Heat pump technology is a new type of energy-saving refrigeration and
heating technology, which has long been mainly used in the field of heating and
air conditioning of buildings. Due to the good performance of heat pump heating
in terms of energy saving, consumption reduction and environmental protection,
sanitary hot water supply systems are increasingly using heat pump equipment as
a heat source. Among them, air source heat pumps that use outdoor air as the
heat source have a simple structure, do not require a dedicated machine room,
are easy to install and use, and have irreplaceable advantages in sanitary hot
water supply. In addition to relatively large air source heat pump hot water
systems, there are now Several brands of small household air source heat pump
water heaters have also been put on the market. However, a major disadvantage of
air source heat pumps is that the heating capacity and heating performance
coefficient decrease as the outdoor temperature decreases, so its use is limited
by the ambient temperature and is generally suitable for areas with a minimum
temperature above -10°C.
Many studies have been conducted at home and abroad on combining heat pump
technology with solar energy to supply domestic hot water. There are two main
ways. One is to directly use air source heat pumps as auxiliary heating
equipment for solar systems, and the other is to use solar thermal energy. A
solar heat pump system in which water is a low-temperature heat source or a
solar collector is used as an evaporator for a heat pump. The former uses direct
solar heating as the main method and is supplemented by air source heat pumps to
solve the problem of continuity of solar heating, but it still cannot get rid of
the influence of ambient temperature on the heating performance of the heat
pump. The latter uses solar energy as the heat source of the heat pump, which
greatly improves the efficiency of solar energy. Utilization efficiency, but
when solar resources are insufficient, other auxiliary heat sources still need
to be added, and the heating capacity of the heat pump is limited by the solar
heat collection, and the scale is generally relatively small. In large-scale
solar central hot water systems, air source heat pumps are undoubtedly an ideal
auxiliary heating equipment. In order to improve the performance of air source
heat pumps in heating operations in low-temperature environments and expand
their use areas, combined with domestic and foreign solar heat pumps Based on
advanced experience in research, we have developed a solar-heat pump central hot
water system suitable for working in low-temperature environments. The system
uses a new low-temperature solar-assisted air source heat pump unit and a solar
heat collection system combined. Solar energy and heat pumps serve as auxiliary
heat sources for each other to maximize the use of solar energy and solve the
problem of insufficient solar energy resources in rainy weather and low ambient
temperatures in winter. The hot water supply guarantee rate is to provide hot
water all year round and all day long.
1Solar-heat pump central hot water system composition
1.1 Basic components of solar-heat pump central hot water system
The main components of the solar-heat pump central hot water system are
solar collectors and solar auxiliary heating air source heat pump units. Other
auxiliary equipment are the same as conventional central hot water systems,
including solar circulation pumps, hot water heating ring pumps, and heat
exchangers. appliances, hot water tanks and controllers, etc.
1.2 Solar auxiliary heating air source heat pump unit 1.2.1 Working
principle of solar auxiliary heating air source heat pump unit
In order to make air source heat pumps operate efficiently, stably and
reliably in low-temperature environments, many scientific research units and
production enterprises at home and abroad have conducted research and
development and improvements. There are mainly three ways to summarize. One is
to rely on external auxiliary heat sources to improve the low-temperature
heating performance of the heat pump. For example, electric heating is used to
increase the temperature of the heat pump heating outlet water, burners are used
to assist in heating outdoor heat exchangers, and phase-change heat storage
materials are laid around the compressor to increase the temperature under
low-temperature conditions. The heating operation output, etc.; the second is to
improve the low-temperature heating performance of the heat pump by improving
the refrigerant circulation system, such as using a two-stage compression air
source heat pump, an air source heat pump with an intermediate air supply
circuit, etc.; the third is to use a frequency conversion system , allowing the
compressor to work at high speed under low temperature conditions to increase
the circulation of working fluid, and at the same time spray liquid into the
working chamber of the compressor to prevent it from overheating, so that the
heat pump unit can operate normally.
The solar-assisted heating air source heat pump unit is based on the first
method mentioned above, as shown in Figure 2. An auxiliary heat exchanger is
added to the evaporator of the unit. When the heat pump is operating for heating
in a low-temperature environment, solar hot water higher than the ambient
temperature flows through the auxiliary heat exchanger and exchanges heat with
the outdoor air entering the evaporator to increase its temperature, thereby
causing the refrigerant to operate at a relatively high temperature. Evaporation
in the environment absorbs heat, which increases the evaporation temperature and
improves the working condition of the compressor.
2. Solar auxiliary heating air source heat pump unit
1.2.2 Performance characteristics of solar-assisted heating air source heat
pump unit
Compared with ordinary air source heat pumps, solar-assisted heating air
source heat pump units have the following obvious characteristics when operating
under low temperature conditions:
(1) COP is significantly improved. Under the same ambient temperature,
solar-assisted heating increases the evaporation temperature of the refrigerant
system, and the heating performance coefficient of the unit is significantly
improved compared to ordinary air source heat pump units.
(2) Prevent frosting on the evaporator and reduce defrosting time. Due to
the heating effect of the auxiliary heat source, the temperature of the air
entering the evaporator is increased, reducing the possibility of frosting. This
prevents frosting on the surface of the evaporator, making it Maintaining high
heat exchange efficiency, at the same time, the number and time of defrosting of
the unit are also greatly reduced, which can save a lot of electric energy and
ensure the continuous and uninterrupted operation of the heat pump unit.
(3) Improve the working environment of the air-conditioning compressor and
extend the service life of the unit. When the ambient temperature is low, the
compression ratio of the air-conditioning compressor increases sharply, and the
exhaust temperature of the compressor often exceeds the allowable working range
of the compressor, resulting in compression Frequent starts and stops of the
compressor will prevent it from working properly. In the long run, it will
damage the overall performance of the compressor and reduce the service life of
the air conditioning equipment. Using solar energy as an auxiliary heat source
to increase the evaporation temperature of the system indirectly improves the
working environment of the compressor. It not only solves the problem of the
compressor not working properly in low-temperature environments, but also
effectively extends the service life of the entire heat pump unit.
1.2.3 Design of solar auxiliary heat exchanger
The auxiliary heat exchanger is located outside the heat pump evaporator.
As a component of the heat pump unit, it is designed and produced simultaneously
with the heat pump unit. It uses a fin tube heat exchanger with the same
appearance size and material as the evaporator. The heat exchange area of the
auxiliary heat exchanger, the air temperature rise and the distance between it
and the heat pump evaporator should be determined based on the auxiliary heat
provided by the solar collector, solar water temperature, ambient temperature,
evaporation temperature of the heat pump unit, exhaust volume and other
parameters. Design Calculation.
1.3 Solar collector
Currently, the solar collectors commonly used in solar water heating
projects mainly include flat-plate solar collectors, all-glass evacuated tube
collectors, U-tube evacuated tube collectors, heat pipe evacuated tube
collectors and DC evacuated tube collectors. Five types of heaters. For
relatively large solar central hot water systems used throughout the year, the
solar collectors are required to have a certain pressure bearing capacity,
relatively high heat collection efficiency, relatively small pipeline
resistance, strong frost resistance, and easy maintenance. Among these types of
solar collectors, although all-glass evacuated tube collectors have high heat
collection efficiency and a large market share, they are not suitable for use in
large-area solar hot water systems because they cannot operate under pressure
and are prone to freezing and cracking. Most of them are used as heat collecting
components of household solar water heaters. The other four types of solar
collectors are all metal heat absorbers, which can operate under pressure and
are suitable for use in large-scale solar hot water projects.
Flat plate collectors are an early type of solar heat collecting device.
They have always been the leading product in the world's solar energy market and
are widely used in various low-temperature hot water heating fields. However,
with the emergence of vacuum tube solar collectors, they have become more and
more popular. Due to the limitations of its own structure, it no longer has an
advantage in heat collection efficiency. Due to anti-freeze issues and heat
collection performance, which is greatly affected by seasons and environment, it
is currently mainly used in areas with higher winter temperatures in the south.
It has poor operating results in cold areas in the north. , not recommended for
use in large hot water projects.
U-shaped vacuum tube collectors, heat pipe vacuum tube collectors and DC
vacuum tube collectors are products developed on the basis of all-glass vacuum
tube collectors. The common features of the three are relatively high heat
collection efficiency. As a heat absorber, metal can operate under pressure.
However, based on a comprehensive evaluation of heat collection efficiency,
leakage prevention, scale prevention, durability, safety, reliability,
installation and maintenance difficulty, etc., heat pipe vacuum tube collectors
are the most suitable for use in The types of solar collectors used in central
hot water supply systems are U-tube vacuum tube collectors and DC vacuum tube
collectors. The heat pipe vacuum tube collector uses heat pipes for heat
transfer, dry connection, and no water leaks in the pipes. It has small heat
capacity, fast heat transfer, freezing resistance, thermal shock resistance,
strong pressure bearing, good thermal insulation, no leakage, and easy
maintenance. Advantages: U-tube vacuum tube collectors and DC collectors use
concentric sleeves in the vacuum tube to directly heat the working medium. In
addition to having the characteristics of high operating temperature, strong
pressure bearing capacity and good thermal shock resistance, their integrated
The thermal efficiency is higher than other forms of collectors, and it can be
installed horizontally, simplifying the installation bracket, reducing the
installation site area, and preventing the collector from affecting the
appearance of the building. It has strong adaptability in combining solar energy
with buildings, but its installation procedure is relatively Heat pipe vacuum
tube collectors are complex, have many interfaces, are prone to leakage during
operation, and have relatively high system maintenance costs.
2Solar-heat pump central hot water system working principle
The combination of solar energy and solar-assisted heating air source heat
pump is used as the heat source of the central hot water system. The purpose is
to complement each other so that the two can complement each other and serve as
backup. When the sunshine is sufficient, solar energy will be used first to heat
hot water and the solar collector will be used to generate The low-temperature
hot water is used as the auxiliary heat source of the solar-assisted heating air
source heat pump, thereby improving the operating conditions of the heat pump
and improving its heating performance. This combination allows both to work
under relatively stable and efficient conditions, ensuring that the system can
supply sanitary hot water all year round. The heating process of an air source
heat pump is essentially the utilization of solar thermal energy contained in
the air. According to the working characteristics of the heat pump, during the
operation of the entire hot water system, only one of the heat supplied by the
heat pump unit operating as an auxiliary heat source A small part comes from
electric energy, so the solar-heat pump central hot water system greatly
improves the utilization rate of solar energy and reduces the consumption of
primary energy.
The operation of the solar-heat pump central hot water system mainly has
the following four working conditions:
(1) Solar heating of domestic hot water will work under this working
condition on most sunny days with good sunshine. At this time, the work of the
solar circulation pump is controlled by the system controller according to the
temperature of the solar collector and hot water tank, and the system will
continue to operate continuously. The heat collected by the collector is
transported to the hot water tank through the intermediate heat exchanger.
(2) Solar-assisted heat pump unit heats domestic hot water. On cloudy or
cloudy days, when the solar heat collection temperature is lower than the hot
water tank water temperature and is insufficient to directly heat domestic hot
water, the heat pump unit starts and uses air as a heat source to heat the hot
water tank. Domestic hot water inside. In autumn and winter, when the ambient
temperature is lower than the economic operating temperature of the heat pump,
the heating efficiency of the heat pump unit decreases and the evaporator
surface is frosted. At this time, the heat pump auxiliary heating cycle starts,
and low-temperature solar hot water higher than the ambient temperature enters
the heat pump unit. In the auxiliary heat exchanger, the passing air is
preheated to improve the efficiency of the heat pump and prevent frosting on the
evaporator, which can save the power consumption of the heat pump unit.
(3) Solar energy and heat pump units heat domestic hot water at the same
time. On a sunny day with good sunshine, if the heat consumption of the hot
water system is greater than the effective heat supply of the solar collector
system or the number of solar collectors is small, the hot water system cannot
be satisfied. If the heat demand is high, the solar energy and heat pump units
will work simultaneously to provide heat to the hot water system.
(4) The heat pump unit directly heats domestic hot water. In continuous
rain and snow weather, the heat required by the hot water system is completely
provided by the air source heat pump unit. At this time, the solar system is in
standby mode and the heat pump unit works alone to heat the hot water tank.
3Solar energy-heat pump central hot water system design
3.1 Determination of the power of solar-assisted heating air source heat
pump unit
In the solar-heat pump central hot water system, the solar auxiliary
heating air source heat pump unit serves as the auxiliary heat source equipment
of the solar heat collection system in fine weather. When solar resources are
insufficient or in rainy weather, it serves as the main heat source of the
system to ensure the normal supply of hot water. Therefore, The heating power
should be determined according to the design heat load of the entire hot water
system. For a full-time central hot water system, the power of the heat pump
unit is determined based on the design hourly heat load of the hot water system.
For a part-time central hot water system, the power of the heat pump unit should
be determined based on parameters such as maximum water consumption, hot water
tank volume, and heating time. OK, please refer to the literature for details.
The rated heating power of the heat pump unit is not less than the design load
of the central hot water system. In areas with relatively cold winters, the
model of the unit can be appropriately increased so that it can operate during
periods of relatively high temperatures throughout the day and in shorter
periods of time. meet the system's heat demand within a certain time.
3.2 Determination of solar collector area
In the solar-heat pump central hot water system, the area of the solar
collector should be based on the design heat load of the hot water system or the
solar heat supply determined according to the actual situation, and the solar
collector area per unit area of the project location should be analyzed and
calculated. The average daily effective heat gain determines the installation
area of the solar collector.
In hot water projects, solar collectors are generally installed at fixed
angles. The effective daily heat gain per unit area changes with the seasons and
the daily solar radiation intensity, and is not a fixed value. The influencing
factors mainly include the installation angle of the collector, system operating
conditions, local meteorological parameters and solar irradiation. Different
collector types have different heat collection efficiencies and their effective
heat gain is also different. Therefore, in practical applications, analysis and
calculation are generally based on the performance parameters such as collector
heat collection efficiency and solar irradiation data provided by the collector
manufacturer. Take the annual average.
3.3 Solar thermal collection system form
For solar-heat pump central hot water systems, the solar heat collection
system serves as both the main heat source for hot water heating and the
auxiliary heat source for the heat pump unit. It should be able to withstand
lower ambient temperatures, so a closed system should be used to circulate the
working fluid in the system. Use antifreeze solution.
4. Practical significance of solar energy-heat pump central hot water
system
4.1 Operation reliability analysis
As the main components of the solar-heat pump central hot water system,
solar energy and air source heat pumps are both energy-saving and
environmentally friendly products with mature technology. The large-scale
utilization of solar energy in domestic hot water systems has a history of more
than 20 years, and the large-scale application of air source heat pumps has a
history of decades [10]. The solar heat pump central hot water system
organically combines solar energy with air source heat pump technology. Without
affecting the original operating functions of the two, the operating efficiency
is significantly improved, thereby ensuring stable and reliable operation of the
system and saving conventional energy consumption of the hot water system.
4.2 Energy saving benefit analysis
According to the solar radiation data in most areas of northern my country,
the collector area of the solar hot water system is determined based on the
average heat consumption of the sanitary hot water system and the average daily
heat gain of the solar collector. In the solar-heat pump central hot water
system, solar energy Direct heating can meet 60-80% of the heat demand of the
hot water system throughout the year, and the remaining 20-40% of the heat is
supplied by the solar-assisted heating air source heat pump unit. The average
COP of the heat pump can reach 3.0, that is, more than 65% of the heat supplied
comes from The solar energy and air heat energy that the collector cannot
directly utilize. During the entire system operation, the solar energy absorbed
by the collectorThe energy utilization rate is close to 100%, and the power
consumption of auxiliary heating only accounts for 7-14% of the total energy
consumption of the system. Compared with conventional energy hot water systems,
it can save at least 85% more energy. From the above analysis, it can be seen
that the solar-heat pump central hot water system is a reliable, environmentally
friendly and energy-saving hot water system. This system only uses solar energy
and a small amount of electric energy, and does not cause any pollution to the
environment. It can be promoted and applied in most areas of my country where
the daytime minimum temperature is above -15°C and the solar radiation is good.
System and hot water supply, saving more than 60% of conventional energy during
heating operation in winter. In most areas of my country with cold winters and
hot summers, this system can also be combined with building heating and air
conditioning systems to maximize energy saving.
5. Project application
In 2004, the solar-heat pump central hot water system was the main research
content of the Chinese Academy of Sciences Olympic Science and Technology
Project's Olympic Village and Olympic Venue Solar Heat Pump Central Hot Water
System Demonstration Research Project. It was mainly used as a demonstration
project at the Beijing Yuetan Sports Center Comprehensive Training Hall. The
function is to supply centralized domestic hot water and provide abundant heat
for auxiliary heating of the swimming pool. The project uses an 800m2 DC vacuum
tube solar collector installed horizontally on the roof, and a solar-assisted
heating air source heat pump unit with an output power of 300kW. After debugging
in December 2004, the system entered the trial operation stage. According to
preliminary test data, the solar thermal collection system of the system can
meet about 90% of the system's designed heat supply on sunny days in winter. The
heat pump unit operates smoothly on rainy days, using low-temperature solar
energy of about 10°C. Hot water heat pump auxiliary heat source, heating
performance is significantly improved. According to the meteorological
conditions and solar radiation data in the Beijing area, the system's annual
comprehensive energy saving rate can achieve the expected goal of more than
90%.
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