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