Category Archives: EUROSUN 2008

Meshing of the New Concepts

Figure 9. Mesh for Tanks 2 to 4 Figure 10. Mesh for Tank 5 Figure 11. Mesh for Tanks 6 and 7

Подпись: Figure 9. Mesh for Tanks 2 to 4 Figure 10. Mesh for Tank 5 Figure 11. Mesh for Tanks 6 and 7

Geometric similarities between Tanks 2 to 4, and between Tank 6 and 7 resulted in the generation of only three new meshes, in addition to those of the original study. These are illustrated in Figures 9 to 11 with a summary of all mesh details provided in Table 2.

Tank ID

Dimensions

(# Faces, plane x # Nodes cyl)

Equiangle Skewness

(< 0.5)

Number of Cells

Подпись: Tank ID Подпись: Number of Cells Подпись: Dimensions (# Faces,plane x # Nodes cyl) Подпись: Equiangle Skewness (< 0.5)

Table 2. Characteristics of Developed Meshes

Validation of the Simulation

Charging and discharging processes where calculated in EES and Matlab and validated with experimental Data. Figure 8 compares the simulated and experimental progress of the charging rate at a charging temperature of -10°C. With the exception of the starting-period, the agreement between measured and predicted values is very good. For the starting period, the simulation predicts an average value without the artificial fluctuations caused by the thermostat.

Figure 9: Comparison of simulated and experimental Charging Rate

 

Nomenclature

Q cond

Conducted heat flow rate,

W

Qlat

Latent heat flow rate,

W

Q kon

Convective heat flow rate,

W

^■ice

thermal conductivity of ice layer,

W/(mK)

A.

-T^ice

Outer surface of ice layer,

m2

rice

Radius of ice cylinder,

m

Ahs

latent heat of solidification,

kJ/(kgK)

mice

Mass of ice,

kg

pice

Density of ice,

kg/m3

aout

heat-transfer coefficient,

W/(m2K)

Ltube

Length of tube,

m

Temperature of liquid water,

°C

Temperature at ice surface,

°C

References

[1] Brendel, T., Spindler, K., Muller-Steinhagen, H.: Aufbau einer Versuchs — und

Demonstrationsanlage zur solaren Kuhlung, DKV Jahrestagung Bremen, 2004, S. 105-118

[2] Hilligweg, A.: Vordimensionierung von Eisspeichern; TGA Fachplaner 6-2003; S. 29

[3] Streit, D.: Freie Konvektion am horizontalen Rohr in einer Kies/ Wasser-Schuttung mit Bildung und Schmelzen von Eis; Dissertation; 1996; S. 59

Separate reactor or integrated reactor

The hydration and dehydration of the TCM material take place in a reactor. This may be a separate reactor or the reactor may be integrated in the storage, as shown in the systems presented in Fig. 1.

If the TCM material is a solid, nowadays integrated reactors are used. This has the advantage that it is not necessary to transport the solid material. However, for large storages such as seasonal storage, this also has disadvantages such as a larger sensible heat loss, especially if the storage is not compartimented since then the whole reactor content has to be heated up before dehydration occurs. Also the required heat exchange area is much larger than for a separate reactor. In a separate reactor, it is possible to heat up only the required amount of TCM, thereby making the charging process much more efficient. Similar benefits occur on discharging the material. Also, the heat- and vapour transfer can be optimised within a separate reactor by stirring of the material.

Maintenance and Implementation of Solar Ponds

A SGSP collects and stores solar energy all over the year being the storage zone designed in a way that the summer stored energy can be used during winter [7]. Temperatures in the Storage Zone can range from some degrees above ambient temperature to values reaching 100 °C, depending on the Solar radiation available in the region and on the design of the storage zone and thus are appropriated for a wide range of applications. In this work we will focus in Aquaculture facilities.

In fact, the decrease of natural catches and the increase of fish demand as a food source, led to a growing interest in intensive aquaculture. In Portugal, Aquaculture growth, in the last decade, with an hundred farms providing 5% of the total consumed fish. Portugal is also the European country with largest per capita consumption and one of the highest in the world, with an annual average consumption of 60 kg/person.

In order to build a Solar Pond the basic conditions are the union of three factors: good solar irradiation good access to salt (typically NaCl but other available salts can be used) and existence of water. In the presence of these conditions the initial cost of construction evolves basically: excavation and waterproofing of a basin and the installation of some kind of heat extraction device. In a second phase the pond must be monitored making it possible to measure salinity and temperature gradients and the state of the Pond in terms of transparency to solar radiation. Those measurements will allow to predict the beginning of instabilities in the gradient zone allowing consequently its reposition. It may be referred that the basic destabilizing process in the gradient zone is the diffusion of salt which is a very slow process which in fact is the reason why solar ponds actually work. However the presence of the opposing temperature gradient can give rise to double diffusive instabilities and from time to time maintenance operations are necessary to correct the salinity gradient to values well above the critical values for the start of instabilities [8].

These corrections can be the extraction of a higher quantity of thermal energy from the storage zone, if the temperature achieved by the Solar Pond is higher then a known value, the injection of salt if the initial imposed profile is getting smooth, as a consequence of the diffusion process or of any convective instability, the injection of low concentration water on top of the gradient zone in the case of he existence of great losses by evaporation and the introduction of wind suppressors at surface, in the case of the existence of wind ripple at the surface [9].

If only the salt diffusion process was involved as a destabilizing process in a SGSP the expected lifetime of these devices without maintenance would be around ten years. However the above mentioned double diffusion instabilities and also the gradual decrease of transparency lead to the necessity of maintenance of the pond at least two times a year.

The idea behind this paper is to propose a different concept for the maintenance of a solar pond making maintenance operations simple standardized and with an annual periodicity.

Monte Carlo radiative transfer simulation of a sola thermochemical reactor

H. I. Villafan-Vidales*, C. A. Arancibia-Bulnes, U. Dehesa-Carrasco

Centro de Investigation en Energia, Universidad National Autonoma de Mexico, A. P. 34, Temixco 62580,

Morelos, Mexico.

Corresponding Author, hivv@,cie. unam. mx
Abstract

The radiative heat transfer in a solar thermochemical reactor, for the thermal reduction of cerium oxide, was simulated with the Monte Carlo method. The model takes into account, in a detailed manner, the directional characteristics and the power distribution of the radiative energy that enters to the reactor. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The optical properties of cerium oxide were obtained from Mie-scattering theory. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the concentrator.

Keywords: Thermochemical reactor, Solar concentrator, Cerium oxide, Radiative transfer.

1. Introduction

Water-splitting thermochemical cycles constitute one of the possibilities for hydrogen production. Greenhouse gas emission can be avoided in these cycles, if solar radiation is used as energy source. Some of them are based on two-step metal-oxide redox pairs like the ZnO/Zn cycle, which has been studied extensively in connection with solar energy utilization [1]. In a recent publication, Abanades and Flamant [2] have demonstrated experimentally a new cycle based on cerium oxide. This cycle consists of the endothermic reduction of Ce2O3 at high temperature (2300 K), where solar energy is used as a source of heat; and of the subsequent steam hydrolysis (700-800 K) of the resulting cerium sesquioxide to produce hydrogen, as in the equations below

2CeO2 (s) ^ Ce2O3 (s) + A O2 (AH = 198 kJ/mol at 2300 K) (1)

Ce2O3(s) + H2O (g) ^ CeO2(s) + H2 (g) (AH = -125kJ/mol at 700 K) (2)

One of the main advantages of this process is that the reduced oxide remains in the condensed phase, while oxygen is released and transferred to the surrounding gas, and the reverse reaction is not carried out during the material quenching. The opposite happens for instance in the reduction of zinc oxide, where zinc and oxygen are produced simultaneously and can recombine if cooling is not efficient [2].

Solar thermochemical reactors are used in the endothermic step of the above cycles. These reactors sometimes feature cavities containing directly irradiated reacting particles. In particular, as in the case of CeO2, cavity receivers may help to compensate for the very low solar absorption of the material particles (scattering albedo of 0.75), although initial demonstrations have been carried out in an all glass reactor. To propose a reactor design that deals with this problem it is necessary to carry out a radiative modelling.

In this work, radiative heat transfer is analyzed within a cylindrical cavity solar reactor by the Monte Carlo method [3]. The reactor is assumed to contain a gas fluidized suspension of radiatively participating CeO2 particles. The model takes into account the radiative characteristics of the particles (absorption and scattering cross sections) as predicted from the Mie scattering theory, and by using the optical properties of CeO2 measured over the UV, visible, and infrared spectrum. The opening of the cavity is assumed subject to concentrated solar radiation from a paraboloidal concentrator.

Screening of inorganic PCMs in the temperature range 120-250°C

Fig. 2 shows inorganic materials which have been proposed as a PCM. For comparison also two metals are shown. The lightweight alkali metal lithium has a high melting enthalpy, whereas metals with a high molar mass, such as tin show low enthalpies (Fig. 2). One major disadvantage of lithium is the handling. Tin has a high thermal conductivity of a few 10 W/mK in the solid and liquid phase around the melting temperature. However, for large scale systems tin is not attractive from an economical point of few due to the low melting enthalpy and the high price per mass. For temperatures below 100°C, salt hydrates are a major PCM option. There are also salt hydrates above 120°C (Fig. 2)[7,24]. However, the release of water and encapsulation at these temperatures can be regarded as challenging.

Metals

(solid-liquid)

Anhydrous Salts

Alkali

Lithium 181°C, 432 J/g

Others
Tin 232°C, 59 J/g

Classified by anions, typical cations are Li, K, Na, Ca, Mg

solid-liquid

Nitrates Nitrites Hydroxides Thiocyanates Chlorates Chlorides

solid-solid

Na2SO4 250°C, 60 J/g KOH 250°C, 110 J/g

Salt Hydrates MgNO32H2O 130°C MgBr2-6H2O 172°C

— Inorganics

Fig. 2. Inorganic PCMs in the temperature range 120 to 250°C.

There are solid-solid and solid-liquid phase transitions of anhydrous salts which can be utilized. As low cost PCM with solid-solid phase transition, sodium sulphate (Na2SO4) has been examined [24, 25]. The transition enthalpy is rather low (Fig. 2) and difficulties with the mechanical stability

have been reported [25]. Other solid-solid PCMs, such as anhydrous KOH, have been also suggested (Fig. 2)[1,5,24].

There are different types of anhydrous salts and salt mixtures with a melting temperature in the range 120 to 250°C. Table 2 summarizes suggested PCMs in thermal energy storage literature classified by their anions [1,3,5,7,8,9,24,26-30]. Additionally, there may be other salts with a suitable melting temperature, such as alkali metal thiocyanates.

Table 2. Suggested anhydrous salts and binary mixtures with a melting temperature from 120 to 250°C.

Group

System

Tm

1°C1

H

[J/g1

KNO3-LiNO3

140

170

Nitrate

KNO3-NaNO3

220

100

LiNO3

253

370

Chlorate

NaClO3

225

210

Chloride

Al2Cl6

192

260

KCl-ZnCl2

230

200

Hydroxide

KOH-NaOH

171

210

Nitrate-Hydroxide

NaNO3-NaOH

254

290

Nitrite-Hydroxide

NaNO2-NaOH

232

252

The scope of IEA SHC Task 32, subtask C, phase change materials (PCM)

The scope, in terms of general system aspects, for Subtask C was the same as that for the whole of Task 32, namely solar heating and cooling systems for residential buildings, principally detached houses for one up to a few families. Buildings with a larger specific heat load (>100 kWh/(m2a) for Zurich climate) are not considered. The main focus was to find storage solutions sized to achieve a significant solar fraction but also for other applications in the heat storage field for domestic housing, especially to reduce the cycling rate of conventional boilers.

Some solutions using PCM have already been tested in full scale pilot plants and some durable commercial products are already on the market for special applications (Cristopia, Rubitherm among others).

Detailed activities included

• the selection of suitable materials,

• the development of storage prototypes and

• the optimization of existing solutions in an integrated system such as the reference combisystem defined by Subtask A.

All solutions with PCM stores were compared to pure water stores.

Fig. 1 shows a classification of processes for PCM storage of heat, in Subtask C only the paraffins (analytical grade) and hydrated salts have been addressed.

Fig. 1. Classification of energy storage materials [1].

Подпись: Fig. 1. Classification of energy storage materials [1].

In terms of temperature, the storage solutions have been limited to temperatures < 85°C, because the maximum needed temperature for the domestic applications with low temperature heating systems is the DHW demand with around 50°C. The phase change temperature of the materials chosen (mainly sodium acetate trihydrate, partly embedded in a graphite matrix to increase the thermal conductivity) is at about 58°C. For some other tests additional PCM with a lower phase change temperature was chosen (paraffin).

Simulation models of the PCM storage component were developed for different types of PCM heat store philosophies, as no validated models were available for the simulation software TRNSYS at the beginning of the task. These models were validated by laboratory tests before being integrated into a system model within TRNSYS. Each Subtask was responsible to develop an appropriate tool, in order to enable an estimation of the performance of a system with the proposed storage concept.

Thermal curtains

The use of thermal curtains during the heating season can lead to energy savings in the order of 22% to 58% [6]. Because AT from inside to outside is relatively small in our location (lower rate of thermal loss), we will assume that, with the use of these techniques, the energy consumption can be decreased only by 20%. Thus, the consumption on the above example would decrease to approximately 25.651,1 kWh/year and the solar contribution would be 24.60,9 kWh/year (93.8%).

2.1. Ground (root zone) heating analysis

Ground heating at low temperatures (max. 40°C) allows to decrease the inside temperature of a greenhouse between 3° to 6°C. According to the Rutgers report [7] this technique can lead to energy savings in the order of 20% in colder regions of the U. S.A.

A typical temperature pattern for a 60cm tall crop with an outside temperature of -12°C would be a floor temperature of 24°C, a canopy temperature of 13°C and a temperature of 9°C at 120cm above the ground [8]. Thus, setting the mean inside temperature of the air in contact with the glaze at 12°C instead of the previous 14°C, the energy consumption for the simple greenhouse in the example will now be 15.632,9kWh/year (Fig. 3), which represents a reduction of 51,2% on the original demand.

This apparently exaggerated reduction results from the fact that the temperature we are now trying to maintain inside the greenhouse is significantly closer to the outside air temperature. Adding this reduction to the 20% obtained with the use of thermal curtains, the energy consumption will decrease to 12.506,3 kWh/year, representing a total reduction of 61,0%. Figure 3 also shows the energy requirements of our standard greenhouse after being fitted with thermal curtains and root zone heating.

Element determination and element concentration across the solidified salt melts in graphite composite

The x-rays emissions with energy characteristics of the composite chemical elements let completed of the qualitative and quantitative elemental analysis on elemental compositions distributed on a sample surface and in a depth of 1-2 microns.

Fig.3. Point spectrum (yellow) of the point area A of the NaNO3 /KNO3 eutectics/graphite NG (20%) composite, compared to the integral spectrum (red) scanned for the sample section. Peak energy levels characterize chemical elements: K, Na, N2, O2, and C.

Подпись: Fig.3. Point spectrum (yellow) of the point area A of the NaNO3 /KNO3 eutectics/graphite NG (20%) composite, compared to the integral spectrum (red) scanned for the sample section. Peak energy levels characterize chemical elements: K, Na, N2, O2, and C.

The EDS spectra were used for a qualitative element analysis of the composite sample. Peak energy levels characterize chemical elements: K, Na, N2, O2, and C. (Fig.3.). Comparable EDS qualitative elemental analysis on both, a point elemental spectrum and an integral elemental spectrum made on the Area A of NaNO3 /KNO3 eutectics/ graphite NG (20%) composite indicate negligible shifting in energy levels.

Different areas and segments of the same composite sample were EDS scanned, but the element determination shows variety of elemental distribution in different sections taken across the solidified salt melts in the graphite matrix. The element concentrations in micro-volumes of the composite were calculated by quantitative element analysis of the peak energy levels from EDS spectra. The calculated data for element concentration of two different area of the same sample were summarized in Table 5.

Table 5. EDS quantitative point elemental analysis of the NaNO3/KNO3eutectics/graphite (20%) composite. Element concentration is measured in weight percentage (wt%).

Point area of the Salt/Graphite Composite

Chemical element [wt %]

N2

O2 Na K

A

16.59

50.77 9.81 21.69

B

9.86

55.7 17.64 11.71

Element concentration presented on Table 5, suggest the element K-rich area in the point area A of the composite sample scanned. The concentration of the element K (from KNO3) is 21.69 wt%, calculated in the fixed point area A of a salt/graphite composite, while the concentration of the

element Na (from NaNO3) is calculated as 9,81 wt%. The calculated element concentrations at a point area B show different values than values for point area A. So, the chemical element concentrations vary depending on spectral area scanned.

The EDS calculated data illustrated clearly the variations in the K and Na element concentrations fixed at different point areas but scanned for the same sample of the salt/graphite composite after its re-crystallization. Therefore, the elemental distribution of salt eutectics is not homogeneous and defines some irregularity in the composite structure and thermophysical properties of the salt/graphite composite.

2. Conclusion

Thermal behaviour of the novel KNO3/NaNO3eutectics/graphite composite depends on intermolecular interaction of both components: salt and graphite. Correlation between thermal and structural analysis of the composite allows identifying the factors for efficient release of heat absorbed and stored.

Solidification of the re-melted KNO3/NaNO3 eutectics in graphite composite led to elemental separation and forming the Na-rich spherical masses and the K-rich layered structure over the graphite plane, clearly proved by methods of x-ray microanalysis. As the products of chemical degradation or chemical reaction were not identified, the inhomogeneous elemental distribution characterizes the salt in salt/graphite composite structure and presents a factor for proper eutectics salt crystallization, finally responsible for structural and thermal stability of composites during repeatable heat charging/discharging process.

References

[1] K. Lafdi, O. Mesalhy, A. Elgafy, “Graphite foams infiltrated with phase change materials as alternative materials for space and terrestrial thermal energy storage applications”, CARBON, 46 (2008) 156-167.

[2] Z. Zhang, X. Fang, “Study on paraffin/expanded graphite composite phase change thermal energy storage material”, Energy Conversion and Management, 47 (2006) 303-310.

[3] S. Pincemin, R. Olives, X. Py, M. Christ, “Highly conductive composites made of phase change materials and graphite for thermal storage”, Sol. En. Mat. & Sol. Cells, 92 (2008) 603-613.

Heat transfer effectiveness

Making sure that air velocity stays below the critical value, a constant sand flow and a steady state temperature field specific for cross flow heat exchangers similar to Fig. 2 can be obtained.

The heat transfer effectiveness is defined as the ratio of actual and maximum possible heat flow. In a system with different products of mass flow rate and heat capacity for the two media, the maximum possible heat flow is defined by the lower value:

s _ Q (4)

(m cp L •( — TS, in)

As for the power plant operation both high sand output temperature and low receiver air heat losses are desired, for the heat exchanger measurements it is aimed for balanced products of mass flow rate and heat capacity for air and sand.

For an effectiveness measurement with sand grain size 1-2 mm the following values were measured: Ta, hi=687 °C, TS, in=25 °C, mA =0.0143 kg/s, mS =0.0205 kg/s, TA, out=62.4 °C, Ts, out=427.5 °C

The lower product of mass flow and heat capacity is defined for air. Further decrease of sand flow is impeded by the valve design, otherwise higher sand output temperatures could be obtained.

The heat exchanger effectiveness comes to

______________ Q_______________ _ mS • cp, S (Tr. f ^ Tin, S ) • (Tin, S — Tref )

‘ (cp )A * ( — Ts,„, ) _ (43kg/s • cp) • (687°C — 25°C)

mS • cp, S(Tr. f ^ T„„.,S) * (Tout, S — Tr. f )

(0.0143kg/s • cp )*(687°C — 25°c) (5)

0.0205 — • 920 J •(427.5°C — 25°C)

0.748

Подпись: 0.748________ s kg • К v___________________ ’ _ 7591W

0.0143^ 4072 — l(687°C — 25°C) 10148W

s kg • К)y ’

The measured heat exchanger effectiveness is clearly below the values obtained from simulation of 0.8-0.9, according to section 2.2. This is partly explained by heat losses on the heat exchanger surfaces and air leakages. Additionally, it is a result of disturbing effects due to the limited heat exchanger width.

Figure 5 shows a front view on the air outflow area and the according air outflow temperatures. As expected the temperatures increase to the bottom, similar to the right line in the middle area of Fig. 2. However, moving on horizontal lines from the centre to the heat exchanger frame the temperatures increase, too. Whereas for determination of effectiveness the air and sand temperatures are measured in the centre section, the energy balance is largely determined by the differing temperatures on the heat exchanger frame region on the left and right side. The temperature increase is attributed to heat conduction effects in the steel frame and air leakages between steel frame and sand.