Application Analysis of Surface Water Source Heat Pump System

1 project overview

A research and development base is located in Tongzhou District, Beijing, 580 acres of land can be built, the use of waste pit pond water production of about 150 acres. The construction area is 95,000 square meters. The main purposes are supporting facilities such as scientific research, development, warehousing, office buildings and experimental houses. The approximate distribution ratio is: research and development - construction area 100,000 square meters, experimental residential buildings -3,000 square meters, Public-07.4 million square meters, municipal -0.22 million square meters. According to the actual situation of the base and the opinions of relevant departments of the hospital, considering the requirements of environmental protection, it is proposed to use the surface water source heat pump system for heating and cooling.

2 the feasibility of using water source heat pump and water source heat pump system options

Surface water source heat pump system is one of the ground-source heat pump system is based on surface water as a source of cooling and heating heating and cooling systems. Due to its environmental protection and energy saving, it has been vigorously popularized and applied both at home and abroad.

Surface water is easily polluted, with high content of impurities such as sediment and algae. The water surface is in direct contact with the air. The water contains high oxygen content and strong corrosiveness. If the surface water is directly supplied to each heat pump unit for heat exchange, it easily leads to Heat pump unit life reduction, heat exchanger fouling and performance degradation, can cause serious pipeline obstruction, it should not be the direct supply of surface water to each heat pump unit heat exchanger. And if the surface water and the circulating water within the building is separated by a heat exchanger, the heat exchanger to small temperature difference heat transfer mode of operation, so that you can use cheap heat exchanger to protect the expensive water source heat pump unit, if the building The possibility of heat recovery exists between the two systems. This system can also automatically heat recovery between buildings. However, due to the heat transfer temperature difference, the energy of surface water can not be fully utilized. Local surface water flow or temperature can not meet the requirements, you can use some auxiliary equipment, such as winter boilers, cooling tower in summer as a peak-shaving equipment, can also help the system meet the requirements.

According to the meteorological and hydrological conditions in Beijing, the average surface temperature of Beijing in summer is generally 25-27 ℃. Considering the temperature difference of 5 ℃ of the heat exchanger, the average inlet temperature of the heat pump unit in summer will not be higher than 32 ℃. If the summer Heat pump cooling water side of the inlet and outlet temperature difference of 5 ℃, the heat pump unit outlet temperature of less than 37 ℃, according to the technical requirements of heat pump units, then the cooling water supply and return water temperature difference is to ensure the normal operation of summer heat pump refrigeration.

In winter, the surface water temperature in Beijing is generally about 4 ℃, considering the temperature difference of 2 ℃ in the heat exchanger, the inlet temperature of the heat pump unit in winter will not be higher than 2 ℃. 5 ℃ Consider the heat pump unit outlet temperature less than -3 ℃, you must add antifreeze in the circulating water. At this time, the COP value of the heat pump unit under heating condition is about 2 to 3 (the COP values ​​of the units produced by different manufacturers are slightly different), and the higher the COP value, the higher the heating capacity of the heat pump unit, When the local surface water temperature is too low (below 4 ℃) or the surface water is not enough, other auxiliary heat sources such as boiler water heaters should be considered to ensure that the heat pump units operate at higher heating capacity. The use of auxiliary heat source such as boiler heat pump unit of chilled water circulating water boiler heating with direct heating compared to other forms of conventional heating energy saving is no advantage, but the use of auxiliary heat source is only in the surface water temperature is too low, or insufficient surface water In extreme conditions, it is used, and in general the number of days in which this happens is small or even non-existent. Consideration of heating by water-source heat pump units throughout the heating season is still energy-efficient compared to the above conventional heating methods, which will be given below With further analysis.

In terms of the system end device, the water source heat pump system is divided into a centralized large water-water source heat pump unit + fan coil and decentralized water-air source heat pump units. Water - water source heat pump unit is the final output of hot and cold water, this type of unit is relatively large, generally the whole project to set up a water source heat pump room, the output of hot and cold water delivered by the pump to the single building, according to need Water and water source heat pump room can be set up in each single building, the circulating water after heat exchange with geothermal water is delivered to each single building, and the water-water source heat pump unit of each single building produces hot and cold then Through the pump to the air-conditioned area of ​​the building, through the fan coil and air handling unit will be hot and cold water energy delivered to each room. Water - air source heat pump unit is a direct evaporation unit, the type of unit is relatively small, generally based on the use of various functions and use of the region is divided into many small systems, circulating water in the central water pump room after the heat exchanger Pumps are delivered to units in individual buildings. The return air in the air-conditioned area is delivered after being warmed / cooled by the unit. No secondary transfer of energy is required. Therefore, no other heat and moisture treatment units, such as fan coils, are required. Both systems have their own characteristics. Water - water source heat pump system is a more centralized mode of air conditioning, has been produced in China, due to the unit is more concentrated so the water source heat pump unit initial investment is small, but the heat pump units need to set up a dedicated engine room; water - air source Heat pump system is relatively decentralized, the current mature products mainly for foreign brands, the initial investment unit slightly higher, but its indoor circulating water pipes do not need insulation, as the unit dispersed to the end, the required engine room area is also small. In terms of the overall cost of the system, the water-air source heat pump unit is more expensive, but the fan coils and air conditioners required for the water-water source heat pump system increase the cost of thermal insulation and increase the cost of the engine room, - Air water source heat pump unit in the form of more expensive, but not much difference between the two. From the operation point of view, as water - water source heat pump unit energy regulation can only be divided into a limited number of stages, and at the same time for cooling and heating must be used four control, so more suitable for rest and rest time is more uniform, the load is more consistent Occasions; water - air source water heat pump unit comes with thermostat, according to the requirements of the independent regulation and operation, but also in the case of two control system to achieve the same time the four heating and cooling functions, it is more suitable for rest Time-diversified and more demanding use of commercial and public buildings.

According to the characteristics of the water source heat pump system and the specific circumstances of this project, we think that the centralized large-scale water-water source heat pump unit + fan coil should be adopted for the following reasons:

1.1 This form of water source heat pump systems used equipment are mature products in the country, the price and after-sales service more easily be protected.

1.2 As the terminal water-air source heat pump unit with a compressor is more disadvantageous in terms of noise control, taking into account the relatively high environmental noise requirements of scientific research offices, the use of a fan coil with no fan and compressor as the terminal equipment For good.

1.3 Since Beijing is located in the north, winter air-conditioning calculation temperature as low as -12 ℃, due to water-air unit on the inlet temperature requirements (generally not less than 5 ℃, or low temperature protection will start), the winter fresh air must be treated using electricity Heating preheating, in the case of sufficient surface water can not effectively use free geothermal resources.

3 hot and cold load estimation and water flow feasibility

According to the specification, the calculation parameters of outdoor air-conditioners in Beijing are as follows:

The average annual temperature of 11.4 ℃

Winter outdoor air-conditioned dry bulb temperature -12 ℃

Outdoor winter air dry bulb temperature calculation -5 ℃

Winter outdoor air-conditioned calculation of relative humidity 45%

Summer outdoor air-conditioning calculation of dry-bulb temperature of 33.2 ℃

Outdoor summer air dry bulb temperature calculation 30 ℃

Summer outdoor air-conditioned wet bulb temperature 26.4 ℃

R & D base building interior design parameters initially set as:

Summer interior design temperature is 25-27 ℃, indoor relative humidity is less than 60%;

Winter interior design temperature of 18-20 ℃, indoor relative humidity is not required.

According to the preliminary calculation, the design cooling load index is about 100W / m2, the design thermal load index is about 50W / m2. According to the first phase of 100,000 square meters and the simultaneous use coefficient of 0.9, the total air conditioning system cooling load is 9000 kW, Heat load 4500 kW. Assuming that the COP value of a water-source heat pump unit under cooling conditions is about 4.3, and the COP value under heating conditions is about 2.6, the amount of heat required to be removed by the surface water under cooling conditions is 11,093 kW. Under heating conditions The amount of heat to be supplied is 2769 kW.

According to "Introduction to R & D Base", the water injection surface of waste pit ponds is about 150 mu with an average water depth of 3.5 m. The total water storage capacity of this pit body is 3.5 × 105 m3. Consider the base of the summer air conditioning running 12 hours a day, the daily heat dissipation to the water body Q = 11093 kW × 12h × 3600 = 479 × 106kJ. The water temperature rise is Q / CM = 479 × 106 / (4187 × 3.5 × 105) = 0.33 ℃. Considering the temperature difference of 5 ℃ of the heat exchanger, the water flow required to remove 11093 kW of heat is: G = 0.86 × 11093 × 1000 / (1000 × 5) = 1908 m3 / h, About 7 days. This shows that summer water temperature is very small, taking into account the water and air and the earth heat exchange, water temperature is smaller or even negligible.

The base winter air conditioning to run 24 hours a day to consider, the daily heat extracted from the body of water for the Q = 2769 kW × 24h × 3600 = 239 × 106kJ. Beijing is located in a cold area where surface water will freeze. Considering the relatively conservative ice thickness during ice age, the water depth will drop to 2.5 m on average. The total water storage capacity of this water body is 2.5 × 105 m3. The temperature drop Q / CM = 239 × 10 6 / (4187 × 2.5 × 10 5) = 0.23 ° C. Considering the temperature difference of 2 ℃ of the heat exchanger, the water flow required for taking away 2769 kW of heat is: G = 0.86 × 2769 × 1000 / (1000 × 2) = 1191 m3 / h, and the entire water body is circulated once for about 210 hours, About 9 days. It can be seen from this that the temperature drop of winter water body is smaller than that of summer and the required water volume is smaller than the effective water quantity that can be provided by reservoirs. Considering the heat exchange between water body and air and earth, the temperature drop of water body is smaller Can even be ignored.

Taking into account the winter may be sustained low temperature, water decreases, leading to water temperature is too low, the water is too small extreme cases, in order to ensure that the normal heating in this case, the need to consider the use of auxiliary heat source to enhance heat pump chilled water temperature, according to The actual situation of the base, the auxiliary heat source can be considered in the form of gas-fired boiler to the most extreme conditions, that is, surface water has been unable to provide heat, all the heat provided by the gas boiler, boiler feed water temperature difference of 25 ℃ to consider, The water flow required for removing 2769 kW of hot water produced by the boiler is: G = 0.86 × 2769 × 1000 / (1000 × 25) = 95 m3 / h.

In addition, the above water flow and other parameters are considered as a whole base as a whole, in the actual construction, for the convenience of regulation and management, the same or similar functions of the single building, respectively, to build power stations, so that pumps, boilers, The specifications of the equipment such as the heater will be reduced.

It can be seen from the above analysis that according to the known conditions, it is feasible to adopt surface water source heat pump system in this research and development base.

4 water source heat pump system on the environment

According to the principle of water source heat pump system, surface water source heat pump air-conditioning system on the use of surface water is limited to heat extraction and conversion, and in the course of the use of surface water strictly limited to any chemical treatment, the limited treatment is limited to It performs physical treatment such as filtration, precipitation and heating and cooling, and the composition of surface water does not change during the entire use. In strict compliance with the above principles under the premise of the operation of water source heat pump system will not bring the region's surface water pollution problems.

In the system design and construction is reasonable, and the surface water source heat pump system under the premise of normal use, the general will not have a negative impact on the environment.

5 Technical and Economic Analysis

5.1 summer conditions

Surface water source heat pump air conditioning system summer operation mode and principle and the most commonly used water-cooled chiller air conditioning system is very similar, from the equipment point of view, water source heat pump air conditioning cooling water system is no longer use the cooling tower to cool, but by Cooling ponds, so from the initial investment point of view, water-source heat pump air-conditioning system eliminates the need for cooling tower equipment investment.

From the operating cost point of view, the water-source heat pump air-conditioning cooling water system depends on the reservoir for cooling, the cooling effect of the cooling tower cooling is better, so the water source heat pump air conditioning COP value may be more than the commonly used cooling tower cooling water cold water Unit air conditioning system to be higher, but the operating cost savings is not obvious.

5.2 winter conditions

According to the requirements of the base, combined with the actual situation of surface water source heat pump air conditioning system, mainly for the system operating costs of heating for technical and economic comparison. According to the local energy conditions, the technical and economic comparison is made mainly on the heating functions of the boiler room + radiator system of the gas district and the groundwater source heat pump + fan coil system on the whole project of the first phase of 100,000 square meters.

Comparison of operating costs

According to the relevant data, the distribution of winter air-conditioning load in Beijing is assumed as follows: the average air-conditioning load accounts for 15% of the total running time at 90%; the average air-conditioning load accounts for 30% of the total running time at 60%; the average air-conditioning load At 30% of the running time accounted for 55% of the total running time. The heating time in Beijing is 129 days according to the standard. Beijing electricity price in accordance with 0.60 yuan / kWh, Beijing gas boiler room heating price in accordance with 35 yuan / m2 calculation.

In this calculation, surface water and system hot and cold water are assumed to be operated at constant flow. The regulation of operation of each fan coil in the building is consistent with that of air-conditioning load. The use of water-source heat pump units is calculated at 60% full load.

Calculated the following table:

It can be concluded from the table that the operation cost of surface water source heat pump system + fan coil system is about 22.1 yuan / m2 per heating season.

6 Conclusion and recommendations

6.1 Under the existing conditions, surface water source heat pump system can be used, and filtering and deaeration preservative work (physical treatment only).

6.2 In the system design and construction is reasonable, and the surface water source heat pump system under the premise of normal use, the general will not have a negative impact on the environment.

6.3 Taking into account the possibility of continuous low temperature in winter, the water volume decreases, resulting in the water temperature is too low, the water is too small extreme cases, in order to ensure the normal heating in this case, the need to consider the use of auxiliary heat source to enhance heat pump chilled water temperature, According to the actual situation of the base, the auxiliary heat source can be considered in the form of gas boiler.

6.4 According to preliminary estimates, the operating cost of surface water source heat pump system for heating system is 22.1 yuan / m2, which is lower than 35 yuan / m2 for gas boiler room heating system. The entire base annual saving operation costs about 1,882,000 yuan.

references

Xu Wei, etc. Ground Source Heat Pump Engineering Technical Guide Beijing: China Building Industry Press, 2001

Lu Yaqing Practical Handbook for Design of Heating and Air-conditioning. Beijing: China Building Industry Press, 1994