Mitsubishi Electric has been supplying VRF systems with water-cooled heat exchangers to the European market for over 7 years. Two modifications of such systems are currently being produced. The WY series assumes simultaneous operation of indoor units in the same mode: cooling or heating, and the WR2 series allows simultaneous operation of indoor units in different modes. It is important to note that the refrigerant circuit in both types of Mitsubishi Electric systems is two-pipe in any section, although the well-known synonym for the second type of systems is «three-pipe systems».
In systems consisting of several compressor-condenser units of the WR2 series, two heat recovery circuits are formed. The first is the movement of heat between indoor units belonging to one refrigerant circuit. The indoor unit operating in cooling mode absorbs heat from the room air, and this heat is not dissipated into the atmosphere, as in conventional split systems, but goes to the indoor units to heat neighboring rooms. It turns out, for example, that a powerful server with high heat emissions partially heats ordinary office premises. The second recovery circuit is formed due to the coolant. If one compressor-condenser unit works primarily for cooling, then it increases the temperature of the coolant. The other, working primarily for heating, will cool the coolant. In such a situation, minimal energy costs will be required to maintain the coolant temperature in the permissible range.
The best system efficiency is achieved in autumn and spring, as well as in regions with large daily temperature fluctuations, when simultaneous cooling and heating of air in individual rooms is required within a single building. In these cases, the system performance factor can reach 7.5, i.e. 1 kW of electric power provides a total cooling and heating capacity of 7.5 kW.
Example of the City Multi WR2 system with a cooling tower and a boiler
Fig. 1. “Traditional” use of water-cooled systems
VRF systems with water-cooled heat exchangers are rapidly gaining popularity due to a number of attractive features.
Firstly, this is the highest energy efficiency, since systems based on them have two heat recovery circuits. Secondly, the possibility of using non-traditional renewable energy sources, in particular low-potential thermal energy of the soil and ground. This feature is in full compliance with the main provisions of the energy strategy of Russia for the period up to 2020. And finally, the use of heat sources located in the buildings themselves (for example, heat emissions from servers) to heat adjacent rooms.
The traditional use of VRF systems with water-cooled heat exchangers is facilities where it is impossible to use VRF systems with air cooling.
For example, air conditioning of high-rise buildings — the long length of the refrigerant line has a negative impact on the energy efficiency of the system. The solution is to use compressor-condenser units with water cooling, placing them closer to the indoor units and reducing the length of the freon lines. Instead, the coolant circuit is lengthened, which is associated with comparatively lower energy costs. As a result, the efficiency of the refrigeration circuit increases and energy consumption decreases.
A less common but relevant application is buildings where no external heat exchange units can be installed. For example, the floors and roof do not allow the installation of multi-ton dry cooling towers, and there is no other place for their placement. Or these can be buildings with an exploitable roof, which is “unprofitable” to give for technological needs, as well as buildings located on the sea coast (high corrosive activity of the air). Finally, objects — architectural monuments: in most cases, it is very difficult to equip them with air heat exchangers. Therefore, the market, saturated with devices with air heat exchangers, has long been ready for systems based on the use of alternative sources of low-potential thermal energy, in particular, the energy of the soil, underground and surface waters.
The surface layers of the globe are a huge accumulator of solar energy.
If the air temperature fluctuates between 0 and 30 °C throughout the year, the soil temperature at a depth of only 3-4 m remains virtually constant at 10.5–11.5 °C. This heat source is ideal for VRF systems and allows for a high energy efficiency factor to be achieved. There are two main options for implementing this solution: systems with an open coolant circuit – water from underground sources rises to the surface and is connected directly to the heat exchanger of the heat pump; systems with a closed coolant circuit include special heat exchangers located underground or under water.
Systems with an open coolant circuit Groundwater is part of the water cycle in nature. Under the action of gravity, water is in continuous motion and, trying to reach the lowest place in the relief, returns to rivers and seas. The aquifer consists of permeable rocks: sand, gravel, pebbles, etc. The temperature of the underground water corresponds to the temperature of the soil and at a sufficient depth is almost not related to fluctuations in the temperature of the atmospheric air. From this point of view, it is a convenient source of heat for the construction of highly efficient heat pumps based on VRF systems.
The location of the aquifer depends on the structure of the rocks and the terrain.
For example, in London, a chalk aquifer 180–245 m thick is located under an 80-meter clay layer. The moisture capacity of the aquifer can be determined using hydrogeological maps. However, to clarify the parameters of the layer and determine the possibility of its use, it will be necessary to conduct test drilling.
In the last decade, the decline of industrial activity in cities has led to an increase in groundwater levels and easier access to them. An interesting project of a VRF system with an open coolant loop was implemented by Mitsubishi Electric partners in the UK. The building of an industrial warehouse in London, built in the 19th century in the Clerkenwell area, was modernized and turned into a first-class modern hotel Zetter. The architects recommended using an air conditioning system that does not contain outdoor units, since, according to their plan, there should be luxurious apartments on the roof — a penthouse. «If we had installed conventional air cooling systems, we would have had to sacrifice one of the suites on the roof, which bring significant income to the hotel,» commented Todd Bilo, the hotel's director of operations. Fortunately, the search for an alternative heat source was successful — an aquifer with a water temperature of 13-14 ° C was discovered under the building at a depth of 130 m. Having raised the water to the surface, it was directed to the intermediate heat exchanger included in the coolant circuit of seven water-cooled compressor-condenser units of the Mitsubishi Electric PQRY-P250YMF-C WR2 series. The choice of a system with utilization is justified by the following reasons. On the one hand, guests of an expensive hotel should be independent in choosing the operating mode: cooling or heating. On the other hand, only such a solution allows achieving maximum energy savings and reducing operating costs. The coefficient of performance of the air conditioning system (COP) is from 3.48 to 6 depending on the operating mode. The average value of the coefficient is 4 — this was the best indicator for air conditioning systems at the time of the project implementation (4 years ago). If this facility is equipped with modern compressor-condenser units using R410A refrigerant, the COP coefficient will be in the range from 4.5 to 7.5 with an average value of 6.5.
Fig. 2. The Portcullis House parliamentary office building (London)
Fig. 3. Air conditioning system diagram
Another European example of using VRF systems with an open coolant circuit is the Steigenberger Kurhaus hotel in The Hague (Netherlands) on the North Sea coast. It is rightfully famous as one of the most fashionable hotels in Europe. The architectural appearance of the building and the sea air exclude the installation of any devices outside the building. The only way out is to use the low-potential thermal energy of groundwater. This required drilling an operational (extracting water from the aquifer) and a injection (returning water to the aquifer) well about 100 m deep. An intermediate heat exchanger connects the groundwater circuit, which has a temperature of 8 °C all year round, with Mitsubishi Electric PQRYP250YMF-C compressor-condensing units. A total of 20 units are installed, to which 250 indoor units are connected. The total cooling capacity of the equipment is 600 kW, and the maximum value of the energy efficiency coefficient is 7.0. In fact, there was another argument in favor of such an energy-efficient solution — this is the state policy of reducing taxes on energy-saving equipment, carried out in the Netherlands.
Fig. 4. Hotel Zetter (London)
Fig. 5. Steigenberger Kurhaus Hotel (The Hague)
Fig. 6. Air conditioning system diagram
Sufficient operational experience has already been accumulated for the above projects, and the operation of the systems does not raise any complaints.
Systems with a closed coolant loop
In some areas, direct use of groundwater is difficult. The water-bearing layer may not have sufficient moisture capacity, or the costs of organizing access to it are too high. In these cases, closed-loop coolant systems are used, which include special underground heat exchangers. They can be vertical or horizontal. The choice of one or another design is determined by the size of the area for installing the heat exchanger, the type of local soil, and the cost of excavation work.
Vertical heat exchangers are used when space for their placement is limited. The heat exchanger consists of a set of U-shaped polyethylene pipes. They are installed in 100-150 mm vertical wells, which are then filled with a mixture of quartz sand and bentonite. To eliminate the mutual influence of such structures, they are located at a distance of more than 5 m from each other. The depth of the vertical heat exchanger can vary within 15-150 m. When constructing new buildings, vertical heat exchangers can be embedded in underground building structures. For example, in a single technological process for the manufacture of bored piles, which is now widely used. For this, holes of the required diameter are drilled in the ground directly on the construction site. The heat exchanger pipes are attached to the reinforcement frame of the pile, which is then lowered into the holes and filled with concrete.
Fig. 7. Slinky heat exchanger
A vertical ground heat exchanger is ready with almost no additional costs!
Horizontal modifications of ground heat exchangers occupy a significantly larger area. This could be a lawn in front of a country cottage or a car park in front of a shopping centre. Horizontal heat exchangers are located relatively shallowly, which is due to the cost of excavation work, and the soil temperature in this location is not optimal in terms of heat pump efficiency. There are several types of horizontal heat exchangers: single-pipe, multi-pipe and spiral. It is recommended to place pipes at a distance of more than 30 cm from each other, and dig trenches with a step of more than 2 m. Spiral heat exchangers of the slinky type allow to significantly reduce the area occupied by the structure. The spiral turns are in the same horizontal plane and are shifted relative to each other. As a result, the area of the spiral slinky heat exchanger is only 20-30% of the area of the single-pipe analogue, but the length of the pipe is doubled.
For buildings located near water bodies, such as ponds, lakes, etc., an underwater version of heat exchangers can be used. Water of the required temperature (10.5–11.5 °C) is located at a depth of about 6 m. A buoy and an anchor connected by a rope are used to position the heat exchanger at the required depth.
The capacity of ground heat exchangers ranges from 6 kW to 10,000 kW, and the service life is 50–75 years. Of course, the use of low-potential heat from the ground and underground waters complicates the design of systems and requires higher capital investments (see Table 1). However, from the point of view of operating costs, nothing better has been invented yet (see Tables 2 and 3), and only such systems can solve specific problems that are beyond the capabilities of traditional air-cooled systems.
Fig. 8. Scheme of a system with a closed underground coolant circuit
|
