Discussion on sea water source heat pump air conditioning system

At present, the energy issue has become the focus of the world's attention. China is a big energy consumer and building energy consumption accounts for nearly a quarter of the country's total energy consumption; therefore, energy conservation and the use of renewable energy have become the focus of research. With a long coastline in China, some coastal cities have begun planning to use seawater source heat pump air conditioning system to provide cold and heat sources to the building in order to save energy, reduce pollution and serve the construction of eco-city. This article will mainly discuss the specific application forms and characteristics of the seawater source heat pump air conditioning system, aiming to provide reference for the specific implementation plan and operation management of the relevant project.

Sea water source heat pump air conditioning system is the use of sea water as a heat source or heat sink, and through the heat pump unit, heat medium or cooling refrigerant, and ultimately provide heat or cooling system. The heat energy contained in seawater is a typical renewable energy source. Therefore, the seawater source heat pump air conditioning system is also a kind of utilization mode of renewable energy, which is a green heating air conditioning system with energy conservation and environmental protection.

The huge sea surface receives the solar radiation moment by moment, and is influenced by the oceanic circulation and the specific climatic conditions around the sea area. Therefore, the sea water temperature value may vary depending on the place. In order to maximize the heating / cooling efficiency of the seawater source heat pump air conditioning system and the economy of the whole system, it is necessary to study various seawater source heat pump air conditioners under different seawater resource conditions in order to meet the needs of hot users. System form and its operation mode.

The seawater source heat pump air conditioning system discussed in this paper refers to a system that can provide both cold and heat sources for multiple buildings simultaneously. Sea water source heat pump air conditioning system can be classified from many angles, here only to discuss by sea heat pump unit set up the different classification, can be divided into three categories:

3.1 centralized seawater heat pump air conditioning system

The so-called centralized seawater heat pump air-conditioning system, is all the seawater heat pump units set together in a unified heat pump room (heat pump room can be set at the beach or user area as needed), the preparation of cold / hot water through the cell outside the network To the user, as shown in Figure 1, it can also be called seawater heat pump central cooling system.

3.1.1 winter system operation mode analysis

(1) The centralized heat pump station directly heats the backwater of the network to the temperature required by the user (for example, the water supply and return temperature of 60/50 ℃), and then sends the hot water to the hot users in turn, and the user and the external network adopt the direct Connection method.

(2) The centralized heat pump station heats the network backwater to a temperature slightly higher than the water temperature actually required by the user (for example, the temperature of the water supply and return of 65/50 ℃), and the user and the network may use a direct connection with a hybrid pump Way (as shown in Figure 2). Compared with the former method, although it slightly lower the COP value of the heat pump unit due to a slight increase in the temperature of the network water supply, the circulating water pump of the network does not need to consider the end user resistance loss of the most unfavorable loop at this time, Meanwhile, due to the increase of the temperature difference between the supply and return of the network, it also reduces the circulating water volume of the network that delivers the same energy, thereby saving the energy consumption of the pump. In addition, And bring the energy loss. Another point worth noting is that this network operation, but also allows users to adjust their own water temperature within a certain range according to their own needs, with more flexibility than the previous approach.

Figure 1 Centralized seawater heat pump air conditioning system Figure 2 has a direct connection of the hybrid pump

3.1.2 Summer system operation mode analysis

(1) The centralized heat pump station directly prepares the chilled water (eg, 7/12 ℃ chilled water) required by each user in summer, and then delivers the chilled water to users in turn. The user and the external network adopt the direct connection without mixing device.

(2) The centralized heat pump station prepares chilled water (eg, 5/12 ℃ chilled water) which is slightly lower than the actual requirement of the user, and then delivers the chilled water to all users. Similarly, in this case, the user and the network may adopt a direct Connection method. This mode of operation has the same characteristics as in the previous winter, and is not repeated.

3.1.3 Advantage of centralized seawater heat pump air conditioning system

Compared with the commonly used methods of setting electric refrigeration units and boiler room hot water heating separately in each building, such systems mainly have the following advantages:

(1) It can take full advantage of the diversity of user load distribution (ie, not all users reach the peak load at the same time), reducing the total installed capacity of the equipment and reducing their initial investment.

(2) COP of large heat pump unit is higher than that of small unit (some specific values ​​can be referred to [1]), which improves the energy utilization efficiency.

(3) Although heat pump units usually still require electricity for cooling / heating, they still have an energy-saving advantage because they operate at higher COP values ​​for much of the year (see [2] for a similar calculation and analysis) ), Which is why seawater heat pump air-conditioning system is an important reason for attention.

(4) Cold and heat source rooms in all users and cooling towers in outdoor areas can be canceled, thus saving a lot of valuable building area, increasing the owners' income, and reducing the layout of the equipment to the structural specialty belt And reduce the cost of structural construction; at the same time, also eliminate the noise generated by these devices; In addition, the cancellation of the cooling tower is usually placed on the roof after the beautification of the appearance of the building, the roof greening program Easier to achieve.

(5) Due to the extensive use of seawater, limited freshwater resources can be saved. The lack of fresh water resources is a worldwide problem. Many coastal cities in our country are severely lacking in water cities and saving fresh water resources is of great significance.

(6) After centralized arrangement of heat pump units, it helps the centralized management and maintenance of professionals, which not only saves running costs but also improves the reliability of cooling / heat supply.

(7) Environmental protection effect is obvious. The aforementioned energy saving and noise reduction all play a direct role in improving the environmental performance of the system.

3.2 decentralized seawater heat pump air conditioning system

The form of such seawater heat pump air conditioning system is shown in Figure 3: All seawater heat pump units are distributed to all users, and the outdoor pipe network system only provides the necessary seawater for each user unit.

Compared with the centralized seawater heat pump air-conditioning system, the heat pump unit of the system is decentralized, the capacity is relatively small, the initial investment will be correspondingly increased, and the COP of the unit will be slightly lower than that of the large-scale unit which is centrally placed, and each user still has to have Cold and heat source room; but the system of each user's heat pump unit is relatively independent, increasing the user's flexibility, as each user according to their specific needs to adjust the level of the heat pump inlet and outlet water temperature, or as mentioned later To the water ring heat pump air conditioning system can be used as a secondary source of cold and heat.

Figure 3 decentralized seawater heat pump air conditioning system Figure 4 with cooling tower users and sea water outside the network connection

3.2.1 winter system operation mode analysis

For buildings with only heat load in winter, heat pump units are operated according to heating conditions. If there is both heat load and cold load in the same building, it may be considered to adopt water ring heat pump system in the building to save energy. In this case, Sea water heat pump unit only as an auxiliary cold and heat source equipment to regulate the imbalance within the building cold and heat load.

3.2.2 Analysis of summer system operation mode

(1) Seawater in the outdoor pipe network is supplied as circulating cooling water to each user's heat pump unit to prepare the chilled water needed by the user. Although the COP of the user unit is slightly lower than that of the centralized large unit, But will still be slightly higher than the COP value when the user is using the cooling tower.

(2) Each user still retains the cooling tower, and the seawater in the extranet provides the cooling circulating water of the cooling tower and its replenishment, as shown in FIG. 4.

Although users still use cooling towers, seawater cooling towers still save a lot of freshwater resources and the seawater flow in the outer net, which serves only the cooling tower, is far less than the amount of water required to cool the heat pump directly with seawater, Minimize the transport of sea water.

3.3 Two-stage coupled seawater heat pump air conditioning system

The system diagram shown in Figure 5. The salient feature of this system is that both the centralized heat pump station and the heat pump unit scattered to all users are also called united seawater heat pump systems. Clearly, this design will increase the initial investment of the system, but it has one of the biggest advantages: when the temperature of the winter sea water is too low (for example, the near-sea water temperature in cold northern areas approaches 0 ° C) User heating (see below) without the need for additional auxiliary heat sources (due to the current heat pump technology, the former two system forms have an auxiliary heat source in the event of such a situation) and a truly separate seawater heat pump for the construction Cold and heat source.

3.3.1 winter system operation mode analysis

The system may have four operating modes in winter:

(1) In the sea water temperature is very low (usually at this time the outdoor temperature is very low, the user heat load larger), centralized seawater heat pump station within a seawater heat pump unit to prepare 15 ~ 20 ℃ warm water through the external network to Each user, and then each user within the two heat pump units and then raise the water temperature to the user the actual desired temperature.

(2) When the seawater temperature is high, hot water can be prepared by the centralized heat pump station at once for each user to be directly used for heating, and the heat pump unit in the user does not work. At this time, the working mode is the same as the centralized seawater heat pump air conditioning system The same mode of operation.

(3) When the sea water temperature is high, only the heat pump units dispersed to each user work to heat the building while the units in the centralized heat pump station stop working. The operating mode at this time is the same as that of the decentralized seawater heat pump air conditioning system in Section 2.2.

(4) Of course, when the sea water temperature is high, one or two heat pump units can work at the same time, and the water temperature inside the external network will be slightly higher than that of 15-20 ℃.

As for the first and second heat pump unit outside the network loop within the best value of the water temperature, two heat pump is cascade work, or single-stage work, as well as the conversion between them, should be more detailed theoretical analysis And experimental verification to determine.

Figure 5 two-stage coupled seawater heat pump air conditioning system

3.3.2 Analysis of summer system operation mode

In addition to the summer generally only single-stage heat pump refrigeration, the rest of the operating mode with the winter time similar to the situation, not repeat them.

In this paper, the heat pump cooling / heating system using seawater as a heat source is divided into three types: centralized, decentralized and two-stage coupled, according to the different locations of heat pump units. The paper summarizes many of the concentrated seawater heat pump systems Advantages, and a brief description and analysis of possible operating modes and characteristics of various types of systems in winter and summer. It is important to point out that all three types of systems have direct access to seawater for direct cooling.

In addition, this article only discusses various types of seawater heat pump air conditioning system and its possible mode of operation, for any given specific actual project, but also from the system energy consumption characteristics, economy and system cooling / heat reliability In terms of more detailed calculations and analysis in order to get the final result, the author will further carry out research in this area.