Category Archives: EUROSUN 2008

Summary and conclusions

The focus of this work was the identification of suitable PCMs in the temperature range 120 to 250°C. First, literature of organic PCMs in this temperature range was reviewed. In summary, it can be pointed out that the long-term thermal stability, the reactivity with air oxygen and the high vapour pressure of organic materials are the major critical aspects. All discussed organic PCMs require hermetically sealed storage systems and this can be considered disadvantageous.

The thermal stability of inorganic materials is typically higher. Besides long-term stability, PCM selected for the steam storage application need to fulfill other requirements such as suitable properties regarding handling and economics. Another aspect is that the entire temperature range 120 to 250°C should be covered with different melting temperatures. Taking these aspects into account, the combination of two alkali nitrate/nitrites to form binary systems is a suitable option. This work selected three alkali metal nitrates (LiNO3, NaNO3, KNO3), two alkaline earth nitrates (Sr(NO3)2, Ba(NO3)2) and two alkali metal nitrites (NaNO2, KNO2). Ternary systems, such as the system KNO3-NaNO2-NaNO3, which offer a further potential but also some complexity were not considered. For the considered seven alkali nitrate/nitrites, the minimum melting temperature of all binary combinations was identified either from secondary literature or through measurements presented in this work (Table 4). It can be concluded that the considered temperature range 120 to
250°C can be covered by these systems with a maximum temperature gap of 20°C (Fig. 5). Known values of the latent heat factors were in a range from 14 to 33 J/(mol K), where LiNO3 systems had exceptionally high values of 36 and 43 J/(mol K). In this work unknown minimum melting temperatures and systems around 170°C were assessed in more detail by phase diagram determinations and melting enthalpy measurements. Although KNO2 can be regarded as an less ideal candidate material, because it is a uncommon substance and strongly hygroscopic, it is the only material of the selected candidates, which forms binary systems with a melting temperature in the range 150 to 190°C. Of the two candidates at around 170°C, KNO2-Sr(NO3)2 has been identified as the favorable system due to its higher melting enthalpy and flatter liquidus line rather than KNO2-Ba(NO3)2.

It can be pointed out that the knowledge about the binary systems varies widely. For some systems essential values such as the eutectic temperature, composition and enthalpy are unknown. The characterization of other systems has progressed much further due to their application as heat transfer carrier, salt bath fluids and molten sensible heat storage media. Examples are the KNO3- NaNO3 and KNO3-LiNO3 systems, where work on aspects such as thermophysical properties, thermal stability and steel corrosion has been reported. Hence, after the selection of PCM with a suitable melting temperature, reported in this work, full qualification of PCMs for

References

[1] H. P. Garg et al. (1985) Solar Thermal Energy Storage, Reidel Publishing.

[2] R. Tamme et al., International Journal of Energy Research, 32 (2008) 264-271.

[3] D. Steiner et al. (1982) University of Stuttgart, Rep. BMFT-FB-T 82-105. (in German)

[4] G. J. Janz et al. (1978-1981) NSRDS-NBS 61 Part I, II and IV.

[5] M. Kamimoto et al., Solar Energy, 24 (1980) 581-587

[6] B. Zalba et al., Applied Thermal Engineering, 23, (2003) 251-283.

[7] A. Hoshi, D. R. Mills, A. Bittar, T. S. Saitoh, Solar Energy 79 (2005) 332-339.

[8] C. E. Birchenall, et al., Metallurgical and Materials Transactions A, 11 (1980) 1415-1420.

[9] H. Kakiuchi (1998) IEA Annex 10, 2nd Workshop, Sofia, Bulgaria (1998).

[10] D. Chandra et al., Z. Phys. Chem., 216 (2002) 1433-1444.

[11] S. D. Sharma et al., International Journal of Green Energy, 2 (2005) 1-56.

[12] T. Ozawa et al., Thermochimica Acta, 92 (1985) 27-38.

[13] Ullmann’s Encyclopedia of Industrial Chemistry (1998), 6. Edition, Wiley.

[14] J. Font et al., Journal of material chemistry, 5 (1995) 1137-1140.

[15] R. Sakamoto et al., Thermochimica Acta, 71 (1984) 241-249.

[16] I. O. Salyer et al., Journal of Applied Polymer Science, 28 (1983) 2903 — 2924.

[17] T. Clauhen et al., Symp. 8. Internationales Sonnenforum ’92 (in German)(1992), 1259-1264.

[18] H. Inhaba et al. Transactions of the Japan Society of Mechanical Engineers. B, 63 (1997) 3382-3389.

[19] Y. Takahashi et al., Thermochimica Acta, 50 (1981) 31-39.

[20] T. Ozawa et al. (2003) Chp. 7: Energy storage, in Handbook of Thermal Analysis and Calorimetry, 2.

[21] P. A.E. Vallejo et al., Proc. of the COMSOL Multiphysics User’s Conference, Boston (2005).

[22] G. Hakvoort et al., Journal of Thermal Analysi and Calorimetry, 69 (2002) 333-338.

[23] D. G. Lovering (1982) Molten salt technology, Plenum Press.

[24] A. Verma et al., Canadian Journal of Chemical Engineering, 56 (1978) 396-398.

[25] R. P. Tye et al., Proc. 7th Symposium on Thermophysical Properties, ASME, (1977) 189-197.

[26] Y. Takahashi et al., Thermochimica Acta, 123 (1988) 233-245.

[27] Y. Abe et al., Proc. 23rd. Intersoc. Energy Conv. Eng. Conf., Denver, 2, (1988) 159-164.

[28] R. J. Calkins et al., Report SAND-81-8184, Sandia Lab. (1981).

[29] Y. Takahashi et al., Thermochimica Acta, 121 (1987) 193-202.

[30] Y. Takahashi et al., International Journal of Thermophysics, 9 (1988) 1081-1090.

[31] I. Barin (1995) Thermochemical Data of Pure Substances, 3. Edition, VCH.

[32] K. H. Stern, J. Phys. Chem. Ref. Data, 1 (1972) 747-772.

[33] R. P. Shisholina et al., Russian Journal of Inorganic Chemistry, 8 (1963) 1436-1438.

[34] E. Thilo et al., Proc. 7th. Conf. on the Silicate Industry, Hungary (1963) 79-85.

[35] M. V. Tokareva et al., Zhurnal Neorganicheskoi Khimii, 1 (1956) 2570-2576.

[36] E. Schumann et al., Berichte der Bunsen-Gesellschaft (in German), 74 (1970) 462-470.

[37] E. M. Levin et al. (1956) Phase Diagrams for Ceramists, The American Ceramic Society.

[38] J. Alexander et al., Industrial & Engineering Chemistry, 39 (1947) 1044-1049.

[39] X. Zhang et al., Thermochimica Acta, 385, (2002) 81-84.

[40] R. W. Berg et al., Dalton Transactions (2004) 2224-2229.

[41] R. N. Grugel et al., J. Material Science Letters 13 (1994) 1419-1421.

[42] P. I. Protsenko et al., Russian Journal of Inorganic Chemistry, 6 (1961) 850-854.

[43] P. I. Protsenko et al., Russian Journal of Inorganic Chemistry, 20 ( 1975) 924-927.

Results of System simulations

Four simulation studies were performed in Subtask C. Three of them were using more or less the reference conditions defined in Subtask A [3]. One of them dealt with a complete different application to reduce boiler cycling by introducing a PCM store.

The simulation results from HEIG-VD in Yverdon-les-Bains, Switzerland concerning the advantage of macroencapsulated PCM in solar combisystems are shown in Fig. 2 [4]. It should be

Fsav, therm(w) [-]

Fig. 2. Difference between pure water and water + PCM system. (PCM gain = Fsavtherm(W+PCM)/Fsavtherm(W) — 1)

Подпись: Fsav,therm(w) [-] Fig. 2. Difference between pure water and water + PCM system. (PCM gain = Fsavtherm(W+PCM)/Fsavtherm(W) - 1)

reminded that the proposed system has been analysed only from the simulation side, where a water tank storage filled only with water or filled with water + PCM (paraffin RT35) is compared.

To evaluate the impact of the PCM on the performances, it is possible to define the energy gain between the Fsav, therm for the tank with PCM (Fsav, therm(W+PCM)) and only with water (Fsav, therm(W)). If this gain is higher than 0, then the PCM brings an advantage. As it can be seen in Figure 3, the gain due to using PCM is low. A decrease of the RATIO according to the increase of the Fsav, therm can also be noticed. But it should be remembered, that when the Fsav, therm is high, the solar installation is oversized. As it can be seen, adding a PCM becomes less interesting when the solar system is oversized. This is due to the fact, that when oversized, the storage of heating is less relevant.

The fractional thermal energy savings fsav, therm are a measure of the percentage of the auxiliary (non-solar) energy input for heating that can be reduced by the solar system. This term does not account for electricity use unless it is used directly for heating. The efficiency of electricity production and distribution pel is 0.4 in all cases. Hereby Qboiler and Qelheater are the energy inputs of the solar combisystem with respective efficiencies pboiler and pel. Qboiler, ref defines the energy input of a boiler of a defined conventional heating system with an efficiency of nboiler, ref [3].

Qboiler __ Qel, heater

fm>er. = 1 — —( (Equation 1)

boiler, ref

According to the additional cost of adding the PCM and the environmental impacts results described in [2], this system with PCM does not show a substantial benefit compare to a storage tank filled only with water.

Only the long term heat storage with subcooled liquid PCM (BYG DTU, Department of Civil Engineering, Denmark, Fig. 3 [5]) shows the possibility to achieve 100 % solar fraction with PCM store volumes of about 10 m3 for a 135 m2 floor area passive houses (15 kWh/m2a space heating energy demand). Water stores have to be far bigger to achieve the 100 % solar fraction. 80 — 90 %

Fig. 3. Simulation model of BYG DTU, Department of Civil Engineering, Denmark [5]

Подпись: Fig. 3. Simulation model of BYG DTU, Department of Civil Engineering, Denmark [5]

solar fraction can be achieved also with water stores of 5 — 10 m3. Taking into account the long term heat losses of water stores the size reduction is far bigger.

At the Institute of Thermal Engineering (IWT), Graz University of Technology, Austria different hydraulic systems were investigated in terms of their ability to reduce boiler cycling operation [6]. In the following a description of the hydraulic systems, which are used in the simulations, is given. Table 2 shows a summary of all simulated concepts.

Table 2: Summary of all simulated system concepts [6]

System

Boiler

type

Type of space heating store

Type of

DHW

preparation

Hydraulic integration and control of boiler

System Category

: no space heating storage, DHW tank

G1a

Gas

No storage

DHW tank

Boiler temp. controlled as a function of the ambient temp., throttle control

G1b

Gas

No storage

DHW tank

Constant boiler temp., flow temp. control via mixing valve

G1c

Gas

No storage

DHW tank

Constant boiler temp., flow temp. control via mixing valve, hydraulic switch

P1

Pellets

No storage

DHW tank

Constant boiler temp., flow temp. control via mixing valve, hydraulic switch, return temp. control

System Category 2: space heating storage, DHW tank

G2a

Gas

Water storage

DHW tank

Constant boiler temp., flow temp. control via mixing valve

G2b

Gas

Water storage + PCM modules

DHW tank

Constant boiler temp., flow temp. control via mixing valve

P2a

Pellets

Water storage

DHW tank

Constant boiler temp., flow temp. control via mixing valve, return temp. control

P2b

Pellets

Water storage + PCM modules

DHW tank

Constant boiler temp., flow temp. control via mixing valve, return temp. control

System Category 3: space heating storage, instantaneous preparation of DHW

G3a

Gas

Water storage

Instanta­

neous

Constant boiler temp., flow temp. control via mixing valve

G3b

Gas

Bulk PCM storage

Instanta­

neous

Constant boiler temp., flow temp. control via mixing valve

The results for the system with water storage (G2a) and for systems with water storage with integrated PCM modules (G2b) are shown in Fig. 4 for different storage volumes. In comparison to the systems without buffer storage the number of start-stop cycles is reduced strongly. Even with the smallest volume of only 25 litres of water a reduction of about 70 % (set temp. 50°C) or 90 % (set temp. 65°C) can be achieved. With increasing storage volumes the number of cycles decreases, whereby the potential for a further reduction is low for volumes above 200 litres. Because of the lower utilized temperature difference the number of cycles is higher with a boiler temperature of 50°C in comparison to 65°C. On the other hand the higher temperatures decrease the annual efficiencies of the condensing boiler by 2-3 %.

The integration of PCM modules (boiler set temp. 65°C in all cases) allows an enhancement of the storage capacity, resulting in a further decrease of the number of start-stop cycles especially with small storage volumes. There are only minor differences between the PCM volume fractions of 50 and 75 %. The integration of PCM modules hardly influences the annual efficiencies of the boiler and the system (Fig. 4, right).

Fig, 4. Gas boiler: annual number of start-stop cycles (left) and annual efficiencies (right) for different
storage volumes for systems with water storage (G2a) and for systems with water storage with integrated

2 6000

0)

£ 5000

0

4000

-Q

0 3000 2000 1000

0

И n cycles eff_boiler eff system

1.2 1.0 0.8 + 0.6 0.4 0.2 0.0

c

<D

О

it=

<D

"to

3

C

c

ro

Fig. 5. Annual number of start-stop cycles and annual efficiency for the systems G3a (water storage) and

G3b (bulk PCM storage)

PCM modules (G2b)

Figure 5 shows the number of start-stop cycles and the annual efficiencies for the system G3a (water storage) and the system G3b (bulk PCM storage). Due to the higher storage capacity of the PCM storage (assuming the same volume of 45 litres) in system G3b the number of cycles can be reduced by 50 % compared to system G3a. The annual efficiency of the boiler is also slightly higher, which is a result of the lower amount of heat produced in start-stop operation due to the higher storage capacity.

2. Conclusion

Phase change materials as heat storage theoretically offer an advantage compared to water stores, when the cycling temperature is close around the phase change temperature and the phase change can be used quite often. The other possible application is the use of the subcooling effect for seasonal storage. However, the investigations reported here showed only little advantages for macro-encapsulated PCM modules in combistores, PCM-stores with immersed heat exchangers and for PCM slurries for heat stores in solar combisystems and residential heating systems. The seasonal storage with subcooled PCM could be in principle a good solution. However the technical expenditure for this system is large.

References

[1] A. Abhat, (1983), Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (1983) 313-332.

[2] W. Streicher, (ed). (2008), Laboratory Prototypes of PCM Storage Units, Report C4, of IEA Solar Heating and Cooling programme — Task 32, “Advanced storage concepts for solar and low energy buildings”, IEA-SHC (http://www. iea-shc. org/task32/publications/index. html)

[3] H. Heimrath, M. Haller, (2008), The Reference Heating System, the Template Solar System, Report A2, of IEA Solar Heating and Cooling programme — Task 32, “Advanced storage concepts for solar and low energy buildings”, IEA-SHC (http://www. iea-shc. org/task32/publications/index. html)

[4] S. Citherlet, J. Bony, (2008), System Simulation Report, System : HEIG-VD-W and HEIG-VD-PCM, Report C6.1, of IEA Solar Heating and Cooling programme — Task 32, “Advanced storage concepts for solar and low energy buildings”, IEA-SHC (http://www. iea-shc. org/task32/publications/index. html)

[5] J. M. Schultz, (2008), System Simulation Report, PCM with supercooling, Report C6.2, of IEA Solar Heating and Cooling programme — Task 32, “Advanced storage concepts for solar and low energy buildings”, IEA-SHC (http://www. iea-shc. org/task32/publications/index. html)

[6] A. Heinz, (2008), System Simulation Report, System: PCM storage to reduce cycling rates for boilers, Report C6.3, of IEA Solar Heating and Cooling programme — Task 32, “Advanced storage concepts for solar and low energy buildings”, IEA-SHC (http://www. iea-shc. org/task32/publications/index. html

DESIGN AND OPERATION OF A SMALL ICE STORE

T. Koller[18] [19], M. Zetzsche1, Prof. Dr. Dr. H. Mtiller-Steinhagen1,2

1 University of Stuttgart, Institute for Thermodynamics and Thermal Engineering (ITW)

2 Institute of Technical Thermodynamic (ITT), DLR
http://www. itw. uni-stuttgart. de/~www/ITWHomepage/Forschung/Kaelte. html

Abstract

A small ice store has been designed, constructed and experimentally investigated at the

Institute of Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart [1]. A simulation program based on EES and Matlab was developed which predicts the experimental data well.

Measurements have shown a higher volumetric capacity (56 kWh/m[20]) than expected from published reports (40 — 44 kWh/m3) [2].The present design offers an efficient solution for cold storage at low cost, and low maintenance and operation efforts.

The chosen type of storage is an ice-on-coil system with external melting. While internal melting would result in higher discharging rates, it is nevertheless not a viable option because of particular requirements on operational mode and design of the present heat exchanger.

The ice store was implemented into the cooling system of the institute building and shows good in-service behaviour. Next steps are further development of the storage design, automation of the operational mode and detailed measurements and evaluation of in-service behaviour.

Keywords: Ice Store, Solar Cooling, Ammonia/Water Chiller, Air-Conditioning

system consisting of absorption chiller, ice storage and solar collector field. This system, which is illustrated in Figure 1, has been taken into operation in spring 2008.

Figure 1: Absorption chiller and Ice store-driven cooling system at ITW

Marstal Fjernvarme

The latest extension of the solar production plant in Marstal means that they in 2008 have a solar fraction of 30% coming from 18.300 m2 of solar collectors combined with a 10.000 m[31] pit heat storage and a 2.000 m3 steel tank. The rest of the fuel is biooil where the production price/MWh is app. 65 €. Marstal has therefore investigated the possibilities of a larger solar fraction. Design cal­culations shows that an extension with another 4.000 m2 solar collectors, a 10.000 m3 steel tank or pit heat storage, a 1,5 MW heat pump (to cool the storages and use outdoor air) will rise the heat production with 4.800 MWh/year and increase the solar/heatpump fraction to 45%. The production price for the extended production is calculated to 66 €/MWh with an annuity factor of 0,1 and without electricity tax. The extension is planned to be realised in 2009.

Investment costs are 2,8 mio. €.

3.1. Strandby

Strandby Varmevsrk is a consumer owned district heating company producing 16.000 MWh/year with natural gas fuelled CHP and boiler. In the summer 2008 Strandby Varmevsrk is implement­ing 8.000 m2 solar collectors, 1.500 m3 steel tank and an absorption heat pump. The solar collectors will cover 18% of the annual heat production and the absorption heat pump will cover another 5%. The absorption heat pump is cooling fluegas from CHP, and boiler and utilising low temperature heat form the solar collectors. The production price for heat is calculated to 55 €/MWh (annuity 0,1).

Investment costs are 2,2 mio. €

Methodology

The research approach has been based on the combination of virtual prototyping techniques (numerical simulation) and the construction and measurement of experimental set-ups and prototypes by means of ISO procedures.

More than 7000 different configurations of ICSs have been evaluated by means of virtual prototyping. Main parameters investigated were: the absorbing surfaces, the covers (from single glazing to transparently insulated covers), different stores (water store, PCM store, hybrid water+PCM store), and different volume of the store. Some of the most interesting configurations in terms of the industrialisation capabilities of the partners, the cost, the thermal performance, and the technical feasibility according to the current state of the technique, have been studied in more detailed resulting into the final OPICS-ICSs developed during the project. Other configurations, however, may also be interesting and may motivate further investigation and development in the frame work of future projects. Some of the configurations studied in the virtual prototyping have been evaluated in more detail, including a detailed design of all the elements, the construction of prototypes, a detailed modelling, and the testing of the prototypes using ISO procedures.

Fig. 1. Pre-industnal prototypes at the tracks of the UPC testing facility. a)Two prototypes OPICS1. b) One prototype OPICS2.

Sizing of the seasonal heat storage tank

1.1. Solar combisystem modelling

The first step in the study consists in the creation of a reference model of a solar combisystem. The system is designed to provide energy for space heating and domestic hot water for a single-family house. It is installed in a low energy house and does not include any long-term Thermal Energy Storage System (TESS). This reference case has been programmed with the transient system simulation program TRNSYS [2]. This software is a complete and extensible simulation environment for the transient simulation of energy systems, including multi-zone buildings. Its modular structure allows the user to build his own systems, from simple domestic hot water systems to the design of

buildings and their equipments, including control strategies, occupant behaviour, alternative energy systems (wind, solar, photovoltaic, hydrogen systems), etc.

The model of the reference combisystem is divided in several sub-sections as depicted in Fig. 2 :

• the solar collector loop with an area of 20 m2 of evacuated tube collectors and counter flow heat exchanger

• the auxiliary heating loop, comprising an electric heater

• the domestic hot water (DHW) demand module, controlled by a load profile and set to provide 200 L of water at 45°C per day, which is the daily consumption of a five-person family

• the building representation section, which corresponds to a low energy single family house of 191 m2 with a bioclimatic architecture

• the space heating loop, modelled with a radiator

• the weather data processing module, which reads the weather data and calculates direct and diffuse radiation outputs for the surfaces of the building and solar collectors with various orientations and slopes.

Weather

Подпись:

Coldpipe (31)

Подпись:Solar collector loop

Evacuated tubes (71)

Weather (109)

Pump II (803)

Low energy house (56a)

Counter flow HX (5 b)

Подпись: Counter flow HX (5 b) Подпись: Pump II (803) Подпись: Low energy house (56a)

Building

Water tank (340)

Diverter (lib)

A

Radiator (362)

How mixer (11 h)

Tempering valve (lib)

Auxiliary heating

Load profile (14b)

Подпись: Water tank (340) Подпись: Tempering valve (lib)

Aux pump (803) Aux coldpipe (31)

Evaluation

Подпись: EvaluationSpace heating

DHW

Fig. 2. Reference solar combisystem layout

This figure is a simplified layout ; all the controllers and regulation devices are omitted.

All these sub-sections are connected with the central stratified hot water tank with a total volume of 2.45 m3. The supply temperature of hot water is set at 45°C, but the setpoint temperature is 60°C in the upper part of the tank. Four double inlet/outlet ports are included in the tank, corresponding to the collector loop, the auxiliary heating loop, the DHW loop and the space heating loop. Five temperature sensors give the values of monitoring temperatures for the control units of the system, such as the protection temperature of the solar collector (Fig. 3).

DP3 input (from the aux. heating)

DP4 input (from the space heating) DP5 input (from the DHW demand)

DPI input (from the collector)

0.15

0.05

DP5 output

^(to the DHW demand)

^ DP4 output

(to the space heating)

*DP3 output (to space heating)

DP1 output (to the collector)

T1 : collector control temperature T2 : not used

T3 : water tank protection from high temperatures T4 et T5 : aux. heater control

Fig. 3. Hot water tank — Relative heights of double ports and temperature sensors 2.2. Heat demand of the house

The simulations have been performed for two different locations in France, Paris and Marseille, in order to evaluate the influence of the climate on the heat demand and the achieved solar fraction. That is why the solar collector area is far over-sized in Marseille, which is located in the south of the country.


Fig. 4. Space heating demand of the modelled house

The building has a space heating demand of 37.2 kWh. m-2 in Paris and 15.4 kWh. m-2 in Marseille. If space heating and DHW preparation are both considered, the demand rise to 53.0 kWh. m-2 and 31.1 kWh. m-2 for Paris and Marseille respectively.

Thermodynamic Limits

Seasonal Storage

Seasonal TES by sorption storage seems to be particularly interesting, because there are no heat losses during the storage period. The stored thermal energy will only be discharged, when the adsorption process will be started. This is only valid, if the sensible heat of the adsorbent is

neglected. About 10 — 15 % of the heat input will be lost by the cooling down of the adsorbent material.

However seasonal storage by sorption systems is strongly influenced by the changes in the ambient temperature between summer and winter. A substantial decrease in the thermal coefficient of performance COPth will be shown in the following example [1]: The charging of the sorption storage will take place in summer time at an ambient temperature of 30 °C (TAC). Discharging will be in winter at -20 °C (TAD). These circumstances lead to a substantial decrease of the ideal ratio of thermal energy output Qout (discharging) and solar input Qin (charging).

In this example the charging and discharging temperature of the storage is 100 °C. In a number of applications the charging temperature is higher than the discharging temperature, which leads to a further decresas in COPth.

If you have a solar application and a Zeolite storage, where you need a charging temperature of at least 160 °C and you are delivering heat to a low temperature heating system running at 40 °C, your thermodynamical maximum COPth can only be 54%.

TCM extruder reactor system

In an extruder reactor (see Fig. 6a), the extruder transports the material and additionally causes stirring, thereby improving vapour and heat transport. The effectiveness of the transport depends on the rotational speed of the screw. The heat can be added or removed by means of either a jacket around the screw tube, or a hollow screw. This would be a convenient way of integrating the means of transport required for the powder and the reactor. If there is risk of the TCM material sticking to the screw, a system with two parallel self-wiping screws may be applied.

0 0.02 0.04 0.06 0.08 0.1 0.12

rotational speed (1/s)

Подпись:700 600 500

*

400 300 200 100 0

Fig. 6. (a) TCM extruder reactor, (b) extruder heat transfer (Jephson model, in (Kalbasenka, 2008))

For the case of dehydration of the TCM, assuming that the heat transport would be the limiting factor for the power, for a 3kW reactor and an effective heat transfer to the wall of about 550 W/m2K, this would lead to a reactor of about 1 m in length and 5 cm diameter, in which about 75% of the open volume would be filled with TCM.

Fundamental geometrical system structure limitations in a closed adsorption heat storage system

Paul Gantenbein,

Institut fuer Solarenergie SPF,

HSR University of Applied Sciences of Rapperswil, Oberseestrasse 10, CH-8640
Corresponding author, paul. gantenbein@solarenergy. ch

Abstract

The power of a closed solid sorbent — gas/liquid sorbate system is limited by the material combination sorbent — sorbate, the external heat sources, the geometrical structure of the fixed sorbent bed and the heat transfer in both the fixed bed to a fixed bed immersed heat exchanger and in the liquid sorbate tank. In the adsorption process the speed uT of the temperature front moving through the fixed bed of spherical zeolite particles is depending of the position z in the fixed bed and is reduced for increasing length z. In a fixed bed of spherical zeolite particles the speed uT is higher and the length of the water vapour mass transfer zone is longer for zeolite particles with a larger diameter dp. After a process cycle time t=200s to t=300s the maximum power — i. e. cooling power in the sorbate tank is reached. For longer cycle times a linear decrease of the cooling power was measured. While in a closed heat storage water vapour flows through the fixed bed of spherical zeolite particles the pressure drop Ap/p and thus the thermal power Pth has a linear dependence of the length z of the fixed bed. This power decrease is directly correlated with a decreasing water vapour pressure gradient. A closed solid — gas/liquid sorption system is more suitable for a heat pump or a cooling machine application than for thermal storage.

Keywords: Solar energy, adsorption heat storage, fixed bed, packed bed, trickle bed, vapour pressure drop, heat pump

1. Introduction

In conventional low temperature solar systems the solar irradiation is converted into heat and stored in hot water tanks. The correlation between thermal losses and the temperature level of the storage device are physically implied. Thus, a loss free heat storage able to release its energy immediately is a desirable characteristic. An exothermic reaction in a chamber with a minimum of two chemical components stored apart from each other would fulfil these two requirements while adsorption of water vapour on sorbent materials like zeolite or Silicagel in a closed system satisfies principally the first requirement. For the desorption of the water from the zeolite solar thermal energy can be used. Beside of solar energy as the high temperature level source the structure of a system will depend on the sorbent — sorbate materials and their undergoing phase transitions as well as on the geometrical structure of the sorbent.

The subject of solid sorbent — vapour sorbate closed heat storage systems has recently gain specific attention because of reports of high energy densities of such a sorption module [1, 2, 3]. But in a laboratory scale system using up to 1.5kg modified zeolite sorbent [4] and in a real sized system with 2×1.1m3 of silicagel sorbent [2] after a strong increase of the output temperature an eminent decrease of this temperature could be observed. So, the output power of the sorption module is strongly depending of the operation time and thus of the dynamic behaviour in the coupled mass and heat transfer.

The measurements of water vapour adsorption on zeolite and silicagel as a function of time and water vapour pressure in a closed system showed a time range of t=300s to t=400s of high mass adsorption [5]. Saha and Boelman [6, 7, 8] were reporting about operational conditions and coefficient of performance COP of an adsorption refrigeration machine working with the sorbent silicagel and the sorbate water. The optimum cooling power output was found to be in a cycle (adsorption / desorption) time of t=250 s to t=300s while the COP of the machine increases up to a cycle time of t=1800s.

Therefore, further work was undertaken for a better understanding of the sorption module fixed bed and to find answers to the questions of what limits the size and structure of a closed heat storage module. In this work the first steps to the design for the power and energy output of a closed low temperature solid sorbent — vapour/liquid sorbate sorption heat storage system depending on the thermodynamic and the geometric parameters are presented. In the following the term fixed bed is used as an equivalent term of packed or trickle bed.

2. Experimental

To measure the dynamic behaviour in the adsorption of water vapour on spherical zeolite 13X particles in a closed system an vacuum tight stainless steel equipment was set up. In Fig. 1 a schematic of the water tank and the fixed bed containing tank is shown. The two tanks are connected through an on/off manually driven ball valve containing pipe of d=40mm diameter. A top view photo a) and the schematic b) arrangement of the pressure and temperature sensors are shown in Fig. 2. With the 10 radially inserted 8mm diameter tubes the pressure in the centre of the fixed bed tube is measured with a capacitive acting sensor. These sensor tubes are in a spiral arrangement with l=50mm equally spaced over the whole length L=500mm of the fixed bed tube and act also as the canal for the four wires of the Pt100 temperature sensors. For a good heat transfer a Zeolith particle was glued on the ceramic oblique cube of the Pt100 sensor. The fixed bed tube has a diameter D=160mm and the particles are fixed at both ends with two stainless steel fabric of s=0.8mm mesh size. The spiral arrangement of the sensor tubes are meant to a perturbation free water vapour flow can be expected.

1)

L

2)

Fixed bed

■10 T(z) P(z)

l:

nz

Sensor 1

d

to 10

p(T)

T(HX in)

Water tank

-^T(HX out)

Fig. 1: Schematic of the experimental setup with the fixed bed of spherical zeolite 13X particles container 1) and the water sorbate tank 2).

1) Sorbent fixed bed (D, L, dp)

2) Water / sorbate tank (p, T)

Diameter D

a) Photo

2 1

b) Schematic

>

Fig. 2: Top view photo a) of the (empty) fixed bed tube of diameter D with the radial inserted tubes to measure the pressure. These tubes act also as temperature sensor cable canal.

The measurements were carried out with two different zeolite 13X fixed beds of average spherical particle diameter of dp=1.51mm and of dp=2.63mm, respectively [9]. This leads to the aspect ratios of approximately D/dp=106 and D/dp=61, and to a bed length to particle diameter ratio of L/dp=3 31 and L/dp=190, respectively. The temperature level i. e. the pressure level in the water sorbate tank was set to T(HX in)=15°C, T(HX in)=20°C and T(HX in)=25°C. The corresponding pressures at the entrance to the fixed bed are listed in table 1 [10]. The hydraulic diameter dh in table 1 is determined with the equation (1) [11],

d = (1 — s’) f (1)

sdp

1

Подпись: sdp
Подпись: 1

in equation (1) s is the porosity of the fixed bed and the shape factor f=1 of spherical particles. The porosity s is determined through the equation (2) [12],

dp

s = 0.375 + 0.34*-^- (2)

Temperature T(HX in) [°C] Pressure p (sorbate tank) [mbar]

15

16.3

20

23.4

25

29.7

Particle diameter dp [mm]

1.51

2.63

1.51

2.63

1.51

2.63

Porosity s [-]

0.41

0.43

0.41

0.43

0.41

0.43

hydraulic diameter dh [mm]

0.168

0.292

0.168

0.292

0.168

0.292

Table 1. Temperature T and pressure p in the sorbate tank — particle and hydraulic diameter in the fixed bed.

The bulk density of zeolite 13X is 650 kg/m3 and the BET surface is in the range of 500m2/g and 800m2/g [9]. zeolite 13X has an average pore size of 1nm. In humid air the water uptake c(p, T) of the this sorbent material is approximately 25 wt.%. The sorbent material fixed bed was dried at room temperature by pumping with a Leybold turbo-molecular pump in series with a Pfeiffer mechanical vacuum pump. The water tank was evacuated to the temperature dependent water vapour pressure

[10] . For pumping out the air dissolved in the deionised water in the lower tank, the procedure was performed three times. The data acquisition was done with an Agilent data acquisition switch unit.

3. Results and Discussion

Modified Phase Change Materials for Industrial Heat Storage Applications

M. Hadjieva1*, M. Bozukov1, Ts. Tsacheva2

1 Central Laboratory of Solar Energy & New Energy Sources, Bulgarian Academy of Sciences,

72 Tzarigradsko Schosse blvd., 1784 Sofia, Bulgaria

2 Institute of Physical Chemistry, Bulgarian Academy of Sciences, G. Bonchev, bl. 11, 1113 Sofia, Bulgaria

* Corresponding Author, sthermal@phys. bas. bg

Abstract

The modified phase change materials (PCM) are the promising new source for cost effective heat storage systems integrated in industrial process solar/heat technology. Application of the modified PCM storage units to heating/cooling technology allows a multiple utilization of solar/thermal energy by repeatable heat cycling process. After years of development and use of typical PCM with diverse heat returns in practice, the modified PCM, which succeeded over some PCM drawbacks or/and add a new function, are a practical choice for heat storage and recovery of solar/thermal energy. Novel modification of the inorganic salt eutectics absorbed within a graphite matrix to produce a highly thermally conductive multifunctional composite was designed for thermal storage at a high temperature cooling technology (up to 250oC). The (Na/K)NO3 eutectics/graphite composite improved thermal conductivity of typical PCM like inorganic salts, (Na/K)NO3 eutectics, which temperature range of phase transition and heat storage capacity are appropriate for solar/thermal storage at the industrial solar steam process. Detailed analysis and control of the specific structuring of salt/graphite composites for long-term thermal stability and function at high temperature heat storage technology of solar concentration electrical plants were in the special focus of this study. Results showed elemental separation and forming the Na-rich spherical masses and the K-rich layered structure over the graphite plane as the effect of the eutectics solidification mechanism that influence strongly thermal storage behaviour of the (Na/K)NO3 eutectics/graphite composite.

Keywords: Phase change material, (Na/K)NO3 eutectics, PCM/graphite composite, latent heat storage technology

1. Introduction

The modification of phase change materials responds to new ideas of solar/thermal industrial technologies to ensure complete energy utilization through heat storage process. Integration of thermal storage system in novel technologies is an ultimate need for well-organized economical functioning and saving heat/energy. Knowledge, collected on typical PCM and thermal storage applications with a varied income in a past century, lets to meet requests of new thermal technology demands. Progress in advanced PCM modification for improving typical PCM behaviour or use additional PCM functions for technology perspectives need a multidisciplinary innovative work to produce efficient storage materials according next generation PCM requirements. The successful paraffin modifications with graphite were developed for achievement of a quality new property (thermal conductivity) and advantage of the PCM heat

storage systems for an industrial process technology. [1,2]. The PCM composites, based on (Na/K)NO3 eutectics and graphite, were elaborated in the framework of EU project DISTOR [3].