Category Archives: EUROSUN 2008

How to compare various storage technologies?

Several criteria can be considered for comparing storage technologies. Task 32 of IEA SHC prepared a detailed list. The first indicator concerns the energy performance of a combisystem with the storage technology. The value of Fsav the fractionnal energy saving has been selected as the best indicator and can be derived from the parameter FSC’.

FSC’ is a dimensionless quantity simultaneously taking into account the climate, the building (space heating and domestic hot water loads) and the size of the collector area, in a way that doesn’t depend on the studied combisystem. First developed within IEA SHC Task 26 [2], the FSC (Fractional Solar Consumption) has been improved in Task 32 to yield to FSC’ which takes into account a possible cooling load also, and the ability of a store to be seasonal.

This means that it has been possible to show that Fsav is a function of FSC’ even if FSC’ is greater than 1.

Energy performance indicators

NRJ1 Fractional energy savings Fsav as a function of FSC’

NRJ2 Comfort for heating and DHW load met without penalties

NRJ3 Comfort in cooling conditions

NRJ4.1 Heat storage material energy density kWh/m3

N RJ4.2 Bulk storage density kWh/m3

NRJ4.3 Storage efficiency

Economical indicators

ECON1 Investment cost per kWh stored

ECON2 Operational costs per kWh discharged

Market introduction

MKT 1 1 if on the market, 2 if within 3 years, 3 in more than 3 years

Environmental indicators

ENV1 Storage material risk (corrosion + toxicity + safety)

ENV2 CO2 saved by the system compared to a reference System integration

INT1 weight of material for the storage unit kg/kWh capacity

INT2 number of separate pieces

INT3 level of skills required to install the storage unit

INT4 need for technical maintenance

Table 1. Criteria considered for comparison of heat storage units within Task 32

In order to assess Fsav in comparable conditions it is necessary to set up a standard simulation framework that many different systems can use. Task 32 defined a complete set of parameters for TRNSYS simulations, for 3 different reference houses (a low energy house with only 30 kWh/m2 for

space heating, 60 and 100 kWh/m2) in 4 different climates (Stockholm ,Ztirich, Barcelona, Madrid). The entire deck of parameters is available through IEA SHC.

Storage technologies integrated into a solar combisystem can be compared using this new method.

2. Conclusion

Water is still the storage of choice for solar combisystems for the years to come. Some important findings for other materials have been discovered by Task 32. Models are now available for more optimisation analysis and for defining the best material a combisystem would need.

Several technologies for advanced storage concepts have been tested within Task 32. Table 2 summarizes them and their status at the end of 2007.

Future work on new materials for heat storage is important since we discover the limits of some promising components.

A new IEA Task will continue the work Task 32 has initiated, but will be more focused on material research.

Solar energy need a dense and long term storage solution if it is to used intensively for house heating.

References

[1] Hadorn J.-C. editor, (June 2005), Thermal energy storage for solar and low energy buildings — State of the Art, a IEA SHC Task 32 book, Printed by Servei de Publicacions Universidad Lleida, Spain, 170 pages ISBN 84-8409-877-X, available through Internet www. iea-shc. org Task32

[2] IEA SHC Task 26 (2004): Solar Heating Systems for Houses — A Design Handbook for Solar Combisystems, W. Weiss and al., James & James, 2004, 313 pages

[3] Task 32 reports are available at http://www. iea-shc. org/task32/publications/index. html

Principle

Material

Institute

Status 2007

Chemical reactions

Closed 2 phase absorption

Mg SO4 7H20

ECN The Netherlands

Material investigation

Sorption

Open adsorption

Zeolite solid

ITW Germany

Laboratory unit

Closed adsorption

Silica gel particles in bed

AEE Austria

System in a house tested — stopped

Closed adsorption

Silica gel and Zeolite beds

SPF Switzerland

Material and bed tested — stopped

Closed 2 phase absorption

NaOH / H2O

EMPA Switzerland

Laboratory unit runing

Closed 3 phase absorption

LiCl

SERC Sweden

commercial

PCM

PCM seasonal storage using subcooling

Na(CH3COO)3 H2O

DTU Denmark

Simulation of concept — Prototype 135 liters

Macroencapsulated PCM in storage tank

Na(CH3COO)3 H2O + graphite

Univ. Lleida, Spain

Lab prototype

Macroencapsulated PCM in storage tank with integrated burner

Na(CH3COO)3 H2O + graphite

HEIG-VD Switzerland

Complete combisystem tested

Microencapsluated PCM slurry

Paraffin,

IWT-TUGraz Austria

Lab prototypes — Stopped for storage

Macroencapsulated PCM in storage tank

Paraffine,

Na(CH3COO)3 H2O with/without graphite

IWT-TUGraz Austria

Lab prototypes

Immersed heat exchanger in PCM

Na(CH3COO)3 H2O without graphite

IWT-TUGraz Austria

Lab prototypes

Water

Simplified combisystem Maxlan system

Water

SPF Switzerland

Simulation proved

Water Stratifier

Fabrics immersed in water

DTU Denmark

Laboratory proved

Table 2. Storage technologies that IEA Task 32 has investigated between 2003 and 2007

The demonstration project and the system design

As part of a demonstration project within the framework of the German “Solarthermie 2000 plus” Programme, a pilot installation of the innovative solar heating and cooling system was installed in the R&D building of the ZAE Bayern, division “Technology for Energy Systems and Renewable Energy”, located in Garching (close to Munich / Germany). The building houses offices and
laboratories for about 75 researchers. The pilot installation will simultaneously serve as field test project for a recently developed compact water/LiBr absorption chiller with 10 kW nominal capacity. The chiller has been designed for chilled water supply/return temperature 15 / 18 °C and allows for utilization of a rather moderate driving hot water temperature 85 / 75°C when operated with an open wet cooling tower. In the pilot installation however, the new latent heat storage (LHS) is used in combination with a dry cooling tower, thereby avoiding the typical disadvantages of a wet cooling tower. The driving heat source for the chiller and for the solar heating system is a solar collector field with an area of about 40 m2, located on the roof of the building. The heating and cooling system is designed to supply about 50 % of the building space with heating and cooling.

Prototype PCM Storage Evaluation

A 15 kWh PCM-based storage prototype was designed, built, and experimentally evaluated with regards to capacity and power properties. Below, details and results from this work are presented and discussed.

3.1. Design Features

In order to reach as high an IPF as possible while minimizing the cost of the PCM storage, a novel design was incorporated into the prototype named HEATPACK. In the HEATPACK, the PCM is enclosed in a cylindrical tank without further packaging material (e. g., balls or bags), and an extended surface tube heat exchanger is submerged in the PCM. The challenge of the concept is to in addition to high capacity achieving a high power of heat transfer, while maintaining low cost. A similar configuration is under examination in e. g. Austria, and has shown excellent power properties as reported within the framework of the IEA Solar Heating and Cooling implementing agreement [4].

One difference though is the cylindrical configuration of HEATPACK, and that the Austrian concept used finned tubes. Other alternative configurations excluding packaging materials are e. g.: the spiral cylinder concept examined by Banaszek et al [5]; or the flat plate heat exchanger concept used in a free-cooling application by Zalba et al [6].

Here, the HEATPACK prototype was built with a volume of close to 140 liters, the specific storage capacity was around 100 kWh/m3, and the IPF was close to 80%. The Austrian concept mentioned above had a storage capacity estimated to 76 kWh/m3 [4]. For a water storage with the same volume, the capacity is approximately 5 kWh assuming a 25 °C temperature difference. The PCM used was, as mentioned before, a commercial salt-based PCM with a theoretical melting point at 58 °C.

Solar assisted district heating system

public pool

fire

station

renovated gym

Подпись: public pool Подпись: renovated gym Подпись: fire station

The solar assisted district heating system with seasonal thermal energy storage in Eggenstein — Leopoldshafen (Germany) is the first system realized with existing renovated buildings. The project was initiated by a major refurbishment of the school, the gym and the existing district heating system. An additional gym with shed roof carrying 600 m2 of flat plate collectors (FC) was built. Together with a public swimming pool and a fire station the district heating system consists of buildings with a gross building area of 12 000 m2. For seasonal thermal storage of the solar heat produced by 1 600 m2 of flat plate collectors a 4 500 m3 gravel-water thermal energy store (TES) is integrated into the district heating system, see Fig. 1.

600 m2 FC

Подпись: 600 m2 FCrenovated school

1000 m2 FC

new gym

gravel-water thermal
energy store

Fig. 1: Solar assisted district heating system with 1600 m2 flat plate collectors and seasonal storage in

Eggenstein-Leopoldshafen (Germany

For backup heating two 600 kW gas boilers and a 30 m3 buffer store are available. Discharging of the seasonal TES down to 10 °C is facilitated by the use of a heat pump with a thermal power of 60 kW. Thus, the thermal capacity of the store is increased by 75% compared to discharging of the heat store to a return temperature of the heating net of 40 °C.

A detailed description of the system concept is given in [1]. Furthermore, optimization of the solar assisted district heating system by means of TRNSYS simulations is presented in [2]. This paper focuses on the design and construction of the gravel-water store and the measurement concept of the system.

Winter period

The effect of the PCM during winter season was evaluated in the experimental set-up during a free — floating temperature test. Fig. 11 shows the outside ambient temperature and the inside ambient temperature of the brick cubicles and Fig. 12 presents the data for the alveolar brick cubicles.

In the second week of December the temperature was very low, never reaching 15 °C (Fig 11a). The inside temperature in the RT27-PU cubicle follows the same tendency as in the PU cubicle with a higher absolute value. At the beginning of the week the temperatures were almost the same but the difference increased to 0.4 °C at the end of the week. Similar results were observed in January, February and March (Fig. 11b to Fig. 11d) with temperature differences around 0.3 °C, especially during the cold hours of the day. This effect can be caused by the low thermal conductivity of the PCM (0.2 W/mK), which works as insulation.

Referring to the alveolar brick and PCM-alveolar brick cubicles, Fig. 12a presents the data of the outside and inside temperatures during the second week of December. A similar effect as in the brick cubicles is observed. Although the temperature of both cubicles presents the same tendency, the SP25- alveolar one is between 0.1 °C and 0.5 °C warmer than the alveolar. Similar results are obtained for January, February and March (Fig. 12b to Fig. 12d). In those cases the effect in more visible at the beginning of the week and decreases with time.

15

10

5 Е

ш

3

0

ш

а.

-5

I-

-10

-15

Подпись:

PU cubicle

Подпись: PU cubicle

c) February

Fig. 11 Comparison of the inside temperatures of PU and RT27-PU cubicles.

Подпись:

RT-27+PU cubicle — — — Ambient temperature |

a) December

b) January

8 0:00 8 0:00 8 0:00 8 0:00 8 0:00 8 0:00 8 0:00 8 0:00 0:00
Period

I PU cubicle………………………. RT-27+PU cubicle ■ • ■ Ambient temperature]

d) March

SHAPE * MERGEFORMAT

Fig. 12 Comparison of the inside temperatures of Alveolar brick and SP25-Alveolar cubicles.

 

In this work the benefits of using PCM in conventional and alveolar brick construction are studied. Both free-floating temperatures and energy consumptions are analyzed for summer and winter periods.

During summer period, a reduction of the energy consumption of the HVAC system was achieved for set points higher than 20 °С. When the set point was reduced the PCM effect decreased since it is not melting properly.

The experimental results of the winter period showed a positive effect of the PCM. The increase of the insulation effect introduced by the PCM results in higher temperatures in the cubicles, especially during the cold hours of the day.

Acknowledgements

The work was partially funded with the project ENE2005-08256-C02-01/ALT, the project 2005SGR 00324 and with a collaboration with the companies Synthesia, Honeywell, Gremi de Rajolers,

Hispalyt, Tealsa, Europerfil, Prefabricats Pujol, Prefabricats Lacoma, Ceramicas Sampedro and Cityhall of Puigverd de Lleida.

Marc Medrano would like to thank the Spanish Ministery of Education and Science for his Ramon y Cajal research appointment.

References

[1] I. O. Salyer, A. K. Sircar, R. P. Chartoff, D. E. Miller, Advanced phase change materials for passive solar storage applications, in: Proceedings of the 20th Intersociety Energy Conversion Engineering Conference, Warrendale, PA, USA, (1985), pp. 699-709.

[2] M. Shapiro, D. Feldman, D. Hawes, D. Banu, PCM thermal storage in drywall using organic phase change material, Passive Solar Journal 4 (1987) 419-438.

[3] D. Banu, D. Feldman, F. Haghighat, J. Paris, D. Hawes, Energy-storing wallboard: flammability tests, Journal of Materials and Civil Engineering 10 (1998) 98-105.

[4] A. M. Khudhair, M. M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conversion and management 45 (2004) 263-275.

[5] B. Zalba, J. M. Marin, L. F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251-283.

[6] A. Hauer, H. Mehling, P. Schossig, M. Yamaha, L. F. Cabeza, V. Martin, F. Setterwall, International energy agency implementing agreement on energy conservation through energy storage, Annex 17 Final report, 2005.

[7] L. F. Cabeza, C. Castellon, M. Nogues, M. Medrano, R. Leppers, O. Zubillaga, Use of microencapsulated PCM in concrete walls for energy savings, Energy and Buildings 39 (2007) 113-119.

Simple weather scenarios for testing predictive control

On account of the aforementioned reasons, the implementation of predictive control (i. e., control taking into account estimates of future loads or conditions) is highly advisable in a house with active and passive storage capacities, such as the ANZEH. At the present time, predictive control is facilitated by the availability of reliable and complete weather forecasts.

To study the response of the ANZEH under different conditions, artificial sequences of 5 days, attempting to represent typical series of cloudy and sunny days, were designed. In these sequences, a sunny day was assigned a daily clearness index (KT) of 0.7, an intermediate day was assigned a KT of 0.5, and a cloudy day was assigned a KT of 0.3. The model of Liu and Jordan, as described in

[5] , was used to determine a typical distribution of hourly clearness indexes (kT) for both conditions. The Erbs model [5] was used to calculate the diffuse fraction of the global horizontal radiation. Finally, the Perez model [6] was used to calculate radiation on surfaces with different orientations.

In the sequences of 5 days used in this study, the last two days were assumed to be cloudy, while the first three days can be either cloudy (represented by a C), intermediate (represented by an M) or sunny (represented by an S). These sequences can be applied to any five consecutive days in the year. After choosing an initial day (from 1 to 365) astronomical angles can be taken into account. The graphs below correspond to the days from January 15th to January 19th.

Figure 4. Global Horizontal Radiation corresponding to scenarios SCCCC, SCSCC and SSMCC.

day

Подпись: day

Temperatures for Montreal were modelled using a steady-periodic curve using an average value and average fluctuation range for the corresponding month, and the design day data (quasi­sinusoidal) proposed by ASHRAE [7]. Wind speed data from a TMY2 (TRNSYS typical meteorological file) [8] corresponding to the dates examined, was used.

day day

4.3 Tank charging, space heating, and ventilation

The tank will only be charged when the top tank temperature is below the setpoint. For all cases, the HX is used directly if the BIPV/T air temperature is at least 3 degrees above the top tank’s temperature. If this is not the case, but the BIPV/T air temperature is above 10 °C, then both heat pumps are used to charge the tank. If the temperature is between 3.5 and 10 °C, then only one heat pump is used. Finally, if the BIPV/T air temperature is below 3.5 °C, one heat pump is turned on using the ground as a source. If the temperature in the house is too high (28.0), then the ventilation rate will be increased from 0.3 ACH (including natural and mechanical ventilation) to 1 ACH per hour.

007 — Simulation of natural convection in a hot water storage tank

HMOUDA Imen1*, RODRIGUEZ Ivette2 and BOUDEN Chiheb1

1Energy in Buildings and Solar Energy, National School of Engineers of Tunis
ENIT B. P. 37 1002 Tunis — Belvedere Tunisia

2Centre Tecnologic de Transferencia de Calor (CTTC), Universitat Politecnica de Catalunya (UPC), C.

Colom 11, 08222, Terrassa, Barcelona, Spain
Corresponding Author e-mail : ih tn@yahoo. fr

Abstract

C (ARa) І4т

as @a(T) = exp

1, where T the dimensionless time, A the aspect ratio of the vertical

Подпись: as @a(T) = exp Подпись: 1, where T the dimensionless time, A the aspect ratio of the vertical

Thermal energy storage tank is an essential component of solar heating systems. In this process, the buoyancy convection plays an important role where it is essential to ensure the availability of energy at the following days. Long-thermal behaviour of cooling an initially isothermal Newtonian fluid in a vertical cylinder by unsteady natural convection has been investigated in this study by scaling analysis and direct numerical simulation. The studied case assumes that the fluid cooling is due the imposed fixed temperature on the vertical side wall where as the top and bottom boundaries are adiabatic. Transient, axi-symmetric and natural convection in storage tank is studied. The unsteady natural convection has been investigated numerically by mean of an appropriate CFD code developed and validated using different benchmarking cases. The long-term behaviour of the fluid cooling in the cylinder is well represented by the average fluid temperature function of time, and the average Nusselt number on the cooling boundary. The scaling analysis shows that the dimensionless temperature is related to the dimensionless time, the Rayleigh number and the aspect ratio of the vertical cylinder. The dimensionless temperature ejj) is scaled

cylinder, Ra the Rayleigh number, and C a proportionality constant. A series of direct numerical simulations with the selected values of A, Ra, and Pr (Pr is the Prandtl number) in the ranges of 1/3< A <3, 6.106< Ra < 6.1010 and 1< Pr <1000 have been carried out to validate the developed scaling relations, and it is found that these numerical results agree well with the scaling relations. The numerical simulations reveal that the flow has considerably different transient features and vigorous flow activities mainly occur in the vertical thermal boundary layer on the side wall.

Key words: CFD, natural convection, cooling-term.

1. Introduction

The phenomenon of laminar convection in fluids in a cylindrical enclosure, driven by density difference, often occurs in technical applications and industrial processes. The fluid motion caused by this phenomenon has a great impact on the working characteristic of devices and processes where it occurs. This problem arises in applications such as: tanks for energy storage. Indeed, buoyancy convection plays an important role in process of thermal energy storage where it is essential to ensure the availability of energy at the following days. Thus, the system response to changing boundary conditions and the understanding of its behavior is of fundamental interest and practical importance.

Several works concerning this subject are available in the literature, involving experimental and numerical analysis. Cotter and Charles [1-2], in two papers, investigated, during the cooling
*

process, the transient natural convection of a warm crude oil contained in a vertical cylindrical tank. The numerical results have been compared with experimental data and a time dependence of Nusselt number for several oil viscosity has been defined. Natural convection was also analyzed by Ivancic et al. [3]. They investigated the case of the cylindrical tank using an adiabatic sidewall while maintaining low temperature at the top and high temperature at the bottom. Different tank aspect ratios (1 — 5) and Prandlt number (10-2 — 105) have been examined.

Oliveski et al. [4] made a numerical and an experimental analysis of velocity and an temperature fields inside a storage tank submitted to natural convection. They studied the effect of aspect ratio, volumes and insulation thicknesses on the thermal performance of the storage tank. Correlations for the Nusselt number were obtained. A good agreement between the numerical and experimental results have been obtained. Adopting the same methodology, these authors [5], compared the one­dimensional results with detailed model and experimental results. They shown that simplified model can agree with experimental only when several computational artifices were included.

In [6], [7], the authors carried out a scaling analysis and direct numerical simulation of the transient processes of cooling-down an initially homogeneous fluid by natural convection in a vertical cylinder. Many direct numerical simulations under different flow situations in terms of Ra, Pr and A were studied. The results show that vigorous flow activities concentrate mainly in the thermal boundary layer along the sidewall.

More recently, Rodriguez et al. [8] carried out a scaling analysis and numerical simulation of transient process of cooling-down an initially homogeneous fluid in a vertical cylinder submitted to natural convection. The obtained correlations can be extrapolated to other situations as they are expressed in term of non-dimensional parameters governing the phenomenon that occurs inside the tank.

In this work, a long-term cooling process of a storage tank submitted to unsteady natural convection is numerically investigated. The the top and upper walls of the stoarge tank are supposed adiabatic while the sidewall is submitted to ambient temperature. A series of numerical simulations with the selected values of A, Ra, and Pr (Pr is the Prandtl number) in the ranges of 1/3< A <3, 6.106< Ra < 6.1010 and 1< Pr <1000 have been carried out to validate the developed scaling relations.

The thermal resistances arise as follows

and

1

^eoSeo

1

h

co co

(14)

(15)

Zi

Z

9

Подпись: and

Zi and Z9 The thermal resistance between the heat source and the external surface of the evaporator and between the external surface of the condenser and the heat sink respectively are given by

evaporative and

Подпись:Z2 and Z8 are the thermal resistances across the thickness of the tube wall in the the condenser respectively, which is determined as

Цр./р,)

2nlek

and

(17)

Подпись: (17)7 = ln(D,/D)

8 2nlck

2.2.1 Internal resistant.

Z3p =

______ 1______

Ф3д°’20°’4 (iDLe)0-6

(18)

Подпись: (18)

Z3 and Z7 are the internal resistances due to pool and film boiling of the working fluid which is divided into Z3p is resistant from pool boiling:

Z3f is resistant from film boiling at the evaporator section :

Z = CQ13

3f " Бі4/У/3ЬєФ 24/3 (19)

when g is gravity (m/s2)

C is constant of cylinder tube C = (1/4)(3/n)43 = 0.235

Ф2 is Figure of Merit:

(20)

Подпись: (20)( 3 2 у ‘ "

Lkl Pl

,l

L is Latent heat of working fluid (kJ/kg)

kl is thermal conductivity of working fluid as liquid phase (W/moC) pl is density of working fluid as liquid phase (kg/m3)

, is viscosity of working fluid as liquid phase (N/m)

: Ф

3

= 0.325 x

0.5^0.3~ 0.7 pl kl Cpl p 0.25, 0.4,, 0.1 pv L Ul

(21)

0.23

Подпись: 0.23 Подпись: (21)

Ф3 Figure of Merit (3)

Cpl is the specific heat at constant pressure of working fluid as liquid phase (kJ/kgoC)

pv is the density of working fluid as vapor phase (kg/m3)

Pv is the vapor pressure of working fluid (Pa)

Pa is the atmospheric pressure 101.3 kPa and the condition for using Z3p and Z3f as Z3 is if Z3p > Z3f so

Z3 = Z3p

if Z3p < Z3f so

Z3 = Z3PF+Z3f(1-F) (22)

when F is the filling ratio that is defined by

Vi

F =

(23)

Подпись: F = Подпись: (23)

ALe

Vl is the volume of working fluid (m3)

A is the area cross section of the pipe (m2)

Z7

(24)

CQ1/3

g1/3LcФ

4/3

2

Подпись: Z7 Подпись: (24)

Z7 is the resistance from film boiling of working fluid at the condenser section

Z4 and Z6 are the thermal resistances that occur at the vapor liquid interface in the evaporator and the condenser respectively. These are always neglected, being exceedingly small.

Z5 is the effective thermal resistance due to the pressure drop of the vapor as it flows from the evaporative to the condenser. It is small compared to Z3 and Z7.

Z10 is the axial thermal resistance of the wall of the container. This is always neglected, being exceedingly small. Due to Z4,Z6,Z5 and Z10 always being neglected because of being exceedingly small, thus the overall thermal resistance is approximated by

(25)

Подпись:Ztotal = Z1 + Z2 + Z3 + Z7 + Z8 + Z9

Weather data

In order to investigate the potential of solar thermal technologies at a residential scale in Wales, average weather data for the city of Cardiff is used in the simulations. The data used is in TMY2 format and is provided by the weather library of TRNSYS. The TMY2 average year for Cardiff is built in METEONORM, based on measurements from a local weather station [7]. Fig 1 shows the difference between predictions using 30 year average data and the weather data built in

Fig 1. Incident (direct and diffuse) solar energy curves (10m2 collector, 45° tilt South) with ECOTECT data and with 30 year average values given by Page and Lebens.

Подпись: Fig 1. Incident (direct and diffuse) solar energy curves (10m2 collector, 45° tilt South) with ECOTECT data and with 30 year average values given by Page and Lebens .

METEONORM. The values represent the incident solar energy on a 10m2 surface orientated towards south and tilted for 45o in Cardiff calculated with the TMY2 weather data and with average weather data of the years 1941-70[8]. To represent the variations found in real weather conditions, one year data for Cardiff taken from actual measurements from a weather station located at the roof of the Welsh School of Architecture [9] during year 2007 is also used in this study.

Chemical and Sorption Storage — Results from IEA-SHC Task 32

C. Bales1*, P. Gantenbein2, D. Jaehnig3, H. Kerskes4, M. van Essen5, R. Weber6, H. Zondag5

1Solar Energy Research Center SERC, Hogskolan Dalama, 78188 Borlange, Sweden
2SPF Hochschule fur Technik Rapperswil, Oberseestr. 10, CH-8640 Rapperswil, Switzerland
3AEE INTEC, Feldgasse 19, A-8200 Gleisdorf, Austria
4ITW, Pfaffenwaldring 6, D-70550 Stuttgart, Germany
5Energy research Centre of the Netherlands (ECN), P. O. box 1, NL — 1755 ZG Petten, The Netherlands

6EMPA Duebendorf, Abteilung Energiesysteme / Haustechnik, Ueberlandstrasse 129, CH-8600 Duebendorf,

Switzerland

* Corresponding Author, eba@du. se

Abstract

Six main groups have studied chemical and sorption storage within IEA-SHC Task 32 “advanced storage concepts for solar and low energy buildings”. Closed and open adsorption systems, two and three phase absorption as well as chemical storage have been studied. The main results of the work are: identification of potentially suitable materials for long term storage of solar heat and publication of material properties; development of new concepts of short and long term storage of solar heat to prototype stage with lab and field tests; development of models for simulation of chemical and sorption storage; simulation of three systems with long term chemical or sorption storage with the Task 32 boundary conditions; and support in the commercialisation of a chemical heat pump with short term thermal storage for solar heating and cooling applications. The main conclusion from the work is that there are a number of promising technologies and materials for seasonal storage of solar heat for single families but that a lot of research is required before it can be become practical and economical.

Keywords: Solar heating, thermal energy storage, sorption, chemical heat storage

1. Introduction

Task 32 of the International Energy Agency’s Solar Heating and Cooling Programme has studied advanced storage concepts for solar and low energy buildings over the period 2004-2007. The Task was split into four subtasks covering the following areas: evaluation and dissemination (A), chemical and sorption storage (B), Phase Change Materials (C) and advanced water storage (D). This paper describes the work performed in Subtask B on chemical and sorption storage, including results from basic research in terms of material and heat transfer characteristics, as well as store and system modelling.

Six groups have been active in this Subtask, as shown in Table 1 below, which also shows the type of technology that the groups have studied. EDF from France have also participated with work on chemical storage towards the end of the Task. The scope, in terms of general system aspects, for Subtask B 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/m2 for Zurich climate) were not considered. The main focus was to be storage solutions sized to achieve a significant solar fraction. In terms of temperature, the

storage solutions have been limited to temperatures < 250°C, with the emphasis on materials suitable up to around 150°C.

The scope in terms of storage concepts included chemical reactions and thermo-chemical storage, which was in practice restricted to sorption processes, both adsorption and absorption. Only one storage solution dealt with in Subtask B has become commercial within the time frame of Task 32, that developed by the Swedish company ClimateWell from Sweden, with over 35 stores/heat pumps having been sold, mostly for solar heating and cooling systems in Spain. A demonstration system of a closed adsorption store was made in the Modestore project, but the materials available for the field test were shown to be not suited for seasonal storage. Three other projects have got as far as design and testing of lab prototypes of sorption stores, and the sixth project was at the stage of material characterisation.

10 reports from the work on chemical and sorption storage are available from Task 32 website at www. iea-shc. org/task32, along with a number of reports on the work on phase change materials, advanced water stores and methods and intercomparisons.

Table 1. Research groups participating in IEA-SHC Task 32’s work on sorption and chemical storage.

Group / Project

Description

ECN and Univ. Eindhoven, Holland.

Compact chemical seasonal storage of solar heat.

Theoretical analysis of suitable chemical reactions in the range 60 — 250°C. Choice of most suitable material and experimental studies of material properties (MgSO4.7H2O). Simple modelling of the chemical heat store and system simulations with the Task 32 boundary conditions.

SERC, Hogskolan Dalarna, Sweden.

Evaluation of thermo­chemical accumulator (TCA).

Measurements on a prototype and commercial TCA chemical heat pump, based on a 3-phase closed absorption process, in the lab. Modelling of the process and of the prototype and commercial machines. System simulations for cooling in district heating and solar cooling systems for Swedish and Spanish conditions.

Institut fur Solartechnik SPF, Switzerland.

Sorption storage.

Solid closed system adsorption process with zeolite or silica gel. Studies of material properties and theoretical analysis. Measurements of heat and mass transfer dynamics.

AEE INTEC, Austria. Modestore (Modular high energy density heat storage).

Design of closed system adsorption heat store with silica gel with all components integrated into one unit. Testing in the lab and in the field. Modelling of the store, design of system and then simulation for full scale domestic seasonal storage for the Task 32 boundary conditions.

ITW, Univ. Stuttgart, Germany.

Monosorp.

Initial study of open adsorption system using zeolite. Heat storage and removal from the store utilises the ventilation heat recovery system and moisture in the house. Measurements on prototype heat store in the lab. Modelling of the store and design of system. System simulations of seasonal storage for German conditions as well as the Task 32 boundary conditions.

EMPA, Switzerland. Closed NaOH absorption storage.

Development of closed two-phase absorption process with NaOH. Measurements on a prototype in the lab.