National University of Architecture and Construction of Armenia, Armenia
Contemporary buildings are large consumers of energy. About 70% of energy carriers are used for heating, cooling, ventilation, air conditioning and other purposes of buildings. As a result, environmental problems and global climate worsening factors arose. To fight the natural problems energy saving technologies, such as solar water heaters, different types of heat pumps and other kinds of renewable energy technologies found rather wide use. But, in spite of advantages the mentioned technologies are characterized by serious disadvantages. At present wide implementation have found closed and open loop geothermal heat pumps. Nevertheless, they have serious disadvantages that limit their use: first, they need underground heat exchangers in form of rather long pipelines laid in special trenches. This requires rather large free area around the buildings which creates problems especially in towns and cities. Second, the construction of heat pump systems is rather complicated and expensive. For overcoming mentioned disadvantages, increasing energy and cost effectiveness of geothermal heat pumps operating in heating and cooling systems of buildings in this article a new structure of local, cheap, energy efficient and sustainable geothermal heat pump is suggested and the method for its calculation and design is developed.
Keywords: local heat pump, Geothermal Water-
As low temperature heat source for a geothermal heat pump instead of the ground can be used appropriate quantity of water, stored in underground tank. The stored water should contain enough heat potential for covering the seasonal heating demand of a house. The ground plays a role of thermal insulation and at the same time of additional heat source. During heating season, the heat pump takes heat from the stored low temperature water and uses it for heating of the house. As a result, the water in the tank gradually is cooled and at the end of the heating season completely turns into ice. The stored ice serves as cooling resource for covering summer time cooling demand of the house. Therefore, in this regime, the heat pump does not operate and saves energy. The cooling of the house is fulfilled by the cold water of melted ice, collected in the tank during heating season. Therefore, in summertime the heat pump does not work and only cold water's circulation pump works and consumes energy. The developed system is investigated on the example of a family house, located in Yerevan, Armenia climatic zone, which is characterized by wintertime -
2. Description of Structure and Work of Developed Water Ice Storage Local Heat Pump
Figure 1 represents the structure of developed water-
In wintertime heating regime water -
Figure 1. Scheme of winter heating and summer cooling system of a house with developed local heat pump
Liquid refrigerant from condenser (8) passes through the expansion valve (10) of heat pump, where drops its pressure and temperature and by pipe (11) enters into the heat exchanger block (12), immersed into the water of underground tank (2). Actually, the heat exchanger block (12) plays the role of the evaporator of heat pump. In the evaporator (12) cooled liquid refrigerant absorbs heat from stored water. As a result, tank water gradually is cooled during whole the season and liquid refrigerant turns into gas and by ? knee tube (13) is exhausted again into the compressor (1). The knee tube provides vacuum, which causes pulsation and helps the mixture of lubricating oil and refrigerant to return from evaporator into compressor. The last technique helps when the compressor is located higher than the evaporator. The compressor (1) takes refrigerant's cold gas from the evaporator (12) and, after compression, pushes it again into the condenser (8). Here the heating water absorbs the condensation heat of the compressed hot refrigerant and is heated up to a temperature, required by the heating system of the house. In the evaporator, cooled liquid refrigerant absorbs heat from the stored water and turns it into gas, which finally is sucked into the compressor and the heat pump cycle replicates.
3. Method for Calculation and Design of Geothermal Water-
The equation (1) allows finding the quantity of water Gw, kg, which should be stored into the tank to provide enough heat potential for covering seasonal heating demand Qhd.seas., kWh/seas. For this purpose, the equation (1) is changed into the following formula:
Actually, should be taken into account that the heat potential of stored water be enough for providing the complete evaporation of liquid refrigerant in the evaporator during whole the heating season. From this point of view it is becoming necessary to establish correlation between condensation and evaporation heats of the heat pump. To find the function of mentioned correlation, the following thermal balance of heat pump is analyzed:
Obtained formula (6) helps to determine the required quantity of low temperature heat source, for producing the needed quantity of the high temperature heat source. In the considered system as high temperature heat source is the seasonal heating demand of the building, which is generated by the condenser of heat pump. Seasonal heating demand Qhd.seas depends on sizes of the house construction characteristics and climatic conditions of the area. The house should have double paned windows and be optimally insulated with δins=0.19m thick insulation material. The value of seasonal heating demand is calculated on the example of a single-
During heating season, the heat pump rejects heat from low temperature water, stored in the underground tank and uses it for building’s heating purposes. As a result, the tank’s water gradually is cooled and at the end of heating season completely turns into ice with tw2 = 5oC. Replacing in the formula (6) Qcond. by Qhd.seas will obtain the equation for determining quantity of water Gw , kg, to be stored in the underground tank:
where 1.07–coefficient indicates that the volume of ice in 7% is higher than volume of water; ρw = 1000 kg/m3 is the density of water.
Special research (Egnatosyan, 2009) shows that any kind of heat pump by its energy efficiency can compete with a heating boiler with COP= 90%, if the heat pump’s COP is at least=3.0. As the suggested water-
4. Determination of Transformation Real Coefficient of the Water Ice Storage Geothermal Heat Pump
For determination of the real volume of the water -
Figure 2. Thermodynamic cycle of investigated water-
i1a=394 kJ/kg; i1=403 kJ/kg, i2=467 kJ/kg, i2b=448 kJ/kg, i2a=428 kJ/kg, i3a=296 kJ/kg, i3=287 kJ/kg, l=64 kJ/kg, qc =171 kJ/kg
5. Operation of the System in Summer Cooling Regime and Method for Calculation
It was stated above that in summer cooling season there is no need in operation of the heat pump, as cooling source is ready in form of the ice, collected on the surface of "heat exchanger -
6. Yearly Energy Consumption by Water-
In winter heating period the consumers of electricity are the compressor of heat pump and water circulation pumps. In summer cooling period only water circulation pumps are in work and consume electricity. So, the energy total consumption during a year NH-
where: NHP.wint – heat pump’s energy consumption in winter season, kWh; NHP.sum – supplemental energy consumption by “air to air” type domestic heat pumps in summer season, kWh; -
As was mentioned above, the value of seasonal heating demand of considered building makes:
Qhd.seas. = 9450 kWh/seas., and real value of transformation rate of heat pump equals to . Therefore,
where: Vh.s.w – volume of the water, circulating by the “condenser -
During summer season, for house’s cooling, the melted ice water with volume Vw.tank =46.3m3 and with initial temperature tin= 0oC, circulates through circuit “tank-
where: tw.fin and tw.in =0oC– final and initial temperatures of water in the tank during cooling season;
Qc.d.seas.= 3490kWh/seas – seasonal cooling demand of considered example of house (Melikyan, Abd Elhaleem, 2010); Gw.tank= 46300kg – designed quantity of water, stored in the tank.
By given above data and equation (18) the following final temperature of water was found:
The calculation shows that the seasonal cooling demand of the house is enough to heat the cooling icy water in fan-
It is clear that the cooling potential of the water is not enough for complete covering of the summer time cooling demand, which makes Q c.d.=3490 kWh/seas. Therefore, available cooling capacity of tank water makes only 38.7% of seasonal cooling demand. The deficit of the cooling potential makes: ΔQcold =3490-
The simplest and cheapest way to solve the problem of the deficit is the installation of two “air to air” split type domestic air conditioners with 1.1 kW electric power each (Carrier, 2016), and 2.2 kW of total cooling capacity. As these air conditioners perform during 919 hours of 1500 hours of the cooling season, consequently, their seasonal energy consumption makes 2.2 kWx919h = 2026 kWh/seas.
Thus, the total annual energy consumption by ice storage heat pump heating -
The yearly consumption of energy, referred to 1 m3 of the house makes 11. 15 kWh/(m3 year). Such a low specific consumption of energy proves the high energy efficiency of developed technology.
The suggested Water-
The water storage tank does not need thermal insulation, as it is located in the ground, which plays the roles of both insulation and partial heat source. The 5575 kWh annual consumption of electricity for both heating and cooling of the house with 500 m3 of volume, that is to say 11.15 kWh/ (m3 year) indicates the high energy efficiency of suggested system. As the system round year operates by energy regeneration principal, the energy wastes are negligible. As a result, the system provides highly efficient wintertime heating and inexpensive cooling in summer period.
Compared with open and closed loop heat pumps the developed local heat pump does not cause environ-
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