Description of main achievements and developments

Major developments of the projects are two pre-industrial prototypes hereafter referred as OPICS1 and OPICS2, see Figure 1. Main features of these prototypes are:

• Prototype OPICS1 : ICS with a TIM-cover, a water store of 75 l and with a modular rectangular shape (of about 1m x 0.75 m x 0.20 m) that facilitates its integration in buildings. Other technical parameters are:

• Maximum relative pressure acceptable in the tank is limited to 1 atm

• Use of a high selective coating in the absorbing surface

• The absorbing area is of 0.75 m2

• The daily solar efficiency after 24 hours of outdoors exposure without draw-off is in the range of 55 %.

• Prototype OPICS2: ICS with a TIM-cover, a PCM store with an internal heat exchanger that permits to transfer energy from the PCM to the thermal fluid, and with a modular rectangular shape (of about 1m x 0.75 m x 0.15 m) that permits the integration in buildings. Other technical parameters are:

• Maximum relative pressure acceptable in the thermal fluid is above 10 atm

• Use of a high selective coating in the absorbing surface

• The absorbing area is of 0.75 m2

• The daily solar efficiency after 24 hours of outdoors exposure without draw-off is in the range of 50 %.

Both pre-industrial prototypes have been modelled, constructed and tested in detail following ISO procedures [4,5]. The tests included measurement of the daily efficiency, measurement of the heat loss coefficient and measurement of the draw off process. The tests have also been modelled using simulation tools [2,3], and good agreement has been observed between the experimental and numerical results. This has proved the credibility of the numerical model.

The numerical model has also been used to estimate the yearly performance of the new prototypes when installed in different climates and for four different demands corresponding to four reference applications:

• Pool heating

• Domestic hot water

• Space heating

• Absorption cooling using a LiBr-H2O machine of simple effect.

Three climates have been considered which corresponded to the cities of

• Barcelona

• Almeria (South of Spain)

• Copenhagen.

In order to analyse the advantages of the new prototypes, simulations of the OPICS prototypes have been compared to simulations of two ICSs using standard materials (no TIM nor PCM) and with a water store equal to the water store of the prototype OPICS1. They are referred as refl and ref2. The reference ICS refl uses a black paint as selective coating (low-medium selective coating), while the ICS ref2 uses the same high selective coating that is used in the prototypes OPICS1 and OPICS2.

Results have shown that, as already known by the solar thermal community, it is very important to use high selective coatings, because the industrial cost can be similar to that of low or medium selective coatings and energy gains are drastically increased.

Results from the yearly performance estimation also show as in all climates and applications, the energy gains in the solar systems are higher when using the OPICS prototypes than when using the standard ICS with a high selective coating, ref2. For applications at low temperature levels as pool heating and domestic hot water, the increase in the energy gains is small, in the range from 0 to 5 %. The explanation is that the use of TIM mainly contributes to a reduction of the heat losses through the cover due to convection effects, and for low temperature levels, thermal losses through the cover are low. For applications at higher temperature level, space heating and absorption cooling with the LiBr-H2O machine of simple effect, as thermal losses through the cover are more important, the reduction of the convection effects leads to a drastic increase of the energy gains. For the space heating application, this increase is in the range from 35 to 50 %. In the absorption cooling application, the reference prototype ref2 is only able to have energy gains in very hot and insulated climates as in Almeria. In other colder climates, even in the Mediterranean climate of Barcelona, no energy gains can be obtained from the ICS ref2. On the other hand, the OPICS prototypes are able to have energy gains in all climates.

2. Conclusions

In the project OPICS Integrated Collector Storage (ICS) devices have been developed using Transparent Insulation Materials (TIM) and Phase Change Material (PCM). Main features of the prototypes are a compact design that facilitates their integration in facades and roofs of buildings and an improved efficiency with respect to the standard ICSs.

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 following ISO procedures.

Pre-industrial prototypes of the OPICS-ICSs have been constructed, tested and modelled in detail.

Results have proved that improved efficiencies can be achieved with respect to standard ICSs, and

that they can be used in applications were standard ICSs are not able to give a reasonable

performance.

Acknowledgements

This work was funded in part by the European Commission under the Fifth Framework

Programme, Thematic Programme: Energy, Environment and Sustainable Development FP5-

EESD, Project CRAFT-1999-70604.

References

[1] J. Cadafalch, A Detailed Numerical Model for Flat Plate Solar Thermal Devices, Sol. Energy (2008).

[2] J. Cadafalch, R. Consul and A. Oliva, Detailed Model for the Virtual Prototyping of Flat Plate Solar Thermal Devices, Proceedings EUROSUN 2006, Glasgow.

[3] J. Cadafalch et al., Optimised Integrated Collector Storage: Low-Cost Solar Thermal Systems for Houses and Offices (OPICS). CRAFT Publishable Report, EU Contract CRAFT-1999-72476, 2005.

[4] ISO9459-2. Solar heating -Domestic water heating systems — Part 2: Outdoor test methods for system performance characterization and yearly performance prediction of solar-only systems, 1995. International Organization for Standarization, ISO 9459-2:1995(E), Switzerland.

[5] ISO9459-5. Solar heating -Domestic water heating systems — Part 5: System performance characterization by means of whole-system tests and computer simulation, 1996. International Organization for Standarization, Draft International Standard ISO/DIS 9459-5, Switzerland.