Date: January 14, 2008
4. Conclusion
Two inlet stratification pipes made of two fabric layers are investigated at different operation conditions. The results are compared to identical tests with a marketed rigid stratification pipe with three holes with “non-return” valves. The thermal performance of the stratification pipes is investigated during heating, stratified heating and cooling tests with volume flow rates of 6 l/min, 8 l/min and 10 l/min.
The investigation shows that the thermal stratification is build up in a very good way with the investigated fabric stratification pipes during all the experiments, especially with the stretchable fabric style 700-12. Further, the investigation shows that the thermal stratification is build up in a better way with volume flow rates of 8 and 10 l/min than with a volume flow rate of 6 l/min. It is expected that fabric stratification pipes can work well for all volume flow rates, provided that the pipe diameter is adjusted to the flow rate.
The investigation also shows that the rigid stratification pipe works very well during the heating test with a volume flow rate 6 l/min and that the thermal performance decreases with increasing volume flow rates. Finally, the investigation shows that, during the stratified heating test and the cooling test, the thermal stratification is not maintained in the same good way as with the fabric stratification pipes.
References
[1] Andersen E., Furbo S., Fan J., Investigations of fabric stratifiers for solar tanks, Proceedings of ISES Solar World Congress 2005, Orlando, Florida, USA, 2005.
[2] Krause Th., Kühl L. Solares Heizen: Konzepte, Auslegung und Praxiserfahrungen, 2001.
[3] Shah L.J. Stratifikationsindløbsrør. Department of Civil Engineering, Technical University of Denmark, DTU, 2002.
[4] www.testfabrics.com
0 100 200 300 400 500 600 700 800
-0.2 0 0.2 0.4 0.6 0.8 1
(T-Ttank,start)/(Tinlet-Ttank,start) [-]
Height from bottom of tank [mm]
30 litre
Solvis 60 litre
90 litre
0 100 200 300 400 500 600 700 800
-0.2 0 0.2 0.4 0.6 0.8 1
(T-Ttank,start)/(Tinlet-Ttank,start) [-]
Height from bottom of tank [mm
30 litre
Style 864 60 litre
90 litre
0 100 200 300 400 500 600 700 800 900
-0.2 0 0.2 0.4 0.6 0.8 1
(T-Ttank,start)/(Tinlet-Ttank,start) [-]
Height from bottom of tank [mm]
start 6 l/min 8 l/min 10 l/min
30 litre
Style 700-12 60 litre
90 litre
Heat of fusion storage with high solar fraction for solar low energy buildings.
Proceedings EuroSun 2006 Congress, Glasgow, Scotland.
Jørgen M. Schultz & Simon Furbo
Solar Low Energy Buildings
J.M. Schultz1* and S. Furbo1
1 Department of Civil Engineering, Technical University of Denmark, Build. 118, Brovej, DK-2800 Kgs. Lyngby, DENMARK
* Corresponding Author, email : js@byg.dtu.dk
Abstract
This paper presents the theoretical investigation on a concept for a seasonal thermal storage based on the phase change material sodium acetate trihydrate with active use of supercooling as a measure to achieve a partly heat loss free thermal storage. The effect of supercooling allows a melted part of the storage to cool down below the melting point without solidification preserving the heat of fusion energy. If the supercooled storage reaches the surrounding temperature no heat loss will take place until the supercooled salt is activated. The investigation shows that this concept makes it possible to achieve 100% coverage of space heating and domestic hot water in a low energy house in a Danish climate with a solar heating system with 36 m² flat plate solar collector and approximately 10 m³ storage with sodium acetate. A traditional water storage solution aiming at 100% coverage will require a storage volume several times larger.
Keywords: Solar heating systems, heat of fusion storage, seasonal storage, sodium acetate 1. Introduction
A key parameter for increasing the yield of solar heating systems is efficient thermal storages that can store large amounts of energy in a reasonable storage volume with limited thermal losses.
Water storages are the most common used storage technology and large improvements with respect to reaching almost ideal stratification as well as efficient control strategies have been achieved.
Combined solar heating systems, i.e. systems for both domestic hot water (DHW) and space heating, require larger storage capacities than DHW-systems leading to larger storage volumes in order to store the energy for longer periods. Storage of heat for longer periods often means that the average storage temperature in periods with surplus of solar energy will be very high leading to higher heat losses reducing the useful energy for space heating. Therefore phase change materials (PCM’s) with a melting point at a useful temperature level are interesting in order to increase the thermal storage capacity at moderate temperatures. One of these phase change materials is sodium acetate trihydrate, NaCH3COO 3H2O, which melts at 58 °C with a heat of fusion energy of approximately 265 kJ/kg [1]. Sodium acetate is capable of supercooling, i.e. it is able to cool down to temperatures below its melting/freezing point without solidification. This is normally seen as a drawback as the heat of fusion energy is not released.
However, as part of the IEA Solar, Heating and Cooling programme, Task 32: “Advanced Storage Concepts for Solar and Low Energy Buildings”, the idea of active use of stable super cooling as a measure to obtain a partly heat loss free seasonal storage is investigated. When first melted the storage will cool down to the surrounding temperature without phase change
preserving the latent heat related to the phase change. In this state the storage will have no heat loss until the phase change is activated. The principle of super cooling is shown in Fig. 1.
Theoretical comparison between sodium acetate storages with active use of supercooling and water storages shows that the benefit of using a PCM storage compared to a traditional water storage is limited with respect to absolute annual energy savings for storage sizes up to 1 m3, but if the same amount of net utilised solar energy should be reached it would require a water storage that is 2 – 3 times larger [2]. But for larger storage volumes - at fixed solar collector areas - PCM
Heat storage capacity of sodium acetate compared to water
0 100 200 300 400 500 600 700 800
20 30 40 50 60 70 80 90 100
Temperature [°C]
Stored energy [kJ/litre]
Melting point = 58 °C
Sodium acetate
Water Supercooling
Fig.1. Illustration of energy content of sodium acetate compared to water as well as the super cooling process.
The theoretical investigations [2] also showed that the effect of the supercooling is limited for PCM storage volumes up to approximately 3 m3 as the heat loss free state is seldom reached.
However for larger PCM storage volumes the heat loss free state may become reality, and it might be possible to achieve very large solar fractions by active use of the supercooling process.
This paper presents the theoretical study of the potential of a seasonal phase change material storage with active use of supercooling for achievement of 100% solar coverage of both domestic hot water and space heating in a low energy single family house in a Danish climate (56° north, approx. 3000 degree days).
2. System description