Design of a Latent Heat Energy Storage. System Coupled with a Domestic Hot Water. Solar Thermal System. Louis Desgrosseilliers. Robynne Murray.
Phase Change Material Selection in the Design of a Latent Heat Energy Storage System Coupled with a Domestic Hot Water Solar Thermal System Louis Desgrosseilliers
Robynne Murray
Alex Sefati
Gina Marin
Jeremy Stewart
Nicolas Osbourne
Mary Anne White, PhD
Dominic Groulx, PhD, P.Eng
Abstract Home and business users of solar domestic hot water (SDHW) heating systems are able to reduce the energy costs and greenhouse gas emissions associated with domestic hot water use. However, the level of deployment of solar thermal technologies, as retrofits and new installations, is limited by the space and weight imposed on the structures for storing the collected energy. Phase change materials (PCMs) are advantageous for daily energy storage with SDHW systems due to their high energy storage density, and mainly isothermal operation. This paper summarizes the initial steps in the development of an energy storage system using PCMs, with emphasis on the material selection and experimental studies used for proof of concept and design optimization. Lauric acid was selected as the PCM based on the melting temperature range that was targeted by studying solar data from an existing solar hot water system in Halifax, Nova Scotia. The proof of concept experiment was done using a finned tube to carry the thermal fluid; these extended surfaces were used due to the low thermal conductivity of PCMs. A validated and optimized design will be built and installed by early 2011 for a pilot study in an existing large scale solar thermal system on an apartment building in Halifax. The entire system will be instrumented in order to acquire continuous data (temperatures, flow rates, pressure, etc.) to fully characterize the system and improve on this first tested prototype. INTRODUCTION Using phase change materials (PCMs) for latent heat energy storage with SDHW systems reduces the volume and weight of thermal storage due to their high energy storage density (Agyenim, 2010). Latent heat energy storage systems (LHESS) have preferable energy storage properties in comparison to sensible heat storage (Fernandez, 2010), and have been shown to store 5-14 times more heat per unit volume than sensible heat storage materials such as water (Sharma et al., 2009; Farid et al., 2004). Louis Desgrosseilliers and Robynne Murray are Masters of Applied Science students in the Department of Mechanical Engineering, Dalhousie University, Halifax, Nova Scotia. Alex Safatli is a undergraduate chemistry student in the Chemistry Department, Dalhousie University. Gina Marin, Jeremy Stewart and Nicholas Osbourne are undergraduate mechanical engineering students in the Department of Mechanical Engineering. Dalhousie University. Mary Anne White is a professor in the Chemistry Department, Dalhousie University. Dominic Groulx is a professor in the Department of Mechanical Engineering, Dalhousie University.
PCMs are the only materials that undergo melting and freezing in a desired temperature range to store and release thermal energy. They are classified into two categories: organic and inorganic PCMs. Organic PCMs in practical use are either fatty acids or waxes (e.g. olefins and alkanes), and in some cases polymers and aromatics. Inorganic PCMs are nearly always salt hydrates, but can include other aqueous solutions or pure substances (e.g. ice). Organic PCMs usually have low thermal conductivity and moderate latent heat of fusion; inorganic PCMs have typically higher heats of fusion and thermal conductivity, but many suffer from cycling instabilities of these properties as well as supercooling. It is imperitive that PCM selection for a LHESS addresess all of these issues. Energy storage using PCMs in combination with solar collectors has been studied mathematically (Qarnia, 2009) and experimentally (Sari, 2003) and shown to be advantageous; however, missing from previous works is a working prototype of a SDHW system for a large scale application (Shukla, 2009). Due to the low thermal conductivity of PCMs in general, design characteristics to enhance heat transfer need to be identified in order to optimize LHESSs. Many different methods of enhancing heat transfer have been studied, and include fins, multi-tube arrays, bubble agitation, metal rings, matrixes and brushes, encapsulation, etc. Fins are the most commonly used, and various studies have compared different fin sizes and orientations. Castell et. al. (2008) used a cylindrical PCM container with external (HTF side) 2mm thick vertical fins and showed that for vertical fins, the longer the fins offer better heat transfer rates. Agyenim et. al. (2009) showed that longitudinal fins are advantageous over circular fins. Gharebaghi and Sezai (2008) developed a mathematical model to compare circular fin spacing based on the finite volume method, and concluded that the closer the fins were together, the larger the increase in heat transfer rate. This paper presents the results of a study aimed at characterizing a short list of PCM candidates for a LHESS to be used in Halifax, Nova Scotia, and experimentally study the performance of the selected PCM using a vertical fin arrangement for the heat exchange. The selected design will be built and installed by early 2011 in an existing large scale solar thermal system on an apartment building in Halifax. PHASE CHANGE MATERIAL SELECTION Primary Selection Data on solar heat collection in a glycol heat exchange SDHW system was obtained from our industry partner, Scotian Windfield. Analysis of the system performance over one year determined that a suitable PCM must have a peak melting temperature in the range of 42-48°C (108-113ºF). Also, freezing should at the minimum occur between 35°C and 40°C (95 and 104ºF). A short list of PCMs meeting the melting criterion is in Table 1. Glauber’s salt remained in the short list since it has a high reported latent heat of fusion and thermal conductivity. In-house calorimetric testing would confirm whether the melting/freezing ranges would be acceptable for the given application. The other desirable properties affecting the selection were low toxicity and cost. Table 1. PCM Short List PCM Type Melting Latent Heat of Source Temperature, °C Melting, kJ/kg (ºF) (Btu/lb) Zalba et al., 2003 Na2SO4∙10H2O (Glauber's salt) Salt Hydrate 32 (89.6) 251 (108) Na2S2O3∙5H2O (Sodium Thiosulfate Pentahydrate) Salt Hydrate 48 (118) 206 (88.7) Kenisarin and Mahkamov, 2007 CH3(CH2)10COOH (Lauric acid) Fatty Acid 44 (111) 181 (77.9) Chemical Properties Handbook, 1999 Final Selection The most promising materials were tested using a differential scanning calorimeter (DSC) to study their melting
temperature ranges and identify materials with significant supercooling 1. The salt hydrates that were tested (Glauber’s salt and sodium thiosulfate pentahydrate) showed supercooling during DSC testing, which is a common phenomenon for these materials (Sandnes, 2006). Figure 1a) shows the DSC curve for Glauber’s salt, confirming its melting range (32 to 38°C [89.6-100.4ºF]) insufficient for the LHESS to be designed. Supercooling was also far too great with more than 40°C (104ºF) of supercooling. Similar results were obtained for sodium thiosulfate pentahydrate, eliminating both inorganic PCMs from further consideration. Furthermore, research is still performed in order to determine their stability over the extended melt/freeze cycles, but reports indicate that most salt hydrates have poor phase stability (hysterisis of the phases) and require additives for reduced supercooling and thickening (Sandnes, 2006), and polymeric encapsulation to prevent metal corrosion (Kenisarin, 2007), making them more costly and more difficult to incorporate into a LHESS design. The DSC curve for lauric acid (crude,
