Category Archives: EUROSUN 2008

Charging processes

During the charging process, as no fluid flows through the absorber, the calculated absorber temperature is directly linked to the composite temperature at the interface by:

— к

Подпись:

c

Подпись:

К / a (Tc — T )

Подпись: (eq.12)

Fig.5: Comparison between experience (interface absorber/polymer (o), interface PCM/polymer (A) and polymer (□) temperature) and simulation (interface absorber/polymer (—) and polymer (—) temperature) in charging

process

Подпись: Fig.5: Comparison between experience (interface absorber/polymer (o), interface PCM/polymer (A) and polymer (□) temperature) and simulation (interface absorber/polymer (—) and polymer (—) temperature) in charging process

With an exchange coefficient of 130 W. m-2.K-1 we obtain the temperature profile plotted in figure 5. The simulated temperature profile at the middle of the composite is situated between the two recorded experimental temperatures.

Development of PCM-based Thermal Energy Storage for Solar Hot Water Systems

V. Martin*, F. Setterwall and L. Hallberg

Ecostorage Sweden AB, Backvagen 7c, SE-192 54 Sollentuna, Sweden
Corresponding Author, viktoria. martin@,ecostorage. se

Abstract

Solar heating in the built environment is a crucial component for minimizing the use of fossil fuel. However, an energy and cost efficient solar system requires some sort of integrated storage. Here, high energy density and high power capacity for charging and discharging are desirable properties of the storage. This paper presents the results and conclusions from the design, and experimental performance evaluation of high capacity thermal energy storage using so-called phase change materials (PCM) as the storage media.

A 15 kWh PCM-based storage was designed, built, and experimentally evaluated with regards to capacity and power properties. The storage had a volume of close to 140 liters such that the specific storage capacity was around 100 kWh/m3 and the ice packing factor (IPF) was close to 80%. The first results from the experimental evaluation show that the storage can provide hot water with a temperature of at least 40 °C for more than two hours, at an average power of 3 kW.

A comparative cost analysis show that, considering the capital (first) cost only, the cost of the PCM-storage is always higher than for a water storage (i. e. IPF=0) although the difference is not very large. However, if the cost of “space requirement” is important, such as in a house, the PCM — solution quickly becomes cost effective as compared to the hot water storage.

Keywords: PCM, phase change materials, latent heat, thermal energy storage, solar hot water

1. Introduction

Solar heating in the built environment has the potential to aid in minimizing the use of fossil fuel. However, an energy and cost efficient solar system requires some sort of integrated storage with high energy density and high power capacity for charging and discharging being desirable properties of the storage. By using conventional hot water storage with a 30 °C temperature difference, it is possible to store about 30 kWhheat/m3 storage. By an appropriate design of thermal energy storage using phase change material-technology (PCM) it is possible to triple the capacity to over 90 kWh/m3. Hence, PCM-storage has been explored extensively over the years, and this work is thoroughly summarized by Hauer et al. [1]. Earlier, most studies focussed on the storage capacity (energy) rather than the importance of also having a large power capacity for charging and discharging. However, one problem with PCM-technology is the relatively low conductivity of PCM resulting in low power for charging and discharging. This important design challenge has recently been addressed in several ways. Recently, for example, Seeniraj and Narasimhan [2] theoretically modelled a finned tube submerged in
multiple PCMs with varying phase change temperature. With the multiple PCMs it is possible to maintain a higher temperature difference between the heat transfer fluid and the PCM throughout the charging process. The results showed a possibility for a much higher rate of melting using this concept, as compared to only a single PCM. Also, the exit temperature of the heat transfer fluid was held constant over time. Pincemin et al [3] also modelled tubes submerged in PCM with added graphite powder to enhance conductivity. To further improve the conductivity, graphite fins were submerged between the tubes. The conclusion was that there is an optimal proportion between the amount of graphite added as powder and the fin spacing for the graphite fins.

For the purpose of developing cost effective PCM-based storage for solar applications, a 15 kWh PCM-based storage was designed, built, and experimentally evaluated with regards to capacity and power properties. The design features were chosen to obtain high power, high storage capacity and low manufacturing cost.