Category Archives: EUROSUN 2008

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 %]


O2 Na K



50.77 9.81 21.69



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.


[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)


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.

Original Hot Tank 141,865 1585 x90 99.17% Study [6] Cold Tank 140,156 1496 x 94 99.74 % Tank 1 same as Hot Tank above Tanks 2 to 4 153,355 2060 x 76 97.81 % Tank 5 198,387 2576 x 76 97.30 % Tanks 6 to 7 600,066 3565 x 169 99.43 % . Simulation Profiles

The charging performance of the tank concepts is to be assessed under both constant and variable inlet conditions. This is illustrated in Figures 12 to 14 and Table 3. The energy curve applied to the variable conditions was taken from TRNSYS simulation data provided by the Drake Landing network. The energy input curve for the Variable Temperature and Variable Flow Rate (VTVFR) case below represents a clear day with a sudden drop in available energy to drive a decrease in the inlet temperature.

The conditional algorithm for the VTVFR case was derived from the sequence of control currently in use at the DLSC. However, the upper limit on the inlet temperature (60 °C) was chosen arbitrarily as the value is strongly dependent upon environmental considerations. Thus, although 60 °C shall be used for this test, it should be noted that future simulations could determine the system’s sensitivity to various upper temperature limits.

Thermal Reservoir



= 25 °C

Figure 12. Constant Temperature and Constant Flow Rate (CTCFR)

Desired Temperature

Tdes = 60 °C


Desired Flow Rate

Q (t)


Energy Profile, Q(t)

Подпись: Energy Profile, Q(t)

initial ’

Подпись: initial ’

Figure 14. Variable Temperature and Variable Flow Rate (VTVFR)

Подпись: Upper Temperature Limit on Delivered Energy = 60 °C

Sorption Storages for Solar Thermal Energy — Possibilities and Limits

A. Hauer

Bavarian Center for Applied Energy Research — ZAE Bayem Dept. 1: Technology for Energy Systems and Renewable Energy Walther-Meissner-Str. 6, D-85748 Garching hauer@muc. zae-bayern. de


Based on the laws of thermodynamics and the sorption theories, possibilities and limits of sorption storages for solar applications can be defined. For the storage of thermal energy closed and open sorption systems as well as solid and liquids sorbent materials can be utilized. Each system has its own advantages and disadvantages. Over the last years a number of R&D activities were performed. Examples are a closed adsorption storage system in Austria for seasonal storage of solar heat and an open liquid absorption storage system for solar cooling in Singapore. The conclusion of this paper is that even the high storage capacities and the possibility of providing heat and cold of sorption storages do not solve all solar thermal storage problems. It is still necessary for each system constellation to find an appropriate application and to carefully check the relevant boundary conditions.

Keywords: Sorption Storage, Adsorption, Absorption, Liquid Desiccant Cooling

1. Introduction

Thermal Energy Storage (TES) is crucial for the efficient use of solar energy. TES systems are able to buffer the variable supply of solar radiation in a short term TES. Seasonal TES systems are able to keep the thermal energy surplus from summer to winter.

A high storage capacity and low thermal losses over the storage period are preferable for TES systems in general. Looking at the three possible technologies for TES — sensible, latent and thermochemical TES — thermochemical systems seem to have optimal properties for solar thermal applications, since they are able to achieve the highest capacities and have no thermal losses.

Thermochemical TES are utilizing reversible chemical reactions. The number of possible reactions for this application from first principle is huge, however only very few are suitable with respect to their reaction temperature. The processes of adsorption on solid materials or absorption on liquids are the most investigated ones. Figure 1 shows the adsorption process schematically.

Figure 1: Adsorption process of water vapour on solids


Adsorption means the binding of a gaseous or liquid phase of a chemical component on the inner surface of a porous material. During the desorption step — the energetic charging step — heat is provided to the sample. The adsorbed components — in this example water molecules — are removed from the inner surface. As soon as the reverse reaction — the adsorption — is started by adding water molecules to the sample, the molecules will be adsorbed and the heat, brought into the system during desorption will be released. The adsorption step represents the discharging process.

Figure 2 shows the examples of liquid and solid open sorption storage systems. In both cases the desorption is activated by a hot air stream carrying the heat of desorption. For the solid a packed bed of adsorbent pellets and for the liquid solution a reactor are blown through, leaving the packed bed dry and the solution concentrated.

air + water

:nt ed


Adsorbent Packed Bed!

air + water


heat of


ДЩД ~~ -|


oncentrated Salt Solution

Salt Solution

Solid Adsorbent Liquid Absorbent

Figure 2: Examples of open sorption storage systems during desorption / charging

TES can be achieved by separating the desorption step (charging mode) from the adsorption step (discharging mode). After desorption the adsorbent and the absorbent can theoretically remain in the charged state without any thermal losses due to the storage period until the adsorption process is activated.


Подпись: Humidifiei


Подпись: Cool Solid Adsorbent Hot

і і 0гУ

Liquid Absorbent Dry Cool

Figure 3: Examples of open sorption storage systems during adsorption / discharging

Figure 3 shows schematically the discharging of open sorption storages. Humid air blown through the storage becomes dry and can be used for dehumidification or, by adding a

humidification step, for cooling (desiccant cooling systems). If solid adsorbents are used the air might be very hot after the adsorption. This heat can be used for heating purposes.

Open and Closed Sorption Storages

The charging process of a sorption TES is a reaction where two components A and B — adsorbent and adsorbed water — will be separated by the input of heat Q and entropy S : AB o A + Bg. The water will be evaporated in this step.

In an open system Bg can be released into the ambience (see figure 4). For the discharging Bg has to be provided to the reactor in sufficiently high concentrations. The water vapour will be adsorbed again and the stored heat can be released.

Figure 5: Schematic view of the thermodynamics of a closed sorption storage

The evaporated component Bg will be condensed during the charging step in a closed (evacuated) system. This is in order to reduce the volume. The heat of condensation and its entropy have to be dissipated into the ambience. For discharging the condensed water has to be evaporated again in order to be adsorbed at the reactor [1].

Closed Sorption Storage Systems

A closed sorption system is shown in figure 6. It is based on the same physical effect as the open storage. However the engineering is quiet different from open sorption systems. Closed system could be more precisely described as evacuated or air-free systems. The operation pressure of the fluid to be sorbed can be adjusted in theses systems. In closed systems chemical components, which do not exist in the atmosphere, can be used, because there is no connection to the ambience.

Figure 6 is showing a closed sorption system using water vapor as adsorptiv. The heat has to be transferred to and from the adsorbent by a heat exchanger. This holds also for the condenser/evaporator. Heat has to be transported to the adsorber and at the same time the heat of condensation has to be distracted from the condenser in order to keep up the water vapor flow from the adsorber to the condenser during the desorption. During adsorption the heat of adsorption has to be taken from the adsorber and the heat of evaporation has to be delivered to the evaporator. Is

this not possible, the sorption process will reach thermodynamic equilibrium and the flow of water vapor comes to a stop.

The main problem in the system design is the heat and vapor transport in and out of the adsorbent. Advanced heat exchanger technologies have to be implemented in order to keep up the high energy density in the storage, which would be reduced by the amount of "inactive" heat exchanger material.

Desorption Adsorption

Charging Discharging

Water Vapor Water Vapor

QDes Qcond QAds QEvap

Figure 6: Closed Sorption system

Thermal energy storage can be realized by closing the valve between adsorber and condenser/evaporator after desorption. The energy density expected is reduced compared to open sorption storages due to the fact that the adsorptive (water vapor in this case) is part of the storage system and has to be stored as well. In the case of Zeolite or Silicagel as adsorbent this is about 30% to 40 % of the weight of the storage material [2], [3].

Closed systems are able to reach higher output temperatures for heating applications compared to open systems. Furthermore they can supply lower temperatures for cooling, e. g. it is possible to produce ice in the evaporator [4].

Reactor boundary conditions

1.1 TCM reactor system

A parametric study was carried out for a TCM system for seasonal storage. A schematic layout of this system is shown in Fig. 2. A 200 liter water tank was added to the system in order to lower the power demands for the TCM storage. In this way, the water tank is used to supply peak power (e. g. shower peaks), while the TCM storage can recharge the water tank afterwards at a lower power level. The collector system is designed in such a way that the water tank is preferentially loaded. Only if the water tank has reached its maximum temperature, the TCM storage is charged with the excess heat of the solar array. The TCM storage consists of 3 vessels: vessel C containing the hydrated salt, vessel B containing the dehydrated salt and vessel A containing the water vapour (in condensed form). On charging the storage, the hydrated salt from container C is fed to a dissociation reactor that is heated by a vacuum collector array, whereby the water vapour is released from the salt. The hot dehydrated salt is fed to B and the water vapour to A, where it condenses. On discharging the storage, the dehydrated salt is fed to an association reactor, where water vapour is again absorbed and the released heat is transferred to a water storage. The water vapour is produced by evaporating the water in tank A by means of heat from the borehole.

Dual Solar Pond Concept (DSP)

1.1. How it works

The concept of a Dual Solar Pond (DSP) settles in the existence of two ponds one near the other that can work alternatively. They will be used as Solar Ponds at different times. In this paper we will consider the use of unused marine salt works for building the ponds, and we will consider a pond with a medium size SZ of 1 m and a NCZ of 1 m making a pond with a total depth of 2 m which is relatively easy to build in old marine salt works.

The single operation consists in create the gradient zone in order to establish the pretended NCZ. With these dimensions a simulation made with the parametric model presented in [7] points to a 21 days start up. For the same conditions but with a deeper SZ (3m), a conventional pond will need about 57 days. During the following months the DSP will work without maintenance until its NCZ degradation. Some time before the second marine salt work must be prepared to run as a Solar Pond. All this is shown in Figure 5.

Then DSP concept consists in recovering traditional marine salt works that will act as two alternating Solar Pond basins. When the first one begins the extraction energy mode the other is at stand by. When the first begins to have instability problems the second needs to be built in. In this way Solar Pond monitoring results to be very simplified. One must only predict when a Pond undergoing instability (not acting more as a Solar Pond) in order to begin, with same advance, to fill the other marine salt work.

The present work shows how to make this prediction concerning the dynamics of the whole process, including energy extraction to Aquaculture applications.

Optical properties of cerium oxide

The optical properties of the cerium oxide particles were calculated from the Mie-scattering theory [4]. From this theory the spectral absorption, scattering, and extinction cross-sections, as well as the scattering phase function can be calculated, with the complex refractive index of the particle material m, and particle radius rp as inputs. The complex refractive index of cerium oxide was

taken from different sources. From the papers by Ozer [5], and by Wiktorczyk and Oles [6], the optical properties for the visible and near IR were obtained. An expression for the IR properties CeO2 was taken from Santa et al. [7]. These latter authors fit the experimental reflectance values by using a quasi-harmonic damped multiple- oscillator model for the dielectric function.

We consider particles of 5 pm radius. The spectral volumetric extinction, absorption, and scattering coefficients (ay, Sv and B, v respectively), are shown in Fig.1. These coefficients are given by

av = Cext,, / V Sv = Cabs,, / V £, = Cscatt,, / V (3)

Where Cext, ,, Cabs, , and Cscatt, , are the spectral extinction, absorption and scattering cross-sections, respectively, and V is the particle volume.

Fig. 1. Spectral volumetric attenuation, absorption, and scattering coefficients for particle radius of 5 pm.

As seen on the graph, the extinction of radiation by these particles is mainly due to scattering; they are not very absorbing. Actually two main extinction peaks are visible, at 11 and 20 pm wavelengths. The first, one corresponds to resonance effects, while the second is due to absorption.












Подпись: Px Подпись: (4)

The absorption (к), scattering (о) and extinction (ДО coefficients of the gas/particle fluidized medium are calculated from the spectral cross sections of the particles by

where fv is the particle volume fraction.

The phase function from Mie — scattering theory, as a function of the cosine of the scattering angle, is shown in Fig. 3. As seen from this graph, the particles scatter radiation mainly in the forward direction, for the UV, Vis, and near — IR, while for the far — IR they have an almost isotropic scattering (28.8 mp), with a slightly backward preference (38 mp).

Screening of nitrate/nitrites and their binary mixtures from 120 to 250°C

In particular nitrate and nitrites and their mixtures promise to be suitable PCMs, if requirements such as handling and steel compatibility are taken into account. Table 2 shows that the number of nitrates and their mixtures, suggested in literature is limited. Hence, the aim of this work it to identify from literature and by own measurements potential binary mixtures of nitrites and nitrates. Table 3 summarizes relevant properties of selected alkali nitrates and alkali metal nitrites which were considered in this work. Due to their hygroscopic behavior, handling of the alkaline-earth nitrates Ca(NO3)2 and Mg(NO3)2 is difficult. Hence, these nitrates were excluded.

Table 3. Selected alkali nitrates and alkali metal nitrites in this work [1,14,31-33]





Hygroscopy in








































n. a.[9]






n. a.



O, Xn









O, T,N





n. a.



O, T,N


In this work, a comprehensive literature survey regarding the minimum melting temperature of the 21 binary alkali nitrate/nitrite systems was carried out [4,34-44]. Table 4 summarizes the result of this survey. Except for a few binary systems, melting enthalpy data of these systems are not known. Also, as can be seen some temperatures could not be identified (Table 4). Hence, additional experiments were carried out with particular reference to temperatures around 170°C,
since this temperature is not covered by known systems. The methods and results of the experiments are described in the next section.




























n. a.

n. a.





n. a.



Table 4. Secondary literature data of the minimum melting temperature of 21 binary alkali nitrate/nitrites.

Two minimum melting temperatures

Projects within Subtask C

There were five PCM related projects included in IEA SHC Task 32. A summary of these projects is given in Table 1.

Three projects dealt with macro-encapsulated PCM containers in water stores. All of these projects include the development of TRNSYS models for the PCM stores:

• At Lleida University, Spain, bottles and filled up heat exchangers of PCM material with graphite matrix for the enhancement of the heat conduction and increase of power input/output were tested. Applications are free-cooling and DHW tanks.

• At the University of Applied Sciences Western Switzerland in Yverdon-les-Bains/Switzerland a parametric study for the use of PCM in heat stores embedded in aluminium bottles for solar combisystems was carried out.

• The Institute of Thermal Engineering at Graz University of Technology performed tests and simulations with different PCM materials encapsulated in plastic tubes and steel containers for stores for conventional boilers to reduce the number of start-stop cycles of the burner.

The two other projects are slightly different:

• At the Department of Civil Engineering, Technical University of Denmark the use of super cooling of PCM materials for long-term heat storage was investigated with simulations. This project showed that a 10 m3 only PCM seasonal storage using the supercooling effect is theoretically possible. Experimental setup assessed some assumptions on heat transfer in a bulk PCM tank

• The Institute of Thermal Engineering at Graz University of Technology performed tests and simulations with PCM-slurries of microencapsulated paraffins for stores for conventional boilers to reduce the number of start-stop cycles.

The above project is also dealing with heat exchangers immersed in PCM material Table 1. Summary of prototype storage units studied in Subtask C.

Type of Technology


Stage of Development



PCM seasonal storage using subcooling


Lab prototype; Simulation model for store developed and seasonal simulations of the system were performed

Technical University of Denmark (DTU), Denmark

Macroencapsulated PCM in storage tank

Na(CH3COO)3 H2O + graphite

Lab prototype; Seasonal simulations of the system were performed, using the model developed by the Institute of Thermal Engineering, Graz Unviersity of Technology

University of Lleida, Spain

Macroencapsulated PCM in storage tank with integrated burner

Na(CH3COO)3 H2O + graphite

Lab prototypes; Simulation model for store developed and validated; Seasonal simulations of the system were performed according to the reference conditions from Subtask A

University of Applied Sciences Western

Switzerland (HEIG — VD), Switzerland

Microencapsluated PCM slurry


Lab prototypes, Development of simulation models for a store filled with slurry with various internal heat exchangers and flow/return pipes and an external heat exchanger with PCM slurry on one or both sides.

Graz University of Technology, (IWT — TU Graz), Austria

Macroencapsulated PCM in storage tank





Simulation model for store developed and validated; Seasonal simulations of the system were performed for various hydraulic schemes for heating systems in order to analyze the reduction of the boiler cycling rate compared to water stores.

Graz University of Technology, (IWT — TU Graz), Austria

Immersed heat exchanger in PCM

Na(CH3COO)3 H2O without graphite

Simulation model for store developed and validated; Seasonal simulations of the system were performed for various hydraulic schemes for heating systems in order to analyze the reduction of the boiler cycling rate compared to water stores.

Graz University of Technology, (IWT — TU Graz), Austria

• The Institute of Thermal Engineering at Graz University of Technology performed tests and simulations with a bulk PCM tank with an immersed water-to-air heat exchanger for conventional boilers to reduce the number of start-stop cycles of the burner.

For a summary of these projects see Table 1; the main results are given in the following chapters.