| |
Master's portal of DonNTU |
Contents
Scientific novelty and practical value
Review of existing methods and development
An expression "geothermal" literally means that it is the energy of the earth’s heat. The main source of this energy is the heat stream from red-hot depths, which is directed to the surface of the land(soil). This heat is enough to melt the rocks beneath the earth's crust, turning them into magma, which we can sometimes see on the surface. Most of the magma remaines below ground and heats a surrounding rock. Underground waters are also heated to 371° C. It occurs at the edges of tectonic plates of the continents, and also in the so-called "hot spots", where the heat is very close to the surface and can be obtained by means of geothermal mal wells.
Geothermal
energy is the energy, which is carried over from Earth’s
depths with help of different types
of heat transfer (conduction and convection). It is assumed that magma’s
heat is carried with help of conduction
through structural layers of the Earth.
Manifestation of the geothermal heat, which has practical importance,
are
stocks of a
hot water in underground
reservoirs and outcropping geysers. Nowadays the geothermal energy is
used for
heating and electricity generation (for making of electric power).
Devices,
that transfer heat energy from the body with lower temperature TH
(teplootdatchika) to the body with higher temperature TB (heat
exchanger), are
called heat
transformators. To
realize
the transformation of the heat it is necessary to spend an external
energy
(mechanical, electrical, etc.). Heat transformers are divided into cold
and heat
pumps.
With
increasing of an
equipment’s energy
intensity, which is installed in residential and social buildings
(electric
boilers, high-powered conditioners, a large amount of electrical
equipment), the
question of energy conservation is becoming increasingly relevant.
A technical condition of power facilities is characterized by critical
level of
depreciation of fixed assets (from 60 to 70%), increase of specific
charges of the
fuel on electric
power production,
and
by increasing of network losses during the transportation of energy
products (electric
power's expenses
during the
transportation in the networks in 1991 were 9%, nowadays –
14%) [1].
Raising of
the level of energy savings is possible by reducing of the energy
intensity. A
heating system requires a major technological transformation, with pervasive
using of combined manufacture of the
heat- and power ehergy, improving economic efficiency.
In Scandinavian
countries, where the
climate is harsher, than our one, an energy consumption of residential
buildings is 120 –
150 kWh / m per year, and energy-efficient – 60
– 80
kWh/m2
(homes and constructions in recent years in
The aim of my work is to study energy efficiency with help of traditional methods and compare them to energy savings by using heat pumps, and also to consider some options of using heat pumps.
Scientific novelty and practical value
Installing of the heat pump, for heating and warming up of the water, allows to significantly reduce electric power consumption. The cost of of the heating system installing, using a heat pump, twice as expensive as the cost of the pump. But during the operation, these costs are already paid off for 2 years and in the connection with this fact the power consumption drops to 60%. Also, it is not necessary to use the heating systems with help of gas or coal plants, and thus there is no spending of these resources.
– Analysis of energy conservation with help of traditional methods, and it’s effectiveness
– Analysis of possible options of using heat pumps
– Analysis of combined heating systems, heat pump and electric boilers.
– Comparison
of energy efficiency with help of a the heat
pump and traditional methods.
All – Ukrainian Scientific Conference "Electrical engineering, electronics and microprocessor technology.
Review of existing methods and developments
In
heat
pumps, as well as in
refrigerations settings,
there is so
–
called
a reverse cycle of heat transmission
from a source with a low temperature to a source with a higher
temperature. It
is necessary to spend some amount of mechanical energy.
The energy balance for both cycles is expressed by 2 equations:
,
KJ / kg
(1)
where Qba –
an energy, that is removed
from the
working body;
Qdc – a thermal
energy, that is
delivered to the working body;
Wcd
– work, which
is spent
on the transmission of heat from low temperature source to a more
high-temperature source. On a figure.
The fig. 1.1 shows a Carnot cycle and the chart of device of an ideal heat pump.
The Carnot cycle consists of isothermal process of DC supply heat Qdc at a low level of temperature Ts, which corresponds to general conditions of heat exchange with the environment, SV isentropic compression, during which the work Wcd is given to the working body, an isothermal process VA Qba removal of heat at a high level of temperature Tg, which corresponds to conditions of heat exchange with a heated space, and the isentropic expansion of AD, during which the working fluid returns energy Wad, and as a result external energy W is given to the compressor; this energy is equal to the energy difference between the Wcd and Wad.
Consider on example the heat pump’s energy balance (Fig. 5.3), where we assume that electric poweris brought to Тl and it is equal to 51 kW. The useful power of the heat pump, which is passed to the consumer from the condenser Qba = 100 kW, is the sum of thermal power of evaporator Qdc = 57 kW, that is got from the environment, and mechanical power of compressor on the compression of refrigerant R = 43 kW. Thus losses of a power in an electric motor of compressor are 5 kW and 3 kW in the drive of cooling ventilator [1].
Fig. 1.2 Example of a chart of the vapor-compressed heat pump’s power balance.
A heat pump’s effectiveness is adopted to determine with help of several different factors. Most often in practice of comparing various cycles and designs of heat pumps it is used the conversion factor (φ), which is defined as the ratio of useful thermal energy generated at the outlet of the heat pump Qba, to the energy expended by the compressor to compress refrigerant W:
φ = Qba / W = Th / (Th – Tl). (2)
Evidently this theoretical conversion factor is expressed by the ratio of shaded on the fig. 1.1 EFVA area of a rectangle to the area of a rectangle DCBA.
Theoretical conversion coefficientы (factors) of an ideal Carnot cycle for heat pump, which are calculated for the values of Te = 313K (40° C), 333 K (60° C) and 353K (80° C), are shown on the Fig. 1.3 [1].
Fig.1.3 The theoretical conversion coefficient of an ideal heat pump
Real conversion coefficients are substantially below,that theoretically possible ones, and it is connected with the irreversibility of processes of heat exchange in the devices and also with their mechanical imperfection.[3]
Heat pumps
can be classified on
these following signs:
• on the principle of action;
• on the source of low-grade heat;
• on the combination of low-grade heat (which is used) with environment, that is heated in heat pumps;
• on types of expended energy.
According
to the principle of action there
are three
types of
heat
pumps :
• the parokompressorny heat pump;
• the heat pump of absorption type;
• the Hybrid heat pump.
There are
following low-grade
heat sources :
• the outdoor air;
• the surface waters (river, sea, lake);
• the groundwaters;
• the soil;
• the sun energy;
• the low-grade heat of artificial origin (waste waters, heated waters or other liquids of technological processes etc.).
According
to the combination
of low-grade heat (which is used)
with environment,
that is heated in
heat pumps,
there
are following
variants:
• the air – the air;
• the air- the water;
• the soil-the water;
• the soil –the air;
• the air-the water;
• the water-the water.
According to the types of expended energy there are heat pumps, that use the electricity (most often), the fuel of one sort or another and secondary sources of energy.
Most often it is used the heat of the soil, which approximately at a depth of 2 meters below the surface has relatively permanent for a whole year the temperature of 8 ... 10 ° C, that is much higher than the outside air temperature during winter and it is below the ambient temperature in summer, it was done before by people [2]. When the heat pump is used in the winter the last uses the heat of a soil or water for heating buildings and in the summer the heat from the building is taken to the soil or water, so the soil operates as the source of a heat in the winter and as the heat receiver in the summer.[5]
From standpoint of thermophysics the soil is an inexhaustible source of the heat. At the "selection" of the Earth’s heat it is used its overhead layer, which is located at a depth of 100 meters from the surface. From point of heat exchange this layer of the soil is situated under the influence of radiant energy from the Sun, radiogenic heat from deep layers of the Earth, convective heat exchange with the air and heat transfer due to different mass transfer processes (rain, melting snow, groundwaters, etc. ).
The principle of heating with help of geothermal heat pump is based on heat collecting from the nature ,that surrounds the building, and transmission of collected heat into the heating system (or hot water) of the building.
To
collect the
heat non-freezing liquid
flows through the pipe, which is located
in the soil or in
a reservoir near the building, to the heat
pump. The heat pump, like refrigerator,
takes away the heat and thus cools the liquid to about 5° C [3].
Degrees, elected by
the heat pump, are given to the heating
system and / or to the heating of hot water and / or a swimming pool
. The
liquid flows through the pipe in the
ground or in water again, restores its temperature, and
comes
to the heat pump again.[6]
The heat pump can use thermal energy, which is accumulated in rocks, soil, water, for heating of buildings: for heating, warming up hot water, swimming pool, winter garden, heated towel rails, anti-icing system, etc. The transformation of low-temperature heat energy (which is accumulated in the nature )into heat for warming up occurs in three circuits. In a ground contour (1) the free heat is transferred from the environment to the non-freezing liquid, and it is served at the temperature of about 0° C to the heat pump [6]. In a circuit of freon (2) the heat pump increases the temperature of generated heat to 100 degrees. In a circuit of the heating side(3) the heat from Freon is transferred to the heating system and then it is distributed throughout the building.
1.The ground circuit(contour)
A - The non-freezing liquid in pipes - a brine - circulates from the heat pump to the heat source (rock / ground / lake / water). An accumulated energy of the heat source heats the brine for a few degrees, for example, from -3° C to 0° C.
B - The brine on pipes comes back to the heat pump’s evaporator. Here the brine gives the thermal energy, cooles by several degrees from 0° C to -3° C. Then the brine returnes to the heat source, and receives power again.
2. The circuit of freon
C - Freon circulates in a closed heat pump’s contour, and passes through the evaporator. Freon has a very low boiling point. In evaporator freon receives the heat energy from the brine, heates from -20° C to -2° C, starts to boil and turns to steam.
D - The steam enters the compressor with the electric drive. The compressor squeezes a steam, the pressure rises, and the steam temperature increases from -2° C to +100° C.
E - The steam from compressor enters the heat exchanger – condenser, where it gives off the heat to the heating system, then the steam is cooled from +100° C to +70° C, and the steam condenses into liquid freon.
F - Freon pressure is still high, and it passes through the expansion valve. Freon pressure drops, and it returns to its initial temperature of -20° C. Freon has done its complete cycle. It returns to the evaporator, and the process repeats.
3. The heating side
G -The thermal energy, which was given by freon in the condenser, is transferred to the water of heating system, or to warming up of hot water, pool, etc.
H - A heating system’s thermal carrier circulates through a closed circuit(contour). With the temperature +40° C it achieves the heat-pump, then heates itself in a condenser to +50° C, and transports the heat to heat water or for radiators / heaters. Having produced the heat to divices and having cooled down to +40° C, the thermal carrier returnes for the next portion of heat to the heat pump.
The geothermal heat pump on the principle of work is like a regular conditioner, but it has a high energy efficiency and the proper power. Unlike air conditioners, geothermal heat pump is adapted for use in all weather conditions and subzero temperatures. The main problem of air conditioners is reducing of productivity and air conditioners stop at subzero temperatures, when heating is most important. And this problem is solved in geothermal heat pumps.