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Research Papers

# Compact Heat Storage for Solar Heating Systems

[+] Author and Article Information
Viktoria Martin

Department of Energy Technology, KTH, Brinellvägen 68, SE-100 44 Stockholm, Sweden

Fredrik Setterwall1

Department of Energy Technology, KTH, Brinellvägen 68, SE-100 44 Stockholm, Swedenfredrik.setterwall@ecostorage.se

1

Corresponding author. Present address: Ecostorage Sweden AB, Bäckvägen 7c, SE-19254 Sollentuna, Sweden.

J. Sol. Energy Eng 131(4), 041011 (Sep 30, 2009) (6 pages) doi:10.1115/1.3197841 History: Received November 23, 2008; Revised April 27, 2009; Published September 30, 2009

## Abstract

Energy and cost efficient solar hot water systems require some sort of integrated storage, with high energy density and high power capacity for charging and discharging being 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 (PCMs) as the storage media. A 140 l $15 kW h$ storage prototype was designed, built, and experimentally evaluated. The storage tank was directly filled with the PCM having its phase change temperature at $58°C$. A tube heat exchanger for charging and discharging with water was submerged in the PCM. Results from the experimental evaluation showed that hot water can be provided with a temperature of $40°C$ for more than 2 h at an average power of 3 kW. The experimental results also show that it is possible to charge the 140 l storage with close to the theoretically calculated value of 15 kW h. Hence, this is a PCM storage solution with a storage capacity of over $100 kW h/m3$, and an average power capacity during discharging of over $20 kW/m3$. However, it is desirable to increase the heat transfer rate within the prototype. A predesign of using a finned-tube coil instead of an unfinned coil show that by using finned tube, the power capacity for discharging can be at least doubled, if not tripled.

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## Figures

Figure 1

Cross-sectional top view schematic of the PCM prototype for high capacity and power hot water storage. Hot water circulates in coils A, B, and C (in series). Cold water circulates in coils D, E, and F (in series).

Figure 2

Schematic of the experimental test facility

Figure 3

Charging curves for the PCM storage prototype (charging temperature of 75°C and flowrate of 0.2 m3/h)

Figure 4

Example temperature profiles within storage during charging of the PCM storage prototype (charging temperature of 75°C and flowrate of 0.2 m3/h)

Figure 5

Charging curves for the PCM storage prototype (charging temperature of 70°C and flowrate of 0.2 m3/h)

Figure 6

Discharging of the PCM storage prototype using various flow rates—hot water temperature (leaving tank) versus time

Figure 7

Discharging of the PCM storage prototype using a 0.2 m3/h flow rate: left axis—power and discharged amount of energy; right axis—temperature of water leaving the tank

Figure 8

Examples of log mean temperature difference versus time during (a) charging and (b) discharging

Figure 9

Examples of heat transfer coefficient versus time during (a) charging and (b) discharging

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