Utilization of Renewable Energy Extracted from Reservoirs in Air Conditioning System：Case study

Utilization of Renewable Energy Extracted from Reservoirs in Air Conditioning System：Case study

Jiang Ai-hua1, Mei Chi1, Shi Zhang-ming,Y ang Shuang-huan1, and Zhu Han-zong2

1 School of Energy Science and Engineering of Central South University, Changsha, Hunan Province, China, 410083 .

2 Liyujiang generation Ltd. of Hunan Huarun, Chenzhou, Hunan Province, China, 423400.

Abstract: This paper presents the principle of water temperature distribution and variation with seasons of large reservoirs, and analyzes a typical example of a new kind of central air conditioning system, which used reservoir’s discharged water as natural energy resource, on design conception, characteristics of performance, and energy conservation effectiveness. The study results shows that most of the reservoirs used for hydro-generation are stratified, and the water temperature changes with the different depths and will not vary when the depth reach more than a definite value (50m), and there is a great deal of useable renewable temperature difference energy in the discharged water, and utilization of the natural energy in air conditioning systems is feasible and worthwhile because that using natural renewable energy source as much as possible will save a large amount of common energy and reduce extraction of pollution.

Key words: utilization, renewable energy,reservoir, central air conditioning, energy conservation.

1 introduction

E nergy cost caused by air-conditioning is going up quickly with the raise of live level of people. Normal air conditioning equipment still exist some problems such as low efficient, loud noise and pollutions. Water source heat pump air conditioning systems and Geo-Exchanges heating and cooling provide both a high level of comfort and low energy costs because it relies primarily on the earth’s natural thermal energy. If the water temperature of rivers is low enough to be used directly in the air condition systems, it will save more energy and be more environmental protective. Studies have shown that big reservoirs store tremendous useable natural temperature difference energy as well as hydrodynamic energy because the temperature of deepwater in most of large reservoirs is invariable throughout the year. In summer or winter, the temperature difference between atmosphere and the discharged water from big reservoirs can reach more than ten degree. This kind of temperature difference energy is renewable and it is very good cooling and heat resource to air-conditioning and heat pumps systems, and if the renewable natural energy can be used widely, a large amount of common energy will be saved and it will alleviate environment pollution.

2 The water temperature distribution and temperature difference energy of reservoirs

2.1 The water temperature distribution of reservoirs

Many studies have shown that the temperature of the ground or groundwater a few meters beneath the earth’s surface remains relatively constant throughout the year, even though the outdoor air temperature may fluctuate greatly with the change of seasons. At a depth of approximately 10m, for example, the temperature of soil in most regions of the world remains steady[1][2]. This is why

well water tastes so cool even on the hottest summer days. Influenced by the soil under the ground, the temperature of the deepwater of reservoirs remains stable throughout the year. According to water temperature distribution, the various kinds of reservoirs may be classified as composite, transient, and stratified. Research data shows that most of the large reservoirs used for hydro-generation are stratified, and the water temperature changes with the different depths, and the degree of stratification varies with reservoirs’ depth, inflow, outflow, thermal-advection, diffusion, etc. However, the deeper the water, the less the change range of the water temperature, and the water temperature remains at a relatively constant when the depth reachs more than a certain value [3][4][5][6].

The Dongjiang reservoir, which is a typical temperature stratified reservoir, is located in Zixing county of Hunan province of China, and its normal impounded area is 160km 2

, normal water –level altitude is 285m, the greatest height of the dam is 157m. Status of water temperature distribution of the reservoir in January and July are shown in figure 1 and figure 2 respectively. The figures show that the water temperature at the depth of 50m beneath the water surface is between 11℃ and 15℃[7].

2.2 Temperature difference energy in the water extracted from reservoirs

For the exploitation of water resources, it is generally that the natural conditions of the certain section of rivers or water body are used to determine that what kinds of projects will be adopted to concentrate the volume and pressure energy of water. At present, there are three main forms of exploitation such as dyke -types, diversion conduit types and composites, and the dyke-type can be subdivided power station in river channel and that at dam toe. In China, most of the hydro-power plants are those at dam toe, not only could they gain strong water power, but also are good for flood prevention, irrigation and developing aquatic products. To obtain high effective utilization rate of the hydro energy and reduce loss, the outlets of power stations at dam toe are usually located at relatively deep place of reservoirs, as it is shown in figure 3[8]

. Because temperature of outflow water extracted from the outlet at deep place of reservoirs is usually invariable in the whole year, and there is obvious temperature differences between the discharged water and atmospheric air which changes with seasons. In winter the water temperature is higher, while, in summer it is Figure 1 The water temperature distribution with depth in Dongjiang reservoir in January

Figure 2 The water temperature distribution

with depth in Dongjiang reservoir in July

opposite, and there is a large amount of usable renewable natural energy in the outflow water. For instance, the outfall of the Dongjiang reservoir is at about 50m under the normal water level, and the highest temperature of water at the outlet is 14.4℃(in October) and the lowest is 11.8℃(in June).

3 The practical central air conditioning system using renewable natural energy

3.1 Outline of the central air conditioning system using natural energy

Liyujiang power station is located near to lower reaches of Dongjiang River, in Hunan province of China. Because of the effect of the Dongjiang reservoir, the downstream water temperature of the Dongjiang River is relatively constant, 13--18℃in the whole year, and 13-- 15℃ in most time of summer. If the indoor designing temperature of the air conditioning is 26℃, practicable temperature difference for refrigeration can reach 11--13℃. A central air conditioning system, which cools the rooms with natural cold energy resource of the river water directly in the summer and supplies heat with hot water heated by vapor extracted from the turbine, was designed and constructed in 1999 in Liyujiang power plant. The servi ce range of the system includes all live districts and office buildings (that including 1236 flats, the office building, kindergarten and the apartment building of single workers). The design conditions are: the general cooling load is 4350 kW and heat load is 3970 kW; the indoor air dry-bulb temperature is 26℃, and wet-bulb temperature is 19.5℃; the outside calculating temperature is 36℃, and the water inlet temperature of coils is 14.5℃. The system has run steadily for over 7 years, and all of technical index are Figure 3 A schematic illustration of a hydropower station 1－the reservoir ；2－head conduit ；3－hydro-plant buildings ；4－hydroturbine 5－generator ；6－tailrace

Figure 4 The block diagram of the air conditioning system

3.2 General layout of equipments and pipelines

The general block diagram of equipments and pipelines of the system is shown in figure 4. In the summer, the system is open, and the exhaust water from refrigeration system drained directly into the Dongjiang River. In the winter, the hot water heating system is closed. The general central air conditioning system is run automatically and self-regulation.

3.3 Characteristic and selections of air terminal devices

Fan-Coil Units，the terminal devices of central or semi-central air-conditioning system, are mainly used to exchange cool or thermal energy with indoor air of rooms. The cooling or thermal capacity of FCUs is influenced by many factors such as the temperature and the flux of medium, the velocity of air passing FCU transversely and the properties of indoor-air, and so on. Especially, the temperature of inlet-water can not only make a strong impact on the capacity of the energy afforded by FCUs, but also affect the economic capability of the whole air-conditioning system directly. As the data given by many handbooks or the manufactories meet the conditions as follows: 26℃dry-bulb temperature and 19.5℃wet-bulb temperature for indoor air, and 7℃water inlet temperature of coils. So it was difficult for the system designers to select FCUs correctly in this instance that water inlet temperature of coils is 14.5℃. Experiment and calculation work were carried out at the same time, and some tested figures of refrigerating capacity for some model machines at the actual spots and calculated when the water inlet temperature of coils was 14.5℃. Both experiment and calculation proved that the refrigerating capacity of Fan-Coil Units meeting the actual condition using renewable natural energy was 50% of that when the water inlet temperature of coils is 7℃, and the problems about selections of air terminal devices were solved[5].

4 Performance and energy-saving effect of the system

4.1 Performance characteristics

The system has run for over 7 years since May 2000, and all performance figures reached expected targets. When the system was running, the room temperature can be maintained below 25℃in summer, and above 20℃in winter. This showed the success of the utilization of renewable natural cooling and heat resource in discharged water from reservoirs.

4.2 Energy saving effect

Firstly, because natural cooling resource was utilized and no main engine for refrigeration was needed, the initial cost of the system was much less than that of a conventional one. The gross investment of the project was only RMB7450000, which saved more than 50% of the cost of a conventional system of the same capacity. Secondly, compared with vapor compression refrigeration systems and absorption chillers, which usually consume a large amount of electricity or mechanical work or heat energy, the system only requires a relatively small amount of electricity to drive the pumps and fans, and nearly 40% of operating cost was saved each year. Further more, this system is of environmental protective as it is operated without any waste gas and refrigerant leakage.

5 Conclusion

Because the temperature of deepwater of high-capacity reservoirs is almost invariable in different seasons, there is a great deal of renewable natural cooling energy or heat energy in the discharged water in the summer and cold winter. The central air conditioning system project

constructed and operated successfully by Liyujiang power station saved much more initial, operating and maintenance cost, and this has shown that the development and utilization of natural energy of the discharged water from big reservoirs are feasible and worthwhile. At the same time, using renewable natural cooling and heat energy source as much as possible will save a large amount of common energy such as electricity, coal, oil or natural gas, etc., and reduce extraction of pollution, and be environmental protective and sustainable.

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Contact of writers

Jiang Aihua:

e-mail : jah65@https://www.360docs.net/doc/721059763.html,