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Abbasov Erkin Sodikovich, doctor of technical sciences, professor Umurzakova Muyassar Äbubakirovna, candidate of Technical Sciences, Associate Professor Fergana Polytechnic Institute, Uzbekistan Eeducation.fer@mail.ru
ABOUT THERMAL EFFICIENCY OF FLAT SOLAR AIR HEATERS
Abstrct. The article discusses issues of thermal efficiency of flat solar air heaters and its enhancement through the intensification of thermal processes occurring on the surface of the absorber. On the basis of the heat balance equation compiled for the air heater, an analytical formula for the thermal efficiency and heat loss of the heater is obtained. The article also obtained the dependence of the thermal efficiency of the heater on the heat transfer parameter. A universal dependence on the calculation of the efficiency of flat solar air heaters is presented.
Keywords: flat solar air heater, temperature, heat transfer, thermal efficiency, heat loss, absorber, thermal energy, coolant flow.
The current state of the power system is characterized by rising fuel prices and, as a consequence, high cost of thermal energy. In connection with the exhaustibility of traditional hydrocarbon energy sources, the release into the atmosphere of greenhouse gases at an alarming scale and noticeable climate change, the prospects for the use of solar energy are becoming increasingly urgent. Consequently, an intensive search for alternative energy sources, in particular, the conversion of solar energy into heat energy is a very important scientific and technical task at this stage of the development of energy. Considering that in the air heating and air conditioning sys-
tems, as well as in the drying equipment, low potential heat is used (up to -70%), the use of solar air heaters is promising and allows to increase the efficiency of the existing heat supply systems and heat and power plants.
Solar air heaters being one of the varieties of solar devices in recent years are becoming increasingly popular because of their obvious advantages: no corrosion of heater elements, ease of maintenance, low heat pumping costs, direct heating of room air bypassing the heating system, availability in manufacturing etc. Figure 1 shows the main types of flat solar air collectors [1-4].
a)
b)
c)
d)
Figure 1. Basic types of flat solar air collectors: a) with air movement above the solar collector, b) -with air movement under the solar collector, c) - with a solar collector made of a perforated sheet and air movement above and below the sheet, d) - with multiple air movement in the collector
Analysis of the works on the development of flat solar air heaters (FSAH) designs show that the main subject of research in this area is to increase the heat transfer from the absorber to the air and, accordingly, the thermal efficiency of the heater.A review of the literature indicates that to date there have been very few published papers devoted to the study of the thermal efficiency of FSAH with free convection and con-vective heat exchange at low and medium Reynolds numbers, i.e. in the modes in which these devices are most often used.
This problem of increasing the thermal efficiency of a solar air heater can be solved by intensifying the processes of convective heat exchange from the heated surface of the absorber to the air flowing in the channel of the solar receiver. An analysis of the literature indicates [5-16] that an increase in heat transfer through the use of surface heat transfer inten-sifiers allows an increase in heat removal from the absorber from 1.3 to 2.5 times compared with a smooth surface.Sur-face intensifiers are a large class, including spherical, cylindrical cone-shaped or other forms of protrusions and grooves, diffuser-confused profiles, as well as spiral and transverse wire inserts and micro-ribs. Moreover, an easily feasible method of wall flow turbulization is a discrete well-streamlined roughness in the form of metal chips, which is a waste of mechanical production and, therefore, widely available for use.
The task ofwall flow turbulization is the periodic destruction of the boundary layer in order to reduce the thermal resistance of heat transfer from the wall of the absorber to the air flow. Such an impact on the near-wall boundary layer does not cause a significant increase in the hydraulic resistance, which has a particularly positive effect on the increase in the efficiency of heat exchange in the solar heat exchanger. The well-known method of evaluating the effectiveness of a solar air heater, based on the use of EF. heater is insufficient especially for a heater with heat transfer intensification, since the traditional formula (1)
n= Q = GCp At / JF JF p
(1)
does not reflect, firstly, the heat transfer efficiency of the heater i.e. efficiency of the processes of convective heat transfer between an absorber with heat transfer intensifiers and air flow. Secondly, the formula (1) does not take into account the hydraulic losses of the device at elevated Re numbers.
As a FSAH scheme selected for the analysis of its thermal efficiency, we choose a flat solar air heater, in which a metal sheet with surface heat transfer intensifiers is used as an absorber of solar radiation (Fig. 2). Provided that the air moves at low speed (which corresponds to the practical operation of the collector), i.e. with small Reynolds numbers, the collector efficiency will be determined only by its thermal efficiency (without taking into account hydraulic losses).
Figure 2. Flat solar air heater: 1 - transparent coating, 2 - absorber, 3 - insulation
The diagram shows t ,t ,tcm - respectively, the temperature of the incoming, outgoing air from the heater and absorber, q-density of falling sun radiation
From the heat balance equation compiled for FSAH in stationary conditions we get
Qi = Q2 + Q3
(2)
Q1 = GCP (tmm -1') - maximal using of heat FSAH W Q1 = GCP (tm -1') - beneficial use of heat of FSAH W Q3 = GCP (tcm -underutilized heat in FSAH due to incomplete heat exchange between the absorber and the air flow of watts.
G, Cp - respectively consumption kg/s b heat capacity K kg/s ° C in the air
Substitution of the above expressions in formula (2) gives tCT - t'«C-t') + (tCm - t") (3)
Consumption and heat capacity ofair is considered constant.
1 =
t -1'
-1' t -1'
(4)
Denoting by
t"-1' tm -1"
S --: and £nm --cm
_ t, _ _ t,
Cm Cm
Get 1 = S+Snom (5)
Or £ = 1 -£„0m (6)
We assume £ - heat efficiency FSAH
£nom - heat loss in FSAH
Thus, Eq. (6) is the formula for the thermal efficiency of FSAH.
IF £nom ^ 0 to s ^ 1 Consequently, at low heat losses, that is, when heat transfer is perfect, the thermal efficiency of PWS tends to its maximum. We show that an increase in heat transfer from the surface of the absorber to the air flow increases the thermal efficiency of the SSWH. The initial equation can be the heat transfer equation.
GCp (f-1') + GCp (t cm - t")=aFAt, (7)
Where At - average logarithmic temperature pressure in FSAH. Ifwe assume that the temperature of the air flow varies slightly along the length of the heater, then the pressure can be calculated by the following formula:
At = [( -1') + ( -1")] /2 (8)
We transform equation (11) to the following form:
(t"-1') t -1"
_V_+ cm
^ aF ^
(t -1') (t -1')
\ cm / \ cm /
GCp
v p
1 + ^nom
Because snom = 1 — s noAynHM s = 2 GC
1 --
2
GCp
aF
(9) (10)
Where
aF
-is a heat transfer parameter of P.
Thus, on the basis of the obtained formula (10), we can draw up the following graph to determine the thermal efficiency of the HRSS depending on the heat transfer parameter
chosen--.
a F
Preliminary analysis of formulas shows that with increase of efficiency in heat exchange processes (rate coefficient heat exchange a ) heat efficiency FSAH s should rise up.
Table 1 shows the results of the calculation of the thermal efficiency of the heating device s depending on the values of
the heat transfer parameter
GC
aF Table 1.
№ GCp aF s s nom
1. 0.5 1 0
2. 0.55 0.9 0.1
3. 0.6 0.8 0.2
4. 0.65 0.7 0.3
5. 0.7 0.6 0.4
6. 0.75 0.5 0.5
7. 0.8 0.4 0.6
8. 0.85 0.3 0.7
9. 0.9 0.2 0.8
In accordance with the table number 1 in (Fig. 2) built dependencies £ = f(GCp/aF).
It should be noted that this relationship is universal for all types of air heaters.
Table
Figure 2. The dependence of the thermal efficiency of s on the heat transfer parameter GCp/oF
The dependence shown in (Fig. 2) was used to compare the thermal efficiency of two FSAHs. The first FSAH had a smooth absorber, the second FSAH consisted of a flat absorber with heat transfer intensification. Experiments on heat transfer were carried out in August 2018 in Fergana separately in FSAH having an absorber in the form of a flat sheet, the width of the flat channel of the absorber was a = 0.5 m, the length of the absorber was L = 1 m. And in FSAH having heat transfer intensifiers.
Metal chips, 1 cm in diameter, spaced 12 cm apart and glued to the walls of a smooth blackened metal sheet across the main direction of the moving air, were used as heat transfer intensifiers. Tables 2 and 3 show the results of experiments on heat transfer from a smooth absorber and from an absorber with heat transfer intensifiers. 2.
№ G kg/s t' C0 t" °C t °C cm a w/m2 °C £1 GC /aF p £1
1. 0.0015 27 41 45 3.8 290 0.8 0.4
2. 0.00275 27 41 49 6.5 506 0.85 0.3
3. 0.0055 27 43 49 12.5 938 0.88 0.24
4. 0.00825 27 43 49 18.3 1440 0.9 0.2
5. 0.0116 27 42 49 24.7 2020 0.94 0.12
Table 3.
№ G kg/s t' °C t" °C t °C '■cm C a w/m2 °C £2 GC /aF p £2
1. 0.0015 30 65 72 4.7 290 0.64 0.72
2. 0.00275 30 63 70 7.7 506 0.71 0.57
3. 0.0055 30 61 69 14.5 938 0.76 0.48
4. 0.00825 30 60 68 21.6 1440 0.79 0.42
5. 0.0116 30 59 67 30 2020 0.77 0.41
The analysis of (tables 2 and 3) shows that the thermal efficiency of FSAH with a smooth absorber is significantly inferior to the thermal efficiency of FSAH with heat transfer enhancers. In (fig. 3) also shows dependences graphically £ = f(Re) for two types of FSAH.
Figure 3. The dependence of the thermal efficiency of on the numbers Re
Conculotion
1. A formula is proposed for calculating the thermal efficiency of flat solar air heaters and the dependence of the thermal efficiency of heaters £ on the heat transfer parameter GCp/aF is constructed.
2. It has been established that the thermal efficiency of SSWN with a smooth absorber is significantly inferior to the thermal efficiency of FSAH with heat transfer intensifiers.
3. The developed method for calculating the thermal efficiency of air heaters can be applied to various designs of absorbers with heat transfer intensification.
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