In most developed and developing nations, nearly 40% of the energy generated is utilized in the building sector, in which nearly 50% of the energy is consumed by building cooling/heating systems. However, the energy requirement for building cooling/heating varies continuously with respect to time. Hence, in hot countries, if the cooling system is integrated with a storage system, the cooling system need not be designed for the peak load requirement. Further, this kind of storage system is very useful and economically beneficial in the scenario of dynamic electricity tariff, being introduced in many countries in the emerging renewable energy scenario to solve the grid stability issues. Further, it is very useful to promote microgrid with distributed renewable power generation. Considering the above, the major objective of the present research is to demonstrate the integration of the air-conditioning system with a sensible heat storage unit for residential applications. An experimental setup is constructed, and experiments were conducted to evaluate the heat exchange behavior during the charging and discharging process by varying the inlet temperature and the mass flow rate of the heat exchange fluid through the circuit. It is observed that the set temperature of the cool storage tank is to be maintained above +5°C to achieve better efficiency during the charging process. During the discharging process, the room could be maintained at the required comfort condition for a duration of 285 min with 29 cycles of operations between the set point temperature limits of 25°C to 28°C. When the inlet brine temperature of the cooling unit reached 20°C, in the next cycle, bringing down the room temperature again to 25°C could not be achieved. The results shown in this work are beneficial for efficiently operating the cooling system and useful in promoting renewable energy in the near future in the building sector. Also, the low-temperature sensible heat storage system is capable of maintaining the storage temperature at approximately +4°C, instead of -4°C normally employed in the case of latent heat-based storage system that allows higher performance in the sensible heat storage system.
The development of a nation mostly depends on the production of power, effective utilization of energy, and environmental condition. In most developed and developing nations, the energy consumption by the building sector is nearly 40% of the total energy produced [
Sebzali et al. [
Kim et al. [
Boonnasa and Namprakai [
Song et al. [
Rosiek and Garrido [
Alva et al. [
It is observed from the literature that the cool thermal storage systems are normally employed for the demand-side management in large building air-conditioning systems. There are no studies reported towards the supply side-management like the integration of solar energy with the chiller operation. Further, it is not seen from the literature about the feasibility of integrating cool thermal energy storage in residential air-conditioning applications. Hence, the objective of the present study is to introduce a small capacity low temperature sensible cool thermal energy storage system with a residential cooling unit which could be integrated with a solar power generation unit. Further, the main problem associated with cool water storage tank is the limitation in bringing down the temperature below 0°C due to freezing which demands increased size of the storage tank. Considering the above, the present work is also focused on mixing an appropriate percentage of monoethylene glycol with water to develop a sensible cool thermal energy storage system to bring down the storage temperature even below 0°C and to study the charging/discharging performance of a low-temperature sensible heat storage (LTSHS) system for room air-conditioning applications.
In the present work, an experimental unit is constructed to charge the cool energy produced by a chiller in the LTSHS tank and discharge the cool brine in the LTSHS tank through a cooling coil unit kept inside the room for space cooling. An experimental investigation is done to study the feasibility of using an LTSHS tank integrated with a chiller system for residential space cooling applications. The experimental setup details and the charging/discharging experiments are presented in this section.
The experimental setup consists of a VCR system, LTSHS tank, a room, and a cooling coil unit as the major components, and the line diagram for the experimental unit is shown in Figure
Schematic view of the experimental setup.
Photographic view of the experimental setup.
The VCR system has a buffer tank of capacity 0.009 m3 filled with brine to accommodate the evaporator coil, heating coil of 2000 W capacity, and a stirrer. A proportionate differential temperature controller (PDTC) was used to maintain a desired constant temperature in the buffer tank. This enabled the supply of a constant temperature HEF from the buffer tank to the LTSHS tank. Valves were provided to control the flow rate of HEF in the charging/discharging circuits. The brine flow from the chiller to the LTSHS tank and from the LTSHS tank to the cooling coil unit was measured using rotometers with the measuring range of 0-1000 LPH and 0-3 LPM, respectively. The cooling coil unit was kept in a room of size
The experiments were performed to study the charging and discharging performance of the developed system. During the charging experiment, brine was circulated from the buffer tank of the chiller unit to the LTSHS tank through a pump, and a flow rate of 400 LPH was maintained by adjusting the valves. The chiller unit was operated at a constant load. HEF temperature of -5°C was maintained in the buffer tank by the PDTC and for varying the heating coil output based on the temperature sensor located in the HEF bath of the buffer tank. The brine temperature was measured at an interval of 10 seconds during the entire experiment using a data logger and stored in the desktop PC connected to the experimental setup, and also, energy meter readings were noted once in every half an hour. The VCR system was operated continuously, and experiments were continued until the brine in the LTSHS tank approached a temperature of -5°C. Figure
Methodology adopted for the experimental investigation.
Discharging experiments were carried out with brine flow by operating the circulation pump between the LTSHS tank and the cooling coil unit. The brine’s uniform mass flow rate was maintained at 3 LPM using the valve arrangement and measured by the flow meter. Average room air temperature at three different locations, one in front of the cooling coil unit and the other two at suitable locations in the room, was considered for the analysis. The brine’s inlet and outlet temperature flowing through the cooling coil unit kept in the room was also measured continuously during the experiment. The pump’s operation controlled the brine flow from the LTSHS tank to the cooling coil unit for maintaining the room temperature within
Technical specifications of system components.
Components | Rated capacity/range/make |
---|---|
VCR system | 1 TR (3.5 kW) |
Evaporator tank capacity | 11 litres |
Refrigerant | R 134a |
HEF circulation pump (chiller unit to LTSHS tank) | 0.25 horsepower (hp) |
HEF circulation pump (LTSHS tank to cooling unit) | 100 Watts |
CTES tank capacity | 212 litres |
Flow meters: | VA-make |
(i) Charging circuit | 0–1000 LPH |
(ii) Discharging circuit | 0–180 LPH |
Thermocouple | T-type |
HEF | HDPE-ethylene |
An estimation of the uncertainties in the measured/derived data was made, and the values are shown in Table
Uncertainty in various measured/derived quantities.
Measured quantities | Accuracy/error (%) |
---|---|
Temperature | ±0.1°C |
Volume (100 ml) | ±0.015 ml |
Mass flow rate | ±2.7% |
Derived quantities | Error (%) |
Instantaneous heat transfer | ±1.85% |
The instantaneous heat transfer, cumulative energy stored, and heat removed from the room were evaluated using the measured temperature values, and the equations used for the estimation of these parameters are presented.
The instantaneous heat transfer (the rate at which the thermal energy is stored in the storage tank) during the charging process is estimated using
where
The cumulative heat transfer (
The heat removed
where
The results of the experiments performed during the charging/discharging of the storage tank and cooling of the room by discharging the cool thermal energy are presented and discussed in this section.
Figure
Temperature-time history of brine at inlet, outlet, and inside of the tank for a mass flow rate of 400 LPH.
Figure
Instantaneous heat transfer and cumulative energy stored in the storage tank for a mass flow rate of 400 LPH.
Figure
Time duration of HEF temperature reduction for every 5°C drop and corresponding COP of VCR.
The brine temperature at the inlet and outlet of the cooling coil units is an important parameter based on which the heat transfer to the room and thereby the cabin temperature will be maintained. These parameters are studied and reported in this section.
Figure
Temperature-time history of the room and the brine inlet/outlet of the cooling coil unit.
The heat transferred from the room by the brine during each cycle of operation is shown in Figure
Heat removed from the cabin by HEF during each cycle of operation.
The heat removal required decreased in the last few cycles of operations because of lesser
In the present work, experimentations were done to find the heat transfer behavior while charging/discharging process in a cool thermal storage tank of capacity 38,000 kJ. The major conclusions arrived from the charging/discharging experiments are summarized in this section.
During the charging process, when the HEF temperature decreases from 35°C to -5°C, the time taken to achieve a temperature drop of 5°C increases appreciably below +5°C The COP of the chiller system also reduces appreciably when the HEF temperature decreases below 5° C Hence, it is recommended to operate the storage system with a temperature not less than +5°C to achieve higher efficiency When the inlet brine temperature of the cooling coil unit reached 20°C, in the subsequent cycle, bringing back the room temperature to 25°C could not be achieved The heat removed from the room is in the range of 300 kJ to 600 kJ during all the cycles of operation
The cool energy released during the discharging process was only 60% of energy charged. This is due to the considerable heat loss incurred through various modes which should be avoided. This ensures the requirement of the distance between the storage tank and cooling coil unit to be very close and the storage tank/piping to be insulated very well to prevent heat loss during the storage period and during the passage of flow.
The major scope and the limitations are as follows.
In the near future, when the renewable energy share increases, the demand for cool thermal storage will increase and this kind of LTSHS system is used to smoothen the fluctuations in the solar operated VCR system and to keep the electricity grid smart as large share of electricity is being deployed in building air-conditioning applications. The economic benefits of this cool thermal storage system will be very high if the dynamic tariff is introduced, as in practise in some of the countries in order to keep the grid smart. However, when the capacity of the cooling unit increases, the requirement of the storage tank size also increases appreciably. Hence, in the thickly populated cities, due to space constraints, implementation becomes difficult.
Combined cool storage
Air conditioning
Heat exchanging fluid
Coefficient of performance
Cool thermal energy storage
Tonnage refrigeration
Low-temperature energy storage
Vapour compression refrigeration
Chilled water energy storage
Low-temperature sensible heat storage
Thermal energy storage
Litres per minute
Vapour absorption refrigeration system
Carbon dioxide
Potential differential temperature controller
Life cycle cost
Resistance temperature detector
Litres per hour.
All data used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interest regarding the publication of this article.