Low-temperature Thermal Storage Research Articles

Ancillary services for the electrical grid by waste heat

This paper proposes a method to provide ancillary services to the electrical grid with waste heat recovery. The thermal flexibility needed to provide variable electrical output is provided by connecting the thermal source to a thermal grid and a thermal storage unit. Large capacity low heat grade thermal storages are cheaper and more readily available than large capacity electrical storage solutions, and hence thermal storage is preferable.In this paper, the dimensions of a low temperature thermal storage buffer to balance the variability of a wind- or solar plant is determined. It is calculated that a spherical storage tank of 25 m diameter and 90–130 °C would be sufficient to balance the unpredictability of a 100 MW plant of either solar- or wind energy.

Performance Assessment of Low-Temperature Thermal Storage with Electromagnetic Control

This study presents electromagnetic-controlled thermal storage (ECTS) that can be directly implemented in strategies of low-temperature waste heat recovery for energy-consuming equipment. A magnetic nanofluid (MNF) prepared from fine iron ferrite ferromagnetic particles is recommended as a latent heat medium (LHM). During electromagnetic induction, local flow fluctuations are generated and thermal convection in the MNF can be enhanced. The achieved results demonstrated that ECTS has a wide operational range and an optimum storage efficiency of 84.46%. Thus, a self-perturbation mode used to enhance thermal energy transportation can be designed for numerous waste heat management applications.

Preparation and characterization of lauric–myristic–palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage

Based on the theoretically calculated mass ratio and the melting temperature of ternary eutectic mixtures, the ternary eutectic mixture of lauric acid (LA), myristic acid (MA) and palmitic acid (PA) was prepared firstly. The differential scanning calorimetry (DSC) results indicated that the eutectic mixture was composed by LA, MA and PA in the ratio of 55.24:29.74:15.02 (by mass). Then the LA–MA–PA was absorbed in expanded graphite (EG), which acts as a supporting material, to prepare six kinds of composite phase change material (PCM). The optimal mass ratio of LA–MA–PA to EG is found. The SEM, FT-IR and DSC comprehensive characterization results show that EG interacted with the capillary tubes and surface tension of LA–MA–PA via its porous structure, instead of any chemical action. At the maximum mass ratio of LA–MA–PA to EG of 18:1 (by mass), the phase change temperature and latent heat of LA–MA–PA ternary eutectic mixture were 31.41°C and 145.8J/g respectively; and the LA–MA–PA/EG composite PCM had a phase change temperature of 30.94°C and a latent heat of 135.9J/g, and the thermal conductivity of the LA–MA–PA/EG was measured as 1.67W/mK, a sharp increase than that of LA–MA–PA. Thermal cycling test showed that the LA–MA–PA/EG composite had a good thermal reliability. Based on all results, it was concluded the prepared LA–MA–PA/EG composite PCM has a proper melting temperature and latent heat for low temperature thermal storage.

Utilization of phase change materials in solar domestic hot water systems

Thermal energy storage systems which keep warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. This effect is known as thermal stratification, and has been studied experimentally thoughtfully. This system stores sensible heat in water for short term applications. Adding PCM (phase change material) modules at the top of the water tank would give the system a higher storage density and compensate heat loss in the top layer because of the latent heat of PCM. Tests were performed under real operating conditions in a complete solar heating system that was constructed at the University of Lleida, Spain. In this work, new PCM-graphite compounds with optimized thermal properties were used, such as 80:20 weight percent ratio mixtures of paraffin and stearic acid (PS), paraffin and palmitic acid (PP), and stearic acid and myristic acid (SM). The solar domestic hot water (SDHW) tank used in the experiments had a 150 L water capacity. Three modules with a cylindrical geometry with an outer diameter of 0.176 m and a height of 0.315 m were used. In the cooling experiments, the average tank water temperature dropped below the PCM melting temperature range in about 6–12 h. During reheating experiments, the PCM could increase the temperature of 14–36 L of water at the upper part of the SDHW tank by 3–4 °C. This effect took place in 10–15 min. It can be concluded that PS gave the best results for thermal performance enhancement of the SDHW tank (74% efficiency).

Ammonia-Water Low-Temperature Thermal Storage System

An analysis of a low-temperature thermal storage system using an ammonia-water solution both as a refrigerant and as a low-temperature thermal storage material is considered. The thermal storage is useable at a temperature of −27°C and higher. The proposed system is designed to shift electric demand from high to low-demand periods. The system utilizes a heat-operated absorption refrigeration system; however, the generator heat is supplied by a self-contained vapor compression heat pump. The heat pump is operated during the off-peak period to recover the low-temperature thermal storage by reprocessing the stored ammonia-water solution to a lower ammonia-water concentration. The ammonia vapor liberated from solution in the dephlegmator is used in the compressor to produce the generator heat. Three different configurations are considered, including a solar-assisted system. The results are compared to an eutectic salt storage system.

Effect of wall conductivity on thermal stratification

The growing desire to find alternate energy sources to meet the ever-increasing energy demands of mankind has led to extensive research in the field of solar energy. Energy devices working on solar energy need energy storage subsystems because of the intermittent nature of solar radiation. Thermal energy storage systems which keep the warm and cold water separated by means of gravitational stratification have been found to be attractive in low and medium temperature thermal storage applications due to their simplicity and low cost. Experimental investigations have been carried out on scaled model storage tanks of various materials and wall thickness to study their effect on the stratification. It has been found that degradation of thermoclines is slower for tanks whose walls have a higher thermal resistance.