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5ИСПОЛЬЗОВАНИЕ СОЛНЕЧНОЙ ЭНЕРГИИ В ОБЕСПЕЧЕНИИ ГОРЯЧЕЙ ВОДОЙ СЕЛЬСКОГО ДОМА И ТЕПЛОЭНЕРГЕТИЧЕСКИЙ АНАЛИЗ СИСТЕМЫ
Салманова Ф.А.,
Доктор философии по техническим наукам, доцент,
Юсупов И.М. Инженер.
Институт Радиационных Проблем, г.Баку
THE USE OF SOLAR ENERGY TO PROVIDE HOT WATER TO A RURAL HOUSE AND THERMAL
ENERGY ANALYSIS OF THE SYSTEM
Salmanova F.,
PhD in Engineering Sciences, assistant professor,
Yusupov I.
Engineer.
Institute of Radiation Problems, Baku DOI: 10.5281/zenodo.7673193
Аннотация
В статье рассматриваются возможности использование солнечного энергии горячего водоснабжения сельского дома. Приводятся результаты теплоэнергетического расчетов плоскими солнечными коллекторами, а также методика определения температуры воды на выходе из плоскими солнечными коллекторами и времени прохождения теплоносителя - воды.
Abstract
The possibility of solar energy usage for the hot water supply of a rural house is discussed in the article. The results of heat and power calculations by flat solar collectors are presented, as well as a method for determining the water temperature at the outlet of flat solar collectors and the passage time of the coolant water.
Ключевые слова: (СВНС) солнечная водонагревательная система, (ПСК) плоские солнечные коллекторы, солнечная радиация, горячая вода, теплоэнергетический расчет, теплоноситель, теплоизоляция, бак-аккумулятор.
Keywords: (SWHS) solar water heating system, (FSC) flat solar collectors, solar radiation, hot water, thermal energy calculation, heat carrier, thermal insulation, storage tank.
Introduction
Azerbaijan is one of the leading countries in the world in terms of solar and wind energy resources. Therefore, the use of these types of energy in our country has great prospects. Currently, in many developed and developing countries of the world, SWHS with various types of collectors, including FSC, are widely used. At the same time, regardless of the number of resources available for fossil fuels and other energy resources, the SWHS is of interest to all countries of the world. First of all, this is because the use of SWHS with FSC for heating and hot water supply allows for saving for future generations of other types of energy resources, such as oil, coal, natural gas, wood, etc. Secondly, the environment is not polluted by both greenhouse and other harmful gaseous wastes since when using SWHS for heat and hot water supply, the heating of the coolant occurs without the process of burning any fuel. In addition, FSC designs are very simple and cheap which is also considered their advantage in comparison with other types of water heating devices. . Finally, FSCs operate with total solar radiation like photoelectric solar energy converters. Therefore, they can function even when the sky is cloudy. Despite all the above advantages of FSC, it is necessary to develop SWHS, taking into account the exact choice of several parameters, which include structural, thermal engineering, hydraulic and thermal insulation parameters, as
well as the operating conditions of SWHS, in general, for them to work with maximum efficiency. Below, we consider both the operating conditions of the SWHS and the method for calculating all of the above parameters.
1. SWH developing terms
Previously, the tasks of supplying hot water to a rural house (dachas, villas, etc.) in which a family consisting of four to five people lived were considered. The house is located in the vicinity of Baku, where the number of sunny days per year is 220^250 with solar radiation intensity of 1600^1800 kW/m2 and an average annual wind speed of ~5.5 m/sec.
FSCs, and as a coolant, in the warm season (except for winter), ordinary water, and in winter, antifreeze are used to ensure the operation of the SWHS. Besides, the plant uses natural (thermosiphon) water circulation.
To ensure uninterrupted operation on cloudy days and at night, SWHS is additionally equipped with an electric heater with a capacity of ~1,5^2,0 kW, powered by a wind energy power unit (WEU) with a maximum power of 3.0 kW, at wind speeds of 10-20 m/s. It is provided for use also from a centralized power supply network on non-solar and non-windy days in the SWHS.
When calculating the installation, we take into account that about 45^50% of the heat load of the system must be provided by solar energy and 20^25% - by
wind energy. The rest of the needs of the rural house are provided by the centralized power supply network.
To solve the problem, we proceed from the accepted condition when the consumption rate is 80-100 l/day per adult and ~ 40-50 l/day per child (8-14 years old).
The performance of the installation can vary widely, from 1 m2 of the active area of the FSC to 4070 litres per day depending on the time of year.
The total amount of hot water for this family is 220-250 l/day.
The area of the SWHS is assumed to be ~5,0 m2. The initial data for solving the problem are the same as in. FSC "sheet-pipe" design, with collector pipe diameters, dm = 12 mm, dou = 14 mm, blackened FSC sheet thickness (with selective coating) 1.0 mm. The thermal conductivity coefficient is K= 45 W /( m -K). Diameter of connecting pipes d£H = 18mm, d'H = 20mm.. Degree of emissivity of the FSC surface ef= 0,90. The number of hours of sunshine during the day is ~ 10 hours. The ambient air temperature is 250C, and the one on the surface of the FSC pipe is ~65-800C. Wind speed V =5.5 m/ sec.
The degree of emissivity of the FSC of the SWHS is ec'= ~ c 0.95. Distance between the FSC plate and the enclosing glass=25mm. The angle of inclination of the FSC of the SWHS to the horizon is p = 40- 420= 40 - 420, provided that they are placed in the South-North direction. An insulating material "glass wool" with a coefficient of thermal conductivity K = 0,04 + 0,05Vt/(m • K)
2. Thermal power calculation of the FSC of the SWHS
The purpose of the thermal calculation of FSC is to determine heat losses through its surface, as well as daily efficiency. To solve this problem, we use the heat balance equation:
q = qn+ Yflnom (1)
Where: q is the density of the heat flux incident on the surface of the FSC, W/m2 ; q„- the useful part of the solar energy spent on heating water in the SWHS, W/m;lqnom - the total heat loss from the upper and side surfaces, as well as from the base of the FSC, W/m2.
These heat losses are respectively determined by the formulas:
I qnom = UL ' Tp - Ta (2)
UL = Ue + US + n (3)
Here; UB is the total coefficient of heat transfer from the upper surface of the collector, W/m2; Us +H -coefficient of heat transfer from the side surfaces and the isolated lower surface of the collector, W/m2.
The value of Ub + n is determined from the expression Ub + n =K. Considering that for this task 8 from =0,045m and, K from =0,045 W/(m- K), we are finding: Ub+ n= 1,0 W/(m2K).
The Uv value can be found from the following formula [8]:
----1- (4)
UB=-
hn.FSC ,hn.FSC+ hair.
convection from the surface of the FSC to the lower surface of the glass enclosure of the FSC; W/(m2K); h2psc is the heat irradiation coefficient of solar radiation from the surface of the FSC, W/(m2K); hf^cm- coefficient of heat transfer from the upper surface of the glass enclosure FSC to air at a given wind speed, W/(m2K); hB ™ cm air - heat transfer coefficient of solar radiation from the upper surface of the glass enclosure of the FSC to the air by radiation W / (m2K).
These values are determined from the following expressions:
h
n.FSC
= h10°c • [1 - 0,0018(7 - 10)]
(5)
Here, h10°c = 1,14 ■
-007- where, AT=Tp - Tc is
the temperature difference on the surface of the ray-receiving plate (Tp) and the enclosing glass (Tc) of the FSC; l is the distance between the ray-receiving plate of the FSC and its enclosing glass; T- the average temperature, in the space between the surface of the plate and the enclosing glass of the FSC which is determined
from the expression: T = Tp+Tc.
The heat irradiation coefficient of solar radiation from the surface of the FSC is determined from the expression:
Li!. FSC
cp
1/^+1/^-1
(6)
h ™ en.cm.
Here: a = 5,7 ■ 10"8W/(m2K-4) absolute black body radiation constant or Stefan-Boltzmann constant; £ph £c- the degree of blackness of the surface of the FSC and the glass, respectively.
The heat irradiation coefficient from the upper surface of the glass enclosure of the FSC to air is determined from the expression:
= 5.7 + 3.8 F (7)
Here: V is the average annual wind speed, it must be taken into account, although, the use of the average annual wind speed is accepted in the literature, however, this can lead to a significant discrepancy between the data obtained from the real ones, since the instantaneous wind speeds in most wind regions of Azerbaijan, including on the Absheron Peninsula and adjacent territories vary widely, and it is the instantaneous wind speeds that most of all affect the heat losses of the FSC.
The heat irradiation coefficient of solar radiation from the upper surface of the enclosing glass of the FSC to air throw irradiation is determined from the expression:
KT™ J (8)
The surface temperature of the enclosing glass of the FSC is determined by the formula:
T_ = T. -
h
JJ.FEC
h
is- FSC ■c.p.
Taking into account the accepted data values for the calculation, from equations (1-9) we find:
Where: h;
n.FSC
- heat transfer coefficient for free
0.31
A"™ = 2,83 Vt /(m'K) ;
M
The total heat losses of the SWHS will be: E«™ = ^ ■ CT, T)= 8.3(65-25)= 332 Vt The useful part of solar energy is:
800 - 332 = 468 vt
We find the daily efficiency of the SWHS using the following empirical formula for the accepted conditions:
46S 800
■100= 58.5%
'swhs q
3. Determining the temperature of the water at the outlet of the FSC
To determine the temperature of water at the outlet
of the FSC, we use the heat balance equation: G-Cp{Te?-Tei) = FR[q-UL(T6i -JJ] (10)
Here: G - daily consumption of water. Assuming the average duration of daylight hours is 10 hours, Fr is the coefficient of heat removal from the FSC, taking into account the ratio of the useful heat used at the average surface temperature of the FSC equals to the water temperature at the inlet to the FSC Tb1 for the actual useful temperature of the FSC. Cp is the average mass heat capacity of water, which is: Gp = 4,2J/(kg • K)
The G value can be found from the following formula:
For the conditions accepted above, we find: G = 72 kg/ (m2/ sec)
We will determine the difference in water temperatures at the outlet and the inlet of the FSC from equation^):
FfVq-UL-%-Tt)\ GG.
ATr=T. -T =
The value of the coefficient FR is determined by the formula:
G-C_
UL
[l-e
■ U,-F'jG-C
] (13)
Here: FI is the FSC efficiency determined by the equal:
1
F' = -
(14)
L
■+---
UL Cs xd„-h,
Where: l is the distance between the FSC pipes (pitch of the FSC pipes, usually w « 100 ^ 150mm); dH is the outer diameter of the FSC pipe, m; Cb is the coefficient of thermal conductivity of the pipe welding material to the FSC (we take Cb - 33.3 W / (m K); hf is the coefficient of free convection when transferring heat from the inner surface of the FSC pipe to the coolant (water) circulating in the pipe under free convection (in this case, we take hf = 300 W / (m2 K); F is the coefficient of ribbing of the FSC surface, which can be found from the following formula:
thm{l-dBK)J2 W=-77-; . , (15)
m{I-d„V 2
m~^ULikSn
(16)
Where: k - thermal conductivity coefficient of the heat-absorbing plate (for a plate made of aluminium, k = 204W / (m deg), bpl - plate thickness (in this case, bpi = 0.0005m).
Considering UL=8.3W/(m2K) from equation (16)
we get:
m— ikS^ = ^/SJ/(204-0.0005) = 9.03
In the equation (1 - dm) / 2- means half of the distance between the FSC pipes (ks). If we take into account that l=0.12m, and dm=0.0012m, then we get: l0.s=0.054m (5.4sm).
Next, using the initial data from the equation (15), (14) and (13), we find F=0.978, F = 0,9 u Ffi « 0,792. We determine ATk=9.5°C by formula (12). The temperature of the water at the outlet of the FSC is determined by the formula:
■55 = 64.5° C
We accept TV2=65°C, i.e. the water temperature at the outlet of the FSC is equal to the average surface temperature of the FSC.
Conclusion
From the analysis of the obtained results, it can be concluded that the proposed method of heat and power calculation of the SWHS can provide a scientific and technical basis for the development and creation of such systems that serve as heating and hot water supply of rural houses, farms, boarding houses, schools, kindergartens, clinics, laundry shops, many strategic facilities, including military facilities located in hard-to-reach places, remote from centralized heating, gas and electricity supply lines, etc., with the separate and combined use of solar and wind energy. The use of alternative energy sources for this purpose in the agricultural sector of the republic (farms, poultry houses, etc.) will save traditional fuel and electricity, and also help improve the environmental situation in the country.
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