Thermostall

MARIE CURIE ACTIONS

Project H2020-MSCA-IF-2015 – GA - 705944

High Performance Seasonal Solar Energy Latent Heat Thermal Storage Using Low Grade, Low Melting Temperature Metallic (THERMOSTALL)

Project duration: 24 months (2016-2018);  Grant value: € 195k

Funded by EUROPEAN COMMISSION Research Executive Agency 

PI: Prof. K Mahkamov; MC Research Fellow: Dr. Carolina Costa;

Aim and Objectives

Energy storage technologies have long been a subject of great interest to both academia and industry. The aim of this project is to develop a novel, cost effective and high performance Latent Heat Thermal Energy Storage System (LHTESS) for seasonal accumulation of solar energy in increased quantities.

The major barrier for currently used organic, salt and salt hydrates based Phase Change Materials is their very low thermal conductivity coefficient and density, which results in the relatively large volumes of storage systems and difficulties in achieving the necessary rates of thermal energy re-charge and discharge, even when using advanced heat exchangers. 

The new approach to overcome the above issues is the deployment of low grade,   low melting temperature metallic alloys (ELMTAs). The ELMTAs are currently produced for application in other industrial sectors (e.g. electronics) and have not been actively considered for the thermal energy accumulation with the exception of very limited studies. Their heat conduction is two orders of magnitude greater than that of conventional PCMs and they are stable and can provide the thermal storage capacity which is 2-3 times greater per unit of volume due to the greater densities.

The project consists of both theoretical and experimental investigations. A range of low grade ELMTAs for application in LHTESS have been selected and their thermophysical properties are being studied using Differential Scanning Calorimetry and thermal conductivity analysis. Thermal cycling tests of such alloys are also being conducted.

Numerical investigations of heat transfer and flow in the LHTESS with ELMTAs will be performed. Experimental studies of heat transfer and flow in a laboratory prototype of the LHTESS with ELMTAs will be conducted. As outcomes of investigations, dimensionless heat transfer correlations will be derived and design recommendations for a practical solar energy seasonal LHTESS with the low grade ELMTA will be produced for a project industrial partner.

Brief description of Literature analysis

Through literature analysis shows that here is no a clear consensus about the use of metallic materials as PCMs (MPCMs) for LHTES. The low thermal conductivity of the common PCMs, leads to the need for bigger heat exchange surfaces or embedding metal or graphite/carbon structures into the PCMs to enhance their performance. Such the approach negatively affects the feasibility of  LHTES [1, 2]. On the other hand, metallic  PCMs (MPCMs) have a significant  advantage in thermal conductivity and this parameter does not require enhancing [3-8]. However, there are a number of research works  [9-13] which highlight  the low heat of fusion per unit weight [4, 6-8, 12, 14], high degree of sub-cooling  and their relatively high cost, compared to that of more conventional PCMs. To date, metal alloys have not been endorsed as effective PCMs although they have also some additional advantages such as high density, low corrosiveness  and small volume change during melting and solidification [17]. 

For high temperature applications aluminium alloys were investigated in [15]  as potential PCMs (>300 °C) because of their high latent heat and good thermal stability.

Information on the lowest reported values of the heat of fusion for   high temperature MPCMs is summarised in Fig.  1. It can be seen that eutectic aluminium alloys have the  highest heat of fusion  in the  group considered. With regard to  the cost and availability of the materials, prices  for Mg, Zn, Si and Al are approximately between 2 and 3 $/kg and these materials  are widely used in various industrial applications [16].

For the mid- temperature range (between 48 and 124 °C), metals with the low melting temperature or alloys based on elements such as Bi, Pb, Sn, Cd, In, Ga, Zn, Sb etc., are available. Low-melting-point alloys are  extensively used in the fields of materials processing, electronics, electrical automatic control, continuous casting simulation, welding etc. [17, 18]. Bismuth is one of the major components of many fusible alloys, which strongly influence the value of melting point, and has  the unique characteristic, namely it  expands during solidification process [19]. Summary of information on melting temperatures and heat of fusion of some mid-temperature MPCMs, investigated in a number of studies, is presented in   Fig. 2.

Many of alloys, which could be deployed in the mid-temperature range, contain harmful elements, such as lead (Pb). Lead is considered as one of the top 17 chemicals, harmful for human life and environment. This is highlighted by the Environmental Protection Agency (EPA) and the European Union's RoHS (Restriction of Hazardous Substances in Electrical and Electronic Equipment). Therefore, lead containing materials should be treated with  special protection  measures put in place. 

Lead-free solders were studied in [20-31] with relevant databases have been created [32-34]. Three most frequently referenced lead-free alloys  to substitute the Pb-Sn eutectic composition solder are Sn–Ag–Cu (near eutectic), Sn–Zn (eutectic) and Sn–Bi (eutectic) composites.  At present,  the Sn– (3–4) wt% Ag–(0.5–0.9) wt% Cu alloy is preferred for application in soldering [21]. This is due to a number of factors, that affect the soldering process, namely mechanical properties, melting point, cost, availability and wetting property.  Furthermore, Pb-free solders are used  because of their  cost competitiveness.  in order to maintain acceptable costs of production in electronic industry. On elemental basis, Pb and Zn are the cheapest metals, whilst  In is more expensive than Ag.

At the low-temperature range (about 40 °C), liquid metals or low melting point alloys have been studied and used for diverse applications, mainly as thermal management in electronic devices. Room temperature liquid metals seem to meet most of the requirements for meeting thermal comfort requirements [6]. Fig.  3 presents summary of gathered information  on the melting temperatures and heat of fusion for such the low-temperature MPCMs.   

Selected ELMTAs and their Characterization

Eight commercially available ELMTAs were selected as potential candidates as PCMs for the seasonal thermal storage (see Fig. 4):  

Alloys listed above are widely used commercially in soldering processes and are described in  the standard specifications for Low Melting Point Alloys ASTM B774 [35] (<183 °C) and  in Soft solder alloys BS-EN-ISO 9453 (<450 °C) [36]. The impurity level in these commercial  alloys is <0.15 wt.%.

Differential Scanning Calorimetry analysis was used to determine the latent heat, phase change temperature and heat capacity of the selected ELMTAs as a function of the temperature.  DSC measurements were performed in argon gas environment using a Setaram EVO131 Differential Scanning Calorimeter.   The DSC calibration was performed following the Standard Test Method ASTM B1269 [37]. During the measurements the DSC apparatus was purged with argon, all the weigh measurements were carried out  with the accuracy of ± 0.01 mg. The calorimeter was calibrated with the melting point and enthalpy of fusion for high-purity Sn. The ELMATs specimens were placed in aluminium crucibles  (30 mm) and samples were thermally cycled 2-3 times with the heating and cooling rates of 2 K/min. The maximum heating temperatures were set about 50 K higher than the melting  temperatures.

The above data obtained for ELMTAs from literature review and experimental characterisation of thermal properties was saved as database.

Figures 5 and 6 show samples of results from  DSC measurements for one of the selected ELMATs, namely Alloy 158F. Bolton 158F and Roto 158F are the same alloy, supplied by two different companies. Heating rate used in measurements was 5 K/min.

Fig. 5 demonstrates the significant temperature difference in the melting and crystallisation processes (75 and 60 °C, respectively) which is caused by the slow rate of crystal structure formation in metallic alloys.

The variation of the specific heat capacity of the Alloy 158F as function of temperature, is shown in Fig.6 (the heating rate used was 20 K/min).   This diagram indicates that the phase change process takes place in the temperature range between 70 and 90 °C. 

Most of MPCMs may have this considerable temperature difference between melting and solidification temperatures, which should be taken into account during thermal storage system design and exploitation phases.

Overall metallic alloys provide much superior performance in terms of heat charging and discharging times. Figs. 7 and 8 below show comparison of this performance during discharging process (solidification) for Tin and Solar Salt based thermal storages with the same capacity. It can be clearly seen that during the same period of time significantly more heat  was delivered to the user when the storage was metallic alloy based.

Current work

Several alloys were selected for further investigations to determine the influence of higher levels of impurity (>1wt.%) on their thermophysical properties as MPCMs.

Presently, measurements of the thermal conductivity as a function of temperature for the selected MPCMS is ongoing. 

The short-term thermal stability experiments (100 cycles) using the DSC technique are being run.

Numerical and experimental investigations of heat transfer and flow in the laboratory prototype of LHTESS with ELMTAs for deriving heat transfer correlations.

Practical Applications

Results obtained will be used in the development of  a novel effective Solar Thermal Energy Storage systems for a small solar power plant Innova MicroSolar - (Project 723596 - H2020-EE-2016-2017/H2020-EE-2016-RIA-IA, 2016-2020). 

Additionally, outcomes will be used in R&D activities of the industrial partner in THERMOSTALL Project.

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Contacts

Emails:   khamid.mahkamov@northumbria.ac.uk  Phone:  + 44 191 243 7510

                carolina.costa@northumbria.ac.uk         Phone:  + 44 191 243 7649