin (W)

333

327

273

876

Transfer Energy to Water, Qc (W)

332

326

271

873

Convection losses (W)

0.72

0.72

0.76

3.38

% of losses

0.22

0.22

0.27

0.39

Convection heat transfer coefficient (W/m2K)

51.05

51.19

50.96

54.66

Finally, figure 9 presents a comparison between experimental and theoretical wall temperature profiles of CAVICAL1 corresponding to the test of the group of mirrors A. As can be seen, the maximum temperature difference between the experimental and numerical values is less than 2 oC, showing a very good agreement.

6. Conclusions

A heat transfer numerical model based on the CFD FLUENT code of CAVICAL1 calorimeter was developed. The mathematical model was able to determined the position where the maximum temperatures occur, the shape of the temperature distributions, the heat concentrated solar power, the convection losses and the convection heat transfer coefficient. The model was validated through a comparison between experimental and theoretical wall temperature profiles of CAVICAL1 having a maximum differences less that 2 oC or less than 6%.

SIMULATION MODEL VALIDATION

Dunkle’s equation was originally applied in SOLSTILL to calculate the convective heat transfer between the basin water and the cover. This is in accordance with the approach used by the majority of previous researchers. The predicted temperature of the water in the still and the measured values are shown in Figure 3 while the predicted and measured distillate produced is shown in Figure 4. As shown in Figure 3, the predicted water temperature closely follows the trends in the measured data. The maximum discrepancy is 8 0C in the

most overcast day (i. e., the second day of testing), while the differences recorded for the rest of the days were up to 5 0C. Similar results were obtained for the predicted distillate from the still. The simulation model over-predicted the experimental result every day with a maximum error of 34 % on the cloudiest day. With the use of Dunkle’s equation, the model overpredicted the cumulative distillate by 14.7% over the 7 day period of testing.

Figure 4. The predicted distillate using the Dunkle model and measured distillate from the

standard still.

ADVANCED SOLAR DRYER FOR SALT RECOVERY FROM BRINE EFFLUENT OF DESALINATION MED PLANT

Manuel Collares-Pereira, INETI — Instit. Nac. Tecnologia e Engenharia Industrial — DER Joao Farinha Mendes, INETI — Instituto Nac. Tecnologia e Engenharia Industrial — DER Pedro Horta, INETI — Instituto Nacional de Tecnologia e Engenharia Industrial — DER

Water desalination is an important idea to alleviate potable water scarcity all around the World and there are already many commercial solutions. An important challenge still persists if one wants to use solar energy as the energy input to the system, thereby taking advantage of the fact that often this problem arises in areas of the world with abundant sunshine and little other energy resources.

The ongoing AQUASOL project [3] is one more attempt at putting solar energy to use in this context, with the objective of improving the economical and environmental performance of a MED desalination plant [1]. Within this project, reported elsewhere, an advanced solar dryer is being studied, allowing for brine concentration and/or ultimate salt recovery from the MED brine effluent. The idea is to add economical value to the investment in a MED plant, by providing one more product — salt — using the fact that the effluent of the MED plant has a higher salt concentration already and that the whole system might be integrated in a classical Saltworks, as one more step in the process.

1. Introduction

In recent decades, an increasing exploitation of water resources has lead to several forms of water shortage in many European regions (and elsewhere in the World), a problem assuming more alarming levels especially in semi-arid climate areas, where water, for human or agricultural consumption is either not supplied or supplied with scarcity and/or lack of quality. Often, in such areas, abundance of sea water and solar irradiation could use desalination as means for a medium-term sustainable process for potable water production. Yet, taking into account the proximity of sensible environments such as marine/tidal ecosystems, eventual negative impacts related with desalination effluent discharges must also be considered.

In light of the arguments above, the undergoing AQUASOL project [3] aims first at the development of a lower cost MED desalination technology with improved energy and environmental performance, promoting the use of solar energy both in the desalination and in the effluent treatment processes. The reduction of energy consumptions in the MED process, together with the exploitation of NaCl as a sub-product resulting from the effluent treatment process through brine concentration in a solar passive dryer, constitute likely means to accomplish a lower water cost objective. This will hopefully increase the competitiveness of MED technology when compared with the more common RO process. The present paper addresses the effluent treatment issue, taking into account the specificities of NaCl production. After development of a new concept of a passive solar dryer, based in the study of a numerical model describing dryer operation under given yearly climate conditions, a prototype is under test, allowing a deeper knowledge of the design and identification of further evaporation enhancement strategies.

The paper is organized as follows: in 2. a description of the dryer prototype is made; in 3. an overview of preliminary evaporation results, as well as comparison with simulation results, after the original numerical model, is presented; in 4. results analysis and numerical model correction is addressed; in 5. a brief idea is given of further prototype developments; in 6 a simulation of yearly results for the corrected numerical model is presented, and in 7 conclusions are presented.

Advanced Storage Concepts for Solar Combisystems

H. Druck, W. Heidemann, H. Mtiller-Steinhagen

Universitat Stuttgart, Institut fur Thermodynamik und Warmetechnik (ITW) Pfaffenwaldring 6, D-70550 Stuttgart Tel.: 0711/685-3536, Fax: 0711/685-3503

email: drueck@itw. uni-stuttgart. de, Internet: http://www. itw. uni-stuttgart. de

Using a typical single family house in Germany as an example, the influence of the solar collector area and the store volume on the energy savings is determined by means of numerical system simulations. Based on these results it is outlined how the system performance can be increased by using advanced storage concepts.

In particular the following storage concepts are investigated:

• hot water stores with improved thermal insulation (e. g. with vacuum insulation)

• stores using phase change materials (latent heat stores)

• thermochemical energy stores (e. g. based on sorption)

In addition to the primary energy savings that can be achieved with the different heat storage technologies and system concepts, the resulting solar thermal heat prices and the energy payback times are discussed.

1 Introduction

Thermal solar systems for domestic hot water preparation and space heating, so-called solar combisystems, are already introduced to the market, and their market share is increasing continuously. Today standardised solar combisystems consist of a solar collector with an area between 10 m2 to 20 m2 and a hot water storage tank with a volume in the range of 0.7 — 1.5 m3. If such systems are installed in a „typical" middle European single family house, they can save approximately 20 — 30 % of the primary energy required for domestic hot water preparation and space heating. In order to increase the energy savings, larger collector areas and/or store volumes are required.

Uncertainty calculation applied to different. regression methods in the. quasi-dynamic collector test

Manfred Georg Kratzenberg1, Hans Georg Beyer2, Sergio Colle1, Dirk Petzoldt1,2
1Solar Energy Laboratory Federal University of Santa Catarina, Department of Mechanical

Engineering, Florianopolis, Brazil

1 Department of Electrical Engineering, University of Applied Science Magdeburg-Stendal,

Magdeburg, Germany

1. INTRODUCTION

Testing the efficiency of solar collectors is a basic pre-requisite to obtain performance characteristics for thermal solar collectors.

In order to quantify those characteristics, we must run a test to determine the efficiency curve coefficients of those solar collectors.

In an international context, standards for the respective test procedures are given by EN 12975 [ 1 ] and ISO 9806 [ 2 ]. In Brazil the standard is given by the NBR 10184. Only the EN 12975 provides different test procedures, the Steady-State Test (SST) and the Quasi-Dynamic Test (QDT). The QDT has the advantage of allowing the execution of more collector tests within the same time period, with the same test equipment and the same test facility as compared to the steady state collector test [ 9 ]. On the other hand the quasi­dynamic test requires a somewhat more demanding effort for the calculation of the collector coefficients.

There has already been made a comparison between SST and QDT [ 7 ], but there have been no statements about the uncertainties. The purpose of the present work is to discuss and evaluate of test procedures in view of the uncertainty of the determination of the collector coefficients for the QDT. The Least Square (LS) and the Weighted Least Square (WLS) regression methods applied in the trend setting QDT are presented in this paper. Uncertainties of the LS and WLS results are calculated. A methodology for the verification of real confidence limit of the LS and WLS uncertainties is presented.

RADIATION & THE COLLECTOR

The transmittance, reflectance, and absorptance of a single cover, allowing for both reflection and absorption losses, can be determined by ray-tracing techniques. For the perpendicular component of polarization, the transmittance, reflectance and absorptance of the cover are [4]:

(2.2.1)

(2.2.2)

(2.2.3)

Similar results are found for the parallel component of polarization. For incident unpolarized radiation, the optical properties are found by the average of the two components.

2.3. TRANMITTANCE-ABSORPTANCE PRODUCT (та)

Some of the radiation passing through the cover system is reflected back to the cover system while the remainder is absorbed at the plate. In turn, the reflected radiation from the plate will be partially reflected at the cover system and back to the plate as illustrated in Figure 2.2.1. In this figure, т is the transmittance of the cover system at the desired angle, a is the angular absorptance of the absorber plate, and Pd refers to the reflectance of the cover system for diffuse radiation incident from the bottom side. It is assumed that the reflection from the absorber plate is diffuse and unpolarized. The multiple reflection of diffuse radiation continues so that the fraction of the incident energy ultimately absorbed becomes [4]:

(та) |eff = та І[(1-а) pd]n =______ ш___

n=0 1-(1- a) Pd

Where pd can be estimated from the following equations at an angle of 60o.

T = °.5 (Tperp + TparraO

(2.3.2)

P= °.5 (Pperp + PparraO

(2.3.3)

G = °.5 (G perp + G parral)

(2.3.4)

On the Determination of the Effective Thermal Capacity of Solar Collectors

W. Eisenmann, F. Pujiula, H. Koln
Institut fur Solarenergieforschung GmbH Hameln/Emmerthal (ISFH)

Am Ohrberg 1, 31860 Emmerthal

Tel.: ++49-5151 / 999-521; Fax: -500, E-Mail: w. eisenmann@isfh. de

1 Introduction

The determination of an effective thermal capacity of solar collectors is necessary for the description of time-dependent processes. The capacity is important for the theoretical simulation of the yield of solar thermal systems (mostly on a daily or yearly basis, e. g. for methods to guarantee or control solar yields). The procedures for the determination of the effective capacity described in EN 12975 give strongly varying results. Depending on the collector type this leads to more or less pronounced differences of the simulated yield, where higher capacities mean lower yields.

This paper investigates which of the procedures for determination of the capacity is most appropriate for the description of dynamic processes, with respect to daily or yearly collector gains.

2 Distributed Capacity and Models with Several Nodes

The basic difficulty of the determination of an effective capacity is that the thermal capacity is spread over the components of the collector, and that the thermal coupling with the temperature fluctuations of the fluid differs from one component to the other. For example, the absorber is generally in close thermal contact with the fluid, while the thermal coupling between the glass cover and the fluid is fairly weak.

Consequently, it would seem appropriate, at least from a scientific point of view, to develop collector models with more than one node (at least one node for the absorber and one for the fluid), see e. g. [2].

Nevertheless, for practical reasons one-node models should be preferred for the present revision process of EN 12975 and for collector tests. One reason is that there is no widespread experience with several-node models. Furthermore, the determination of the parameters of these models would most probably be quite difficult and inaccurate.

All the procedures for the determination of the collector capacity described in EN 12975 are based on one-node models.

RADIANT FLOOR HEATING SYSTEMS SUPPLIED BY. SOLAR COLLECTORS. THERMAL AND ECONOMIC ANALYSIS

N. Arcuri, R. Bruno, S. Ruffolo,

Department of Mechanical Engineering — University of Calabria
87036 Rende, Cosenza
e-mail:natale. arcuri@unical. it

01

Abstract

A procedure developed in a TRNSYS environment has allowed to estimate the thermic performances of a radiant floor system supplied by solar collectors, connected to a house. A suitable control strategy has been employed to use in the best way the incident solar energy. For three representative localities of Italian territory the curves of solar fraction have been obtained, as well as efficiency of the system and some economic indices in relation to the variation of collectors’ surface and tank’s storage volume.

1. INTRODUCTION

The increasing greenhouse gas concentration and the successive risk of climatic changes can be mostly attributed to the use of fossil sources in order to produce energy. Reducing their emissions implies turning to a system of energetic supplying based on the use of renewable energies, especially solar energy. To get a remarkable energetic saving, several ways can be used to reduce the building thermal requirements. This can be surely obtained by using active solar systems. Using solar energy to domestic hot water (DHW) has demostrated that solar technology is fully developed and it can be used for house heating too. The present work has considered a solar combisystem to produce hot water both for house heating and domestic hot water to satisfy a fraction of yearly energy requirement, that it is function of the system size. Among heating systems the radiant floor is the best to exploit solar energy since thermal power must be supplied by a flow rate at low temperature that may be produced by a solar system with satisfactory efficiency. A comparison between radiant floor heating system with fan coil system has evidenced energy requirement savings in the period of heating equal to 24% for Cosenza, 21% for Rome and 18% for Milan [1,2].

The present work develops an energy analysis about a house heated by radiant floor. The radiant floor is supplied by a secondary circuit powered by a storage tank that is ulteriorly supplied with thermic energy by a field of solar collectors. The tank acts as a thermic accumulator and regulates inlet temperature by a mixing valve. When heat level of tanking fluid is not enough to satisfy energy requirement, an independent system supplied by traditional fuel is actived. A heat exchanger in the storage tank provides domestic hot water with an auxiliary heating system to get wished temperature. Radiant floor has been designed by a stationary method, according to UNI EN 1264 standard. The building thermic design is suited to N°10/1991 italian Law. The system building-radiant floor has been studied by the simulation code TRNSYS [3] which has made possible to estimate the fraction of energy requirement from the solar source in relation to the collectors’ surface and of the tank’s storage volume variation. To simulate the climatic variability, an appropriate procedure has been used, generating hour values of solar radiation and outside air temperature starting from respective monthly average data.