Proceedings of REHVA Annual Conference 2015 “Advanced HVAC and Natural Gas Technologies”
Riga, Latvia, May 6 – 9, 2015
Air Heating Solar Collector
for Hemp Drying
Ilze Pelēce1, Semjons Ivanovs2, Ādolfs Ruciņš3, Oskars Valainis4
1,4
Latvia University of Agriculture, 2,3Research Institute of Agricultural Machinery
Abstract – Attempts to use solar energy for drying of hemp (and II. MATERIALS AND METHODS
other agricultural production) have been made in this work. Air is
heated in solar collector approximately to 40 – 60 degrees Celsius, New construction of solar collector – cylindrical solar
which also results in decreasing of the humidity of this air from 60 collector (Fig. 1) has been investigated.
% (normal ambient air humidity) to 18 - 8 % respectively. Then
such hot dry air is used for drying of hemp or other agricultural
production.
A new construction of air heating solar collector – a cylindrical
solar collector has been worked out.
Keywords – Air heating, production drying, solar collector, solar
energy
I. INTRODUCTION
Along with the decrease of the reserves of fossil fuels, as well
as the increasing impact of the use of fossil fuels on the climate,
on the world scale more attention has been paid to renewable
sources of energy, including solar energy.
In Latvia, solar energy is also used, mostly in solar collectors
for hot water production [1], [2]. Another possible use of solar
energy in Latvia is for drying of agricultural production. Mainly Fig. 1. Cylindrical solar collector.
air heating solar collectors have been used for this purpose.
However, due to geographical and climatic conditions in Latvia It consists of two cylinders: inner is black-painted metal
there are some specific features in comparison with traditional (galvanized 0.5 mm thick steel sheet, coated with black mat
solar energy using countries [3], [4]. Therefore, traditional flat silicon color), and outer is transparent 1 mm thick PET material,
plate solar collectors are not efficient enough in Latvia and new coated with UV-protective film. Diameter of the inner cylinder
constructions of solar collectors must be investigated. One of is 0.59 m, of the outer 0.67 m, the length of both cylinders is
such new constructions can be a cylindrical solar collector, i.e. 1.3 m. Both cylinders are mounted on one axis, which has been
a solar collector with cylindrical absorber. The main advantage pointed to the Polar Star to ensure perpendicular striking of
of such construction is the capability to receive solar energy solar beams to collector surface throughout the entire day. Ends
from all sides, which means to receive also diffused radiation of cylinders are closed with metal discs, from inside covered
and the radiation reflected from clouds, it is also better in with 3 cm thick Rockwool heat insulation. There are openings
comparison with the flat plate collector with regard to receiving in the discs for the inflow of cold air and outflow of the heated
energy in the morning and evening. Similar constructions of air. These openings are positioned so that inflow is from the
air-heating cylindrical solar collector for drying of agricultural bottom of the lower end of the cylinder and outflow is at the top
production have already been investigated by other authors [5], of the upper end of the cylinder. Such positions allow
[6]. But in these investigations a cylindrical solar collector was convection-provided flow of the air without fan. There are two
placed vertically, and energy output and effectiveness of the openings in every end allowing using the heated air both from
collector were not investigated, taking only temperature the inner cylinder and from the space between the metallic
measurements. cylinder and the transparent one, either together or separately.
In our work, a new construction of solar collector – Temperatures were registered using HOBO logger (Fig. 2).
cylindrical solar collector mounted on the axis pointed to the This logger is capable to measure air temperature and relative
Polar Star – has been investigated. Continuous measurements humidity and two external temperatures using sensors
of temperature of the outflowing air allow determining the daily connected with cables. We used two loggers of that kind to
course of temperature and its dependence on meteorological measure ambient air temperature and relative humidity, inflow
conditions, and calculating energy output and effectiveness (as air temperature, outflow air temperature and relative humidity
defined in our previous works, e.g. [7]) of the collector. and temperature inside the cylinder and in space between the
In this work, mainly the same collector has been investigated; cylinder and outer transparent cover.
its use in hemp drying will be considered later.
doi: 10.7250/rehvaconf.2015.032 217
Proceedings of REHVA Annual Conference 2015 “Advanced HVAC and Natural Gas Technologies”
Riga, Latvia, May 6 – 9, 2015
Fig. 5. 97W flexible PV module.
Energy outcome Q(J) from the solar collector has been
Fig. 2. Temperature and air humidity logger HOBO. calculated by (1):
Solar energy was measured using pyranometer CMP 6 from 𝑄 = 𝑐 ∙ 𝑚 ∙ ∆𝑇, (1)
“Kipp&Zonen”(Fig. 3), corresponding to 1-st class of ISO.
Global solar energy was registered after every 5 minutes. where c is specific heat of the air, J kg-1 K-1; m is mass of the
heated air, kg; ΔT is the difference between inflow and outflow
air temperatures, K. The specific heat of air is calculated from
(2)
i2 R
c , (2)
2
where i is number of degrees of freedom, R is universal gas
constant, J K-1; µ is molecular mass, kg. The mass of heated air
by (3) can be calculated
pV
m , (3)
RT
Fig. 3. Pyranometer CMP 6.
where p is atmospheric pressure, Pa; V is the volume of the
Analysis of solar radiation data also gives a view on heated air, m3; T is air temperature, K. Volume of the heated air
nebulosity. can be obtained by multiplying velocity of the air flow with
Air flow was measured using anemometer (Fig. 4) cross-sectional area of inflow (or outflow) tube of the collector.
Calculating in a similar way the energy amount received by
water vapor at such little relative humidity of air gives
approximately 100 times smaller value than that of the air and
therefore can be neglected.
The obtained data are compared with theoretical calculations
using methods explained in our previous works [7].
III. RESULTS AND DISCUSSION
A. Daily course of collector’s temperature
Fig. 4. Anemometer.
As it was expected, the cylindrical solar collector receives
solar energy longer during the day rather than the flat one.
Measurements have been carried out with and without forced Theoretically calculated daily course of power of cylindrical (a)
flow. Without forced flow it is convectional flow. For full and flat (b) solar collectors is shown in Fig. 6.
autonomy of the drying process the forced flow is ensured with
fan powered by a solar cell. The fan with 12 V direct current
electrical engine was used. This voltage was ensured using 97W
flexible PV module (Fig. 5).
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Proceedings of REHVA Annual Conference 2015 “Advanced HVAC and Natural Gas Technologies”
Riga, Latvia, May 6 – 9, 2015
From this picture it can be seen that in the afternoon global
solar energy (curve c) significantly reduces from time to time,
1200 which indicates the presence of clouds, but temperatures of the
a collector (both inner cylinder and middle space) decrease only
1000 slightly.
Another interesting fact to be observed in this picture (Fig.
Power, W m-2
800 8) is that when clouds appear, solar energy among the clouds is
higher than that of the clear day. It means that we receive not
600 only direct radiation, but also reflected from the clouds, which,
b
in general, moves in other direction than the direct one.
400 Therefore, the cylindrical solar collector, which receives energy
from all sides, has advantages in comparison with the flat one.
200 It might seem that the capability to receive radiation from all
sides, when really radiation comes only from one side at the
0
time, will result in large temperature difference between sun
00:00 06:00 12:00 18:00 00:00
Time side and the dark side, but infrared pictures show that it is not
Fig. 6. Theoretically calculated daily course of power of cylindrical (a) and flat so. Investigations show that at temperature of outgoing air 80⁰C
(b) solar collectors. temperature difference of several places of collector surface
does not exceed 10⁰C, such difference is too small to show in
At midday the amount of received energy is the same for both grayscale pictures.
(with equal area), but in the morning and evening the cylindrical
collector receives more energy than the flat one, and it starts to B. Effectiveness
receive the energy earlier in the morning and ends later in the Effectiveness of solar collector is defined [7] as ratio of
evening. Such situation has been observed energy output from the collector to global solar energy on
100 1000 horizontal surface of the same area as the collector has. It is not
the efficiency that is usually used for characterizing solar
Temperature, ºC
Solar energy, W
80 800 collectors. Efficiency is the ratio of energy output to solar
60 600 energy received by the collector. It does not depend on
a positioning of the collector, but effectiveness does.
40 400
Furthermore, solar energy received of the collector is hard to
20 b 200 evaluate. Global solar energy on the horizontal surface can be
0 0 directly measured by pyranometer, but for the calculation of
03:00 09:00 15:00 21:00 energy received by a slope surface solar coordinates must be
Time known as well as beam and diffused radiation separately. In
contrast to efficiency, effectiveness can be greater than one.
The simplest way to evaluate the effectiveness of the solar
Fig. 7. Daily course of temperature of the air outflowing from the cylindrical
solar collector (a) and global solar energy (b). collector is to plot daily sums of energy output of the collector
via those of global solar energy, then to draw linear trendline
also experimentally. Fig. 7 shows daily course of temperature taking into account that the intercept must be zero, because
of the outflowing air from the cylindrical solar collector (a) in when solar energy is zero then also energy output from the
comparison with that of global solar energy (b), at almost sunny collector is zero. The slope of this trendline is the effectiveness
day of July 5. of the collector.
Other interesting fact is that the cylindrical solar collector For calculating the energy outcome of the solar collector, air
works rather well also on a partly cloudy day (Fig. 8). flow was measured. Without fan (convection flow) it was 1±0.7
m/s. With photovoltaic powered fan the flow was 3.9±0.5 m/s.
Then calculations show the power of the collector 100 – 200 W
on a medium cloudy day without fan and 600 – 1100 W with
the fan. It means that convectional flow without fan is not strong
enough for the effective use of such solar collector, but with the
fan the power of the cylindrical solar collector is sufficient for
the drying of the agricultural production.
The plot of the daily sum of collector output energy with
respect to those of the global solar energy on horizontal area
(Fig. 9) without fan for 20-days period gives the slope of the
trendline, i.e. effectiveness 0.26 with determination factor of
Fig. 8. Daily course of temperature of space between the cylinder and
transparent coating (a) and inner cylinder (b), and of global solar energy (c). the trendline R2 = 0.88. The same plot for the collector with fan
(Fig. 10) gives effectiveness 1.5 with determination factor R2 =
219
Proceedings of REHVA Annual Conference 2015 “Advanced HVAC and Natural Gas Technologies”
Riga, Latvia, May 6 – 9, 2015
0.85 for space between the cylinder and transparent cover (for ACKNOWLEDGMENT
14-days period) and effectiveness 1.3 with determination factor This work is supported by the ESF “Attraction of Human
0.86 for the inner cylinder (measurements of 8-days period). Resources to Science (2nd round)” project “Development of the
innovative technologies for the accumulation and
production of heating and cooling” No
2013/0064/1DP/1.1.1.2.0/13/APIA/VIAA/050
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Fig. 10. Dependence of the daily sum of energy output from cylindrical solar
collector with the fan on the daily sum of the global solar energy, a – space
between cylinder and outer transparent cover, July 9 to 23, b – inner cylinder,
July 24 to 31.
Further investigations will be carried out with adding the
reflectors to the cylindrical solar collector, and using it for
hemp drying.
IV. CONCLUSIONS
The power of the cylindrical solar collector with the diameter
0.6 m and length 1.3 m is approximately 600 W on a medium
cloudy day and up to 1100 W on a clear day, using solar energy
(photovoltaic) powered fan. Such power is sufficient for drying
of agricultural production and therefore the cylindrical solar
collector can be used for this purpose in Latvia.
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