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Energy analysis of heat surplus storage systems in plastic tunnels
Renewable Energy 35 (2010) 2656e2665
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Energy analysis of heat surplus storage systems in plastic tunnels S. Kurpaska*, H. Latala Institute of Agricultural Engineering and Computer Science, University of Agriculture in Krakow, Balicka 116 B, Krakow 30-149, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2009 Accepted 14 April 2010 Available online 21 May 2010
This paper presents the findings of a theoretical analysis and experimental verification on the storing of heat excess in soil and liquid accumulators located in a foil tunnel. There was positive verification of the formulated macroscopic heat exchange model in both accumulators (maximum error 81%) and the quantity of heat stored in them was defined. During the experiments, under existing weather conditions, the amount of stored heat stood between 6 MJ and 45 MJ in the liquid accumulator and between 9 MJ and 130 MJ in the soil accumulator. The quantity of heat supplied from the accumulator to the interior of the tunnel during discharging, which stood between 0.6 MJ and 46 MJ, was also described. The COP was determined for the tested system both for the accumulator charging process and the discharging of the soil accumulator. Furthermore, the quantity of heat used for heating up heat originating from the discharging of the accumulator whilst heating the tunnel for favourable and unfavourable surrounding climate conditions was determined. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Heat Accumulator Foil tunnel
1. Introduction Cultivation with the use of tunnels and greenhouses leads to higher emission of harmful substances brought about by the combustion of fossil fuels. Research [7] has shown that in the Polish climate, about 70% of production costs are connected with heating of tunnels and greenhouses. This excess heat storage and the unfavourable thermal conditions have a negative effect on plant cultivation. Bot [5] presented an analysis of possible heat use reduction in greenhouses. Sen [22] examined modern trends of converting solar energy into other forms of energy by concentrating on ecology and emphasizing the potential threat to the modern natural environment. Singh and Tiwari [24] demonstrated the effectiveness of solar energy in the heating of greenhouses. One area of research, which focused on the reduction of heat consumption in heated garden structures, attempted to demonstrate the use of solid body heat-storage accumulators for heating purposes (stone deposit, soil, liquid tank). This system involves the pumping of hot air during surplus energy periods and recycling it when the structure requires more heat. The energy impact of operating this kind of system was analysed by e.g. [6,8,14,17,19]. In turn, work carried out by [11,25] involved researching the system of storing heat in liquid accumulators or bodies subject to phase change.
* Corresponding author. E-mail address: [email protected] (S. Kurpaska). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.04.011
An alternative to the storage of surplus heat in solid bodies or liquids is presented by research on heat stored in bodies subject to stage change as well as change in crystal structure which occurs during heat in-take and return. Research [1e4,8,10,12e15,21,23,25] has shown that this method may be successfully used as a means of reducing the amount of heat employed in heated structures. Ozgener and Hepbasli [19] have drawn up an international review on findings relating to the solar-assisted ground-source system for greenhouse heating. Ozturk and Bascetincelik [20] analysed the system in which excess heat was stored in an accumulator using deposit heat volume. Opdam [18] presented experimental research findings, which focused on the storing of heat excess accumulators using change in deposit concentration. Garcia et al. [9] made a comparison of different heating systems used in greenhouses together with currently used heat sources corresponding with the excess heat-storage system. The above clearly indicates that many specialists worldwide have carried out work on heat-storage. Heat is stored in various carriers (soil, water, stone deposits) and structures. Specialist literature allows one to conclude that the work effectiveness of these systems depends on the geographical latitude of the analysed system, the parameters of pumped air and the deposit. Hitherto international research has concentrated on heat-storage exclusively in one type of accumulator. Research on heat-storage in two types of accumulators is therefore fully justified. This research will allow the energy effectiveness of the examined processes to be established, but it also ought to give an answer to the practical legitimacy of using both types of excess heat-storage generated inside the structure. This paper offers an analysis of these systems.
2. System description 2.1. Experimental set-up This analysis covers the work effectiveness of an associated excess energy storage system resulting from the passive heating of a structure with solar energy. The first system refers to soil heat storage, whilst the second to liquid tank heat storage (Fig. 1). The heat acquisition system comprises a number of perforated coils through which air is sucked from the upper sections of the tunnel by means of a ventilator, from where it is directed to the soil accumulator, or the aireliquid exchanger, or the liquideliquid exchanger. The liquideliquid exchanger is located in the water tank. The soil accumulator comprises a number of perforated coils located in the soil. Ventilator capacity is regulated by setting the number of rotor revolutions in the power transmission system installed in the A/C current frequency inverter. The heated air was directed to the soil or water accumulator depending on the position of the ball valve. The change of this position was connected with the assumed temperature in the soil accumulator. When the assumed temperature in this accumulator was achieved, air was pumped into the water accumulator. The standard i.e. the horticultural basis of greenhouse production (peat, tree bark and perlite) was used as a deposit for the soil accumulator. In this research use was made of an electrically-powered (2.2 kW) ventilator with maximum airflow of 0.4 m3 s1 and with maximum static pressure of 4.5 k Pa. The soil accumulator comprised perforated tubes with diameter of 50 mm. These tubes were located in the deposit at a depth of 0.3 m and at a distance of 0.6 m from one other. Average temperature and humidity in the soil collector was determined on the basis of gauge point values located at the following depths: 0.05 m, 0.15 m and 0.25 m and at a distance from the axis of the perforated tube equal to: 0.0 m, 0.1 m, 0.2 m and 0.3 m. By this virtue the isolated space constituted a repeatable element of the analysed accumulator. This kind of solution was employed on the basis of an analysis carried out on the horticultural base with the use of warm air [16]. The liquid accumulator was a reservoir with known cubic capacity. Average temperature in this accumulator was determined on the basis of sensor indicators located at a distance of 0.1 m from the bottom and upper levels of the liquid. The liquideliquid exchanger was placed inside the liquid accumulator. Heated air was sucked up from the upper part of the tunnel and pumped to a liquideair heat exchanger where the circulating water was heated.
energizing perforated pipe
The unitary decrease of humidity soil, cm3·cm–3·hour –1
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0,0001 0,00008 0,00006 0,00004 0,00002 0 0
50
100
150
200
250
300
A quantity of the air pumped into the soil, l·s–1 Fig. 2. Dependency of unit change of base humidity on the amount of air pumped into the soil accumulator.
The following levels were recorded at regular intervals of 1 min: a) the liquid stream flowing through the impulse flow meter, b) water and air temperature measured with the use of copperconstantan thermocouples, c) heated air and inside air humidity measured by a capacitive sensor, d) collector deposit temperature and humidity measured with the use of a Time Domain Reflectometry device, e) wind speed measured with the use of a wind anemometer, f) electricity consumed by the circulating pumps and ventilator measured with the use of a wattmeter. An analysis was performed on structures where plants had not been cultivated, however, both the humidity and the temperature of the soil accumulator deposit were determined at the level recommended for potentially cultivated plants; the heat storage process in the liquid accumulator took place once the set temperature value in the soil accumulator had been exceeded. 2.2. Analysis In formulating the presented dependencies, it was accepted that the processes taking place in the accumulators may be defined by means of macroscopic heat and mass exchange in soil accumulators. The following simplification assumptions were drawn up: a) Referring to humid air
heat screen impulse flowmeter circulating pump
B physical parameters of humid air are solely a function of its humidity and temperature, B the process of mixing air inside the structure with out-flowing air or air pumped
ventilator
W
manual valve I
water tank
into the soil accumulator takes place under stable surrounding pressure conditions, B heat proper to dry air remains stable during flow processes,
shut-off valve
b) Referring to heat accumulators B soil air humidity may be calculated on the basis of the generally accessible Kelvin
II air-liquid exchanger
liqiud-liquid exchanger
formula, B thermo-physical properties of bodies storing heat are stable during each cycle, B processes taking place in bodies (heat, soil water movement) are described through the macroscopic model, B convection heat exchange takes place on the surface of the soil, whilst surface
soil collector Fig. 1. Plastic tunnel energy storage system.
evaporation is not noted, B the decided upon heat exchange process in the chosen accumulator is subject to analysis.
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Heat stored in the soil collector, MJ
2658
140 120 100 80 60 40 20 0 1
6
11
16
21
26
31
36
41
46
51
56
61
66
71
Cycles of the experiment, units Fig. 3. Heat stored in the soil accumulator during the experiment.
2.2.1. Energy storage in the soil accumulator Heat storage for established heat exchange may be defined as follows:
the surrounding air inside the structure. This relationship, following conversion was as follows:
Q_ wej ¼ Q_ gl þ Q_ wyj þ Q_ teta Q_ wn þ Q_ str
twyj ¼
(1)
Energy reaching the soil (Q_ wej ) is the product of the stream of air pumped into the soil, the density, temperature and heat of the air proper and time:
ahtwew þ ltgl l þ ah
(4)
a e heat transfer coefficient in free convections flow (for horizontal
where: mpow e mass stream of air, kg s1; rpow e air density, kg m3; Cp-wej e heat specific of air entering the soil, J kg1K1; twej e entry air temperature from the soil, K; s e time, s. The stream of heat transferred from the soil together with outlet air (wyj) is directly proportional to the stream of air, its density and proper heat, as well as the temperature of outlet air from the soil in a given period:
surface), W m2 K1; h e depth of analysed layer, m; twew e air temperature inside in the tunnel, K; tgl e soil temperature, K; l e thermal conductivity of the soil, W m1 K1. Specific heat of outlet air from the soil (Cp-wyj) was calculated according to the previously shown models as heat proper of air entering the soil (Cp-wej). Calculations indicated that relative humidity of outlet air from the soil is equal to the state of saturated air. The only difference during calculation was that rather than entry temperature (twej) it was outlet air temperature that was considered (twyj). Increase in stored energy in the soil (Q_ wej ) was calculated as follows:
dtwyj Q_ wyj ¼ mpow rpow Cpwyj ds
dtgl Q_ gl ¼ abhrgl Cpgl ds
dtwej Q_ wej ¼ mpow rpow Cpwej ds
(2)
(3) 1 1
Heat from balance equation, MJ
where: Cp-wyj e heat specific of outlet air from the soil, J kg K ; twyj e outlet air temperature from the soil, K. In defining the temperature of air leaving the soil (twyj) use was made of the relationship being the result of boundary conditions (type III) of heat exchange between the surface layer of the soil and 160 140
where: a e distance between pipes in the soil collector, m; b e unit length of pipe in the soil, m; rgl e soil density, kg m3; Cp-gl e specific heat of the soil, J kg1K1; tgl e soil temperature, K. Heat entering the tunnel from the soil (Q_ wn ) is the product of experimental tunnel surface, the coefficient of heat takeover between the top layer of the soil and the surrounding air, and the difference in soil outlet air temperature and air temperature in the tunnel:
d twyj twew Q_ wn ¼ aFtun ds
120 100 80
emax=81%; σ=16.9 MJ
20 0 0
20
40
60
80
100
120
(6)
where: a e heat transfer coefficient in free convections flow (for horizontal surface), W m2 K1; Ftun e area of soil surface in the tunnel, m2. Heat used on water evaporation in the soil (Q_ teta ):
60 40
(5)
140
160
Heat established through experimentation, MJ Fig. 4. Comparison of calculated and measured values for heat storage in the soil accumulator.
dmsw Q_ teta ¼ L ds
(7)
where: msw e soil water mass change, kg; L e latent heat of vaporisation of water, J kg1 K1.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665 10 9
700
COPgl coefficient, [-]
Solar radiation intensity , W m–2
800
2659
600 500 400
W m–2
300
100 24
30
36
42
48
7 6 5 4 3 2 1
15 45 80 110 145
200
8
0 0
2
4
6
54
The impact of air temperature, oC
8
10
Time storage, h Fig. 6. Temporary changing of the COP during cycle one of the experiment.
Fig. 5. The impact of air temperature and solar radiation intensity on the density of stored heat stream in the soil accumulator for air streams above 210 l s1.
relationship of stored energy in the soil to energy used during storage: In analysing the process of discharging the soil accumulator air temperature was marked out within the structure which would have existed had the interior of the structure not been supplied with heat. Temperature was defined by balancing the heat gathered inside the structure with losses which take into account all mechanisms of heat loss. This balance may be expressed as follows:
Cpwew rpow Vðt0 t0þDs Þ ¼ Fosl kosl
d t0 t20þDs tot ds
(8)
where: Cp-wew e heat specific of air inside, J kg1K1; V e volume of the tunnel, m3; t0 e initial air temperature inside the tunnel, K; Fosł e area of cover surface of the tunnel, m2; kosł e heat transfer coefficient of the tunnel cover, W m2 K1; tot e outside air temperature, K. After converting this relationship, air temperature inside the structure at the end of the given time period (t0þDs) is expressed by the following formula:
t0þDs ¼
2Cpwew V rpow t0 Fosl kosl Dsðt0 tot Þ 2Cpwew V rpow þ Fosl kosl Ds
(9)
where: t0þDs e air temperature inside the structure at the end of the given time, K; t0 e initial air temperature inside the tunnel, K; Cp-wew e specific heat of air in the tunnel, J kg1 K1; Ds e time period, s. In order to establish the correctness of this relationship, theoretical air temperature during accumulator discharge was defined as follows: in addition to calculated temperature value (t0þDs) thermal gain resulting from soil accumulator discharge was added. Air temperature resulting from soil accumulator discharge (t0þDs-gl) is expressed as follows:
t0þDsgl ¼
abhrgl Cpgl tgl0 tgl0þDs Cpwew V rpow
DQgl Pw s
(10)
(11)
The effectiveness of storing heat in the soil base (COPgl) is the
(12)
COPgl e coefficient of effectiveness of storing heat in the soil accumulator, e; DQgl e change of heat storage in the soil accumulator, J; Pw e power of engine to running the fan, W; sw e fan operating time, s. 2.2.2. Heat storage in the water accumulator In the analysis it was accepted that energy stored in the tank (Q_ zb ) is an additive heat function supplied by the coils (Q_ wez ) and heat penetrating the tank walls (Q_ wn ). The assessment may be expressed as follows:
Q_ zb ¼ Q_ wez þ Q_ wn
(13)
With time the tank walls (s) supply heat to the inside of the structure in proportion to their surfaces (Fzb), the heat penetration coefficient (kzb) and differences in temperature between average w2 water temperature in the tank (tw ¼ tw1 þt 2 ) and temperature inside the tunnel (twew) in the form of:
dðtw twew Þ Q_ wn ¼ Fzb kzb ds
(14)
where: Fzb e area of the tank walls, m2; kzb e heat transfer coefficient of the tank walls, W m2 K1; tw e average water temperature in the tank, K; twew e air temperature inside in the tunnel, K; tw1 e water temperature at the upper part of the tank, K; tw2 e water temperature at the bottom of the tank, K. The tank wall heat penetration coefficient is calculated as follows:
kzb ¼
where tgl0, tgl0þDs e initial (tgl0) and final (tgl0þDs) temperature of soil accumulator discharge in the analysed time period (Ds), K. Coefficient kosł takes into account heat loss mechanisms in the structure according to Kurpaska [14]. Once the balance constituents have been calculated (equal to 1), the heat loss stream (Qstr) may finally be calculated as follows:
Q_ str ¼ Q_ wej DQ_ gl Q_ wyj DQ_ teta Q_ wn
COPgl ¼
Fzb
mw Cw ðtw0 twk Þ tw0 þ twk twew0 þ twewk s 2 2
(15)
where: mw e mass water in the tank, kg; Cw e specific heat of water, J kg1K1; tw0 e initial water temperature, K; tw-k e final water temperature, K; Fzb e area of the tank walls, m2; twewe0 e initial air temperature inside the tunnel, K; twewek e final air temperature inside the tunnel, K. Starting from the basic balance, heat stored in the tank (Q_ zb ) is the product of water mass in the storage tank (mw), specific heat of water (Cw) and the difference in water temperatures at the end (twk) and at the beginning (tw0) of the considered period of time. The following description may be used:
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18
COPgl coefficient, [-]
16 14 12 10 8 6 4 2 0 1
4
7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70
Cycles of the experiment Fig. 7. Values of the COP during the cycles of the experiment.
dðt tw0 Þ Q_ zb ¼ mw Cw wk ds
(16)
Heat taken from the coil (wez) is directly proportionate to the intensity of liquid flow in the coil (Vp), proper water heat (cw) and the difference in supply and return temperature of water in the coil (tz, tp) in the given period of time:
d tzas tpow Q_ wez ¼ Vp Cw ds
(17)
0 Heat taken from the coil (Q_ wez ) may also be calculated from the coefficient product of water supply through the coil (kw), coil surface (Fw) and the difference between the average temperature of t þt the water feeding the coil ð zas 2 pow Þ and the average temperature of wk Þ during a given period of time: the water in the tank ðtw0 þt 2
0 Q_ wez
tzas þ tpow tw0 þ twk d 2 2 ¼ kw Fw ds
kw
COPw ¼
Qzb Pw sw þ Pp sp
(18)
(19)
(20)
where: Pp e power of the circulation pump, W; sp e pump operating time, s. The total amount of stored heat in the analysed accumulators, the amount of heat supplied to the inside of the structure during soil accumulator discharge and heat demand by the analysed structure was calculated on the basis of momentary values of specific constituents stemming from the work of the systems. The following model demonstrates this relationship:
Zs
where: Fw earea of the coil, m2; kw e heat transfer coefficient of the coil, W m2 K1; tzas e inlet water temperature, K; tpow e outlet water temperature, K; tw0 e initial water temperature, K; twk e final water temperature, K. After comparing models 17 and 18 and after conversion the following relationship of the coil heat supply coefficient was obtained:
2mw cw tzas tpow ¼ tzas þ tpow Fw ðtw0 þ twk ÞFw
The effectiveness of the heat storage process in the water tank (COPw) is defined in a similar manner as in the case of soil heat storage analysis, as the relationship of energy stored in the tank to energy used for driving the ventilator and pumps:
Qtot ¼
qðtðsÞÞds
(21)
0
where: q(t(s)) e heat stream, W. These errors were calculated on the basis of the following standard formulae relative error as:
e ¼
jEzm Eobl j 100 Ezm
(22)
root mean-square error:
s¼
n X ðEzm E
2 obl Þ
i¼1
!0;5
n
(23)
where: Ezm e measured energy, J; Eobl e calculated energy, J; n e number of comparisons. At the same time regression analysis was carried out by calculating the parameters of the line regression.
3. Results and discussion
Fig. 8. The quantity of stored heat in the water tank during the experiment.
The experimental research was carried on a 54 m2 plastic tunnel, a 27 m2 soil accumulator and a 0.7 m3 liquid accumulator. During accumulator charging the level of soil water decreases, as air passing through soil pores and capillaries causes soil water to evaporate, thus decreasing the level of soil humidity. As soil humidity decreases heat is gathered from passing hot air and this causes its temperature to decrease. In turn, decrease in the
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2661
50
Heat from balance equation, MJ
40
30
20
e max =77%
σ =5.5MJ 10 Fig. 11. The COP on the given day of the experiment.
0 0
10
20
30
40
50
Measured heat, MJ Fig. 9. Comparison of calculated and measured values for heat storage in the water tank.
temperature of passing air brings about a decrease in the temperature gradient between the air and soil particles, which in turn influences the intensity of receiving air, which influences the effectiveness of the storing process. For this reason Fig. 2 presents the dependency of unit change of base humidity on the amount of air pumped into the soil. Change in base humidity in relation to the amount of pumped air was as follows:
Qjedn ¼ 3 107 Vpow 1 106 R2 ¼ 0:77
; with use range : 50 Vpow 240; ls1
During research, soil humidity was kept at the same level through irrigation.
this comparison demonstrates considerable similarity which is evidence that the chosen method and adapted simplifications were appropriate. In order to define the amount of heat obtained in the accumulator on the basis of varying experimental factors for further purposes energy effects were calculated in terms of heat stream units. As it is not necessary to present all of the possible relationships, Fig. 5 has been used to offer an example of the impact of pumped air temperature and the sum of solar radiation intensity on changes in the stream of stored heat in the soil accumulator for maximum values of air volume streams pumped into the accumulator. In order to make use of the findings a multiple regression model was prepared; this model defines the density of the heat stream stored in the base as variable experiment factors. This relationship is expressed as follows: 1;31 1;07 qakum ¼ 0; 18R1;05 þ 2; 5 zew þ 0; 057Vpow 1396; 08tzas
R2 ¼ 0; 81; with use range : 0 Rzew 709 Wm2 ; 20 tzas 51; 7 C; 50 Vpow 239 l s1
3.1. Heat stored in the soil accumulator Fig. 3 shows the amount of heat collected during the experiment. The differences in the amount of stored heat in the accumulator stem from different experimental conditions and the time of storage. This figure presents average values of various heat constituents when charging the soil accumulator. Calculated heat losses (constituting approximately 18% of stored heat in the soil accumulator) are the result of gauge procedure (depth of accumulator layer was limited to 0.5 m) and the method applied on temperature of air leaving the soil accumulator. Fig. 4 presents a comparison between heat stored in the accumulator (gauged) and heat from assessment. As can be noted,
50
Temperature of pumped air, o C
COPw coefficient, [-]
3,5
The energy effectiveness coefficient in the store process determines whether it is worthwhile to adopt the analysed accumulator. Fig. 6 presents the COP time variable coefficient for the one in the experiment. It must be noted that the average value of this coefficient throughout the charge cycle stands at 3.5. In turn, Fig. 7 presents calculated average coefficient values obtained for the entire research cycle.
3 2,5 2 1,5 1
46
42
38
COPw 0,5 1 1,5 2,5 3,5
34
0,5 0 0
1
2
3
4
5
6
30 80
100
120
140
160
180
200
220
240
260
280
300
Air volume stream, l·s–1
One cycle charging time, h Fig. 10. Actual change of the COP during water accumulator charging.
Fig. 12. Dependence of the COP on air volume stream and temperature of pumped air for minimum sum of solar radiation.
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without discharging
Air temperature inside the tunnel, oC
discharging 30 25 20 15 10 5 Fig. 15. Amount of heat supplied to the inside of the tunnel during the experiment.
0 0
2
4
6
8
10
12
14
Time of discharging, h Fig. 13. Temporary change in air temperature inside the tunnel with and without discharging the soil accumulator.
Statistical analysis has shown that the value of the COP significantly correlates with the introduced research factors. On the basis of the COP one may state that together with the increase in temperature of pumped air and the sum of solar radiation the effectiveness of energy grows, whilst with the increase in pumped air stream volume, effectiveness decreases.
3.2. Heat stored in the water accumulator Fig. 8 presents the quantity of stored heat in the water tank during the experiment. The difference in the quantity of collected heat stems not only from the conditions of the surroundings, but also from the set temperature of the soil, which the soil accumulator e operating with this system e had to attain. This is because the steering system charged the liquid accumulator only after the arbitrary temperature had been reached in the soil accumulator. The amount of collected heat in the water accumulator is defined as follows: 3:76 0:77 qzb ¼ 0:578R0:706 þ 0:2 zew þ 14:11Vpow 4:41DT
R2 ¼ 0:82
kw ¼ 13:35m233:1 þ 2:9 109 w
tzas þ tpow 2
5:64 þ0:24
R2 ¼ 0:84 with use range: 0.019 mw 0.3 kg s1; 24.44 48.63 C. Much the same as in the case of the soil accumulator the utility of this method of heat storage is decided by the COP. The time sequence of the averaged COP for a given storage cycle is illustrated in Fig. 10. Concerning the presented cycle of charging, the coefficient value clearly demonstrates energy cost-effectiveness of storage, as the average value of the COP is 1.2. This cycle was carried out with the following parameter values: the sum of solar radiation 4.03 kWh, average temperature inside the tunnel equivalent to 43.8 C. Fig. 11 presents the sequence of the COP throughout the experiment. Heat storage effectiveness was between 0.12 and 7.5. Presuming it unnecessary to present all of the possible cases, the analysis was limited to establishing the impact of the following decision variables: solar radiation amounts, stream volume of air
Heat calculated from the model, MJ
with the use range: 2.3 DT 12,1 K; 89 Vpow 290 l/min; 170 Rzew 786 W m2. where: Vpow- stream of air volume, l min1 whilst DT is the difference in air temperature between air pumped into the exchanger and air inside the tunnel, K.
Confirmation of the correctness of the chosen model is the graphic depiction of obtained experimental data presented in Fig. 9. The x-axis presents the amount of heat stored in the liquid accumulator (calculated with a relationship of 16), whilst the y-axis presents the amount calculated from the equation of 13. From a cognitive point of view it is undoubtedly interesting to assess heat penetration coefficient value for the liquideliquid coil located in the water accumulator. For the obtained gauge and calculation results (relationships 18 and 19), a multiple regression model was found which permits the value of the analysed coefficient to be calculated in the form of easily measured parameters. This model is as follows:
50 40 30 20
emin=0.3%; emax=61%
10
σ=3.32 MJ 0 0
10
20
30
40
50
Heat established through experimentation, MJ Fig. 14. Comparison between measured and calculated air temperature in the tunnel during soil accumulator discharge.
Fig. 16. Comparison between measured and calculated amount of heat supplied to the inside of the tunnel during soil accumulator discharge.
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COPgl coefficient, [-]
7 6 5 4 3 2 1 0 0
2
4
6
8
10
12
14
Time of discharging, h Fig. 17. Time variability of the COP in the analysed discharge cycle.
pumped through the ventilator and temperature of pumping heated air. The above decision variables were chosen purposefully, as pumping temperature is strictly correlated with temperature inside the structure. Furthermore, detailed analysis indicated that heat transferred through penetration is several times smaller than heat transferred from the liquideliquid exchanger. Fig. 12 gives an example of the impact of air volume stream and temperature differences on COP value in the case of low levels of solar radiation: 0.7 kWhe1.5 kWh. Statistical analysis indicates that the COP value significantly correlates with experimental factors, whilst the tendency of change in the COP indicates that together with increase in pumped air temperature and radiation value energy effectiveness increases, whilst with increase in pumped air volume stream, effectiveness decreases.
3.3. Discharging the soil accumulator Fig. 13 presents time change in air temperature inside the structure with and without supplying heat to the soil accumulator. Temperature without discharging was calculated through model (10), whilst temperature with discharging was a gauged value. Air temperature in the tunnel with discharge ranges was between 18 C and 24 C, whilst without discharge between 16 C and 24 C. One may also note that, as a result of soil accumulator discharge, air temperature in the tunnel increased by about 4 C in comparison to the same situation without discharge. During discharge the experiment was carried out for the following
22
COPgl Temperature of pumped air, o C
21
0,1 0,4 1 1,8 3
20 19 18 17 16 15 14 50
100
150
200
250
300
Air volume stream, l·s–1 Fig. 18. COP dependence on stream volume and temperature of charge air.
Fig. 19. Time sequence of gauged parameters during an exemplary cycle (favourable weather conditions).
21 twyj 23.6 C; parameter values: 10.7 tot 19.8 C; 1 1 0.1 Vw 1.15 m s ; mpow ¼ 167.7 kg s . Fig. 14 presents a comparison (for one 24-hour cycle) between measured and calculated temperature inside the structure during soil accumulator discharging (calculated from models 9 and 10). This comparison demonstrates considerable similarity, whilst differences occur as the result of the not so fully reliable measuring equipment and the use of simplifications in defining matters occurring in the soil (e.g. use of the macroscopic model). Fig. 15 presents the amount of heat (calculated on the basis of relationship 9) for the entire experimental cycle supplied from the soil accumulator to the inside of the structure. The amount of heat supplied from the soil energy accumulator is approximately between 6 MJ to almost 45 MJ of heat. One may state, therefore, that thanks to discharge of the soil accumulator it is possible to save in terms of heat demand in the tunnel between 6 MJ and 45 MJ depending on the day. In order to check whether the proposed accumulator discharge model defines this process correctly (relationships 8 to 12), a comparison was made between the measured amount of heat and the calculated amount of heat from the model. The results of this analysis are presented in Fig. 16. When analysing the values of compatibility meters, one may unequivocally state that the proposed discharge model displays a high level of convergence. Change in the COP heat discharge effectiveness coefficient (calculated on the basis of relationship 12) from the soil accumulator in terms of time for the given day of the experiment, is presented in Fig. 17. By analysing values introduced in order to assess the effectiveness of the COP it was found that during the analysed discharge cycle value unity (in other words the kind which e from the energy point of view e disqualifies the discharge process) was only attainable at the final stage of discharge. On the basis of analyses carried out for other cycles this tendency was observed in all cases of discharge. For this reason one may decide to include an additional criterion in the soil accumulator discharge process (not only heat requirement by the greenhouse) i.e. the difference in temperature between the pumped-in air and the soil accumulator. Fig. 18 illustrates the impact of the volume stream and the temperature of charge air on the value of the heat receipt coefficient (COP) from the soil accumulator. The highest COP values are attained with charge air temperature of about 14.5 C.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665
50
40
40
32
30
24
20
16
10
8
0
5 0
4
8
12
16
20
24
28
32
36
Heat provided from the soil collector, MJ
Heat storaged in the water tank, MJ Heat storaged in the soil collector, MJ
2664
Duration of experiment, h Heat provided from the soil collector Heat storaged in the soil collector Heat storaged in the water tank Fig. 20. Time sequence of analysed components of heat during the experiment (unfavourable weather conditions).
For the purpose of the presentation two typical days were chosen with high radiation and high temperature of surroundings, as well as low radiation and surrounding temperature. Figs. 19 and 20 illustrate the time sequence of gauged parameters and specific constituent parts of the heat storage system (calculated on the basis of model 21) during an exemplary cycle with favourable weather conditions, whilst Figs. 21 and 22 are for low temperature values within the structure. Heat demand by the structure was calculated for internal temperature of 12 C. Based on an analysis of total work cycles of the analysed accumulators one may explicitly state that heat storage in the water accumulator is justified, as the heated water may be used both for technological and welfare purposes. In establishing work parameters it is necessary to find the best stream volume of air pumped by the heat exchanger, in order to ensure that the value of the COP is the highest. In turn, charging and discharging (and by the same virtue supplying heat to the inside of the structure) is only justified when night temperature inside the structure is lower than that assumed for plants. In analysing the presented research findings (in both work cycles of the considered system i.e. under favourable and unfavourable conditions), one may
Fig. 21. Time sequence of gauged parameters during an exemplary cycle (unfavourable weather conditions).
Fig. 22. Time sequence of analysed components of heat during the experiment (unfavourable weather conditions).
state that during favourable climate conditions there is no justification to store heat in the soil accumulator (it would not be logical to do this) as there is no demand for heat by the structure. In order to sensibly implement this process it is necessary to make use of surrounding temperature values and thus foresee the need to supply previously stored heat in the soil accumulator. During unfavourable weather conditions it is sensible to store heat and discharge the soil accumulator. In the presented case heat supplied from the soil accumulator covers about 20% of heat demand by the examined structure.
4. Conclusions 1. Constituent parts of heat balance occurring during the heat surplus storage process in the plastic tunnel may be defined by means of the macroscopic model. On the basis of research maximum relative errors of gauged and calculated values are between 61% (discharging of the soil accumulator) and 181% (liquid accumulator storage), whilst the average square error stands between 3.32 (discharging) and 16.9 (soil accumulator storage) of MJ heat. 2. During storage the amount of stored heat in the heat accumulator under consideration stood at between 6 MJ and 45 MJ (liquid accumulator) and between 9 MJ and 130 MJ (soil accumulator), whilst during soil accumulator discharging heat was supplied at between 0.6 MJ and 46 MJ. 3. The change tendency of the heat storage effectiveness coefficient (COP) both in the soil and the liquid tank indicates that together with the increase in pumped air temperature and radiation value the effectiveness of energy grows, whereas with growth in stream volume of pumped air effectiveness decreases. 4. In the soil accumulator discharge process the highest value of the COP was attained with an air stream of between 130 l s1 and 190 l s1 and temperature of air pumped into the soil of about 14.5 C. 5. When discharging the soil accumulator it is necessary to take into consideration air temperature inside the structure, whilst the discharge process should take place when the value of this temperature is lower than that recommended for plants.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665
References [1] Badran AA, Jubran BA, Qasem EM, Hamdan MA. Numerical model for the behaviour of a salt-gradient solar-pond greenhouse-heating system. Applied Energy 1997;58(1):57e72. [2] Bascetincelik A, Ozturk HH, Paksoy HO, Demirel Y. Energetic and exergetic efficiency of latent heat storage system for greenhouse heating. Renewable Energy 1999;16(1e4):691e4. [3] Beshada E, Zhang Q, Boris R. Performance of solar greenhouses under Manitoba’s winter weather conditions. Paper No. 05-071. Annual of Meeting of CSBE-SCGAB (Can. Soc. Bioengineering); 2005. [4] Beshada E, Zhang Q, Boris R. Optimizing the Design of Solar Energy Greenhouses. Paper No. 06-108. Annual Conference of CSBE/SCGAB Edmonton, Alberta (Can. Soc. Bioengineering); 2006. [5] Bot GPA. The solar greenhouse; technology for low energy consumption. ISHS. Acta Horticulturae 2004;633:29e33. [6] Chen W, Liu W. Numerical and experimental analysis of convection heat transfer in passive solar heating room with greenhouse and heat storage. Solar Energy 2004;76(5):623e33. [7] Critten DL, Bailey BJ. A review of greenhouse engineering developments during the 1990s. Agricultural and Forest Meteorology 2002;112:1e22. [8] Fath HES. Heat exchanger performance for latent heat thermal energy storage system. Energy Conversion and Management 1991;31(2):149e55. [9] Garcia JL, De la Plaza S, Navas LM, Benavente RM, Luna L. Evaluation of the feasibility of alternative energy sources for greenhouse heating. Journal of Agricultural Engineering Research 1998;69(2):107e14. [10] Gillet AC. The use of heat pumps in solar heating in Belgium. Renewable Energy 1999;16(1e4):647e51. [11] Jewett TJ, Jackson HA, Bruno L, Shaw MB, McGugan CA. Evaluation of an air conditioning water chiller/heat pump system with heat storage for solar heating of greenhouses. Acta Horticulturae 1984;148:691e8. [12] Kurklu A, Bilgin S, Ozkan B. A study on the solar energy storing rock-bed to heat a polyethylene tunnel type greenhouse. Renewable Energy 2003;28 (5):683e97.
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[13] Kurklu A. Energy storage applications in greenhouses by means of phase change materials (PCMs): a review. Renewable Energy 1998;13 (1):89e103. [14] Kurpaska. S. Energetics analysis of heat surplus storage systems for heated _ plastic tunnels (in Polish). Inzynieria Rolnicza 2003;9(51):309e16. [15] Kurpaska S, Lata1a H. Effectiveness of heat pump working with solar _ collectors in hybrid system (in Polish). Inzynieria Rolnicza 2008;6 (104):97e104. [16] Kurpaska S, Slipek Z. Mathematical model of heat and mass exchange in garden subsoil during warm-air heating. Journal of Agricultural Engineering Research 1996;65:305e11. [17] Lindenberger D, Bruckner T, Groscurth HM, Kümmel R. Optimization of solar district heating systems: seasonal storage, heat pumps, and cogeneration. Energy 2000;25(7):591e608. [18] Opdam J, Schoonderbeek G, Heller E, De Gelder A. Closed greenhouse: a starting point for sustainable entrepreneurship in horticulture. ISHS. Acta Horticulturae 2005;691:517e24. [19] Ozgener O, Hepbasli A. A review on the energy and exergy analysis of solar assisted heat pump systems. Renewable Energy 2005;11(1):65e71. [20] Ozturk HH, Bascetincelik A. Energy and exergy efficiency of a packed-bed heat storage unit for greenhouse heating. Biosystems Engineering 2003;86(2):231e45. [21] Ozturk HH. Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating. Energy Conversion and Management 2005;46:1523e42. [22] Sen Z. Solar energy in progress and future research trends. Progress Energy and Combustion Science 2004;30(4):367e416. [23] Sethi VP, Sharma SK. Survey and evaluation of heating technologies for worldwide agricultural greenhouse applications. Solar Energy 2008;82:832e59. [24] Singh RD, Tiwari GN. Thermal heating of a controlled environment greenhouse: a transient analysis. Energy Conversion and Management 2000;41:505e7. [25] Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 2003;23(3):251e83.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Energy analysis of heat surplus storage systems in plastic tunnels S. Kurpaska*, H. Latala Institute of Agricultural Engineering and Computer Science, University of Agriculture in Krakow, Balicka 116 B, Krakow 30-149, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2009 Accepted 14 April 2010 Available online 21 May 2010
This paper presents the findings of a theoretical analysis and experimental verification on the storing of heat excess in soil and liquid accumulators located in a foil tunnel. There was positive verification of the formulated macroscopic heat exchange model in both accumulators (maximum error 81%) and the quantity of heat stored in them was defined. During the experiments, under existing weather conditions, the amount of stored heat stood between 6 MJ and 45 MJ in the liquid accumulator and between 9 MJ and 130 MJ in the soil accumulator. The quantity of heat supplied from the accumulator to the interior of the tunnel during discharging, which stood between 0.6 MJ and 46 MJ, was also described. The COP was determined for the tested system both for the accumulator charging process and the discharging of the soil accumulator. Furthermore, the quantity of heat used for heating up heat originating from the discharging of the accumulator whilst heating the tunnel for favourable and unfavourable surrounding climate conditions was determined. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Heat Accumulator Foil tunnel
1. Introduction Cultivation with the use of tunnels and greenhouses leads to higher emission of harmful substances brought about by the combustion of fossil fuels. Research [7] has shown that in the Polish climate, about 70% of production costs are connected with heating of tunnels and greenhouses. This excess heat storage and the unfavourable thermal conditions have a negative effect on plant cultivation. Bot [5] presented an analysis of possible heat use reduction in greenhouses. Sen [22] examined modern trends of converting solar energy into other forms of energy by concentrating on ecology and emphasizing the potential threat to the modern natural environment. Singh and Tiwari [24] demonstrated the effectiveness of solar energy in the heating of greenhouses. One area of research, which focused on the reduction of heat consumption in heated garden structures, attempted to demonstrate the use of solid body heat-storage accumulators for heating purposes (stone deposit, soil, liquid tank). This system involves the pumping of hot air during surplus energy periods and recycling it when the structure requires more heat. The energy impact of operating this kind of system was analysed by e.g. [6,8,14,17,19]. In turn, work carried out by [11,25] involved researching the system of storing heat in liquid accumulators or bodies subject to phase change.
* Corresponding author. E-mail address: [email protected] (S. Kurpaska). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.04.011
An alternative to the storage of surplus heat in solid bodies or liquids is presented by research on heat stored in bodies subject to stage change as well as change in crystal structure which occurs during heat in-take and return. Research [1e4,8,10,12e15,21,23,25] has shown that this method may be successfully used as a means of reducing the amount of heat employed in heated structures. Ozgener and Hepbasli [19] have drawn up an international review on findings relating to the solar-assisted ground-source system for greenhouse heating. Ozturk and Bascetincelik [20] analysed the system in which excess heat was stored in an accumulator using deposit heat volume. Opdam [18] presented experimental research findings, which focused on the storing of heat excess accumulators using change in deposit concentration. Garcia et al. [9] made a comparison of different heating systems used in greenhouses together with currently used heat sources corresponding with the excess heat-storage system. The above clearly indicates that many specialists worldwide have carried out work on heat-storage. Heat is stored in various carriers (soil, water, stone deposits) and structures. Specialist literature allows one to conclude that the work effectiveness of these systems depends on the geographical latitude of the analysed system, the parameters of pumped air and the deposit. Hitherto international research has concentrated on heat-storage exclusively in one type of accumulator. Research on heat-storage in two types of accumulators is therefore fully justified. This research will allow the energy effectiveness of the examined processes to be established, but it also ought to give an answer to the practical legitimacy of using both types of excess heat-storage generated inside the structure. This paper offers an analysis of these systems.
2. System description 2.1. Experimental set-up This analysis covers the work effectiveness of an associated excess energy storage system resulting from the passive heating of a structure with solar energy. The first system refers to soil heat storage, whilst the second to liquid tank heat storage (Fig. 1). The heat acquisition system comprises a number of perforated coils through which air is sucked from the upper sections of the tunnel by means of a ventilator, from where it is directed to the soil accumulator, or the aireliquid exchanger, or the liquideliquid exchanger. The liquideliquid exchanger is located in the water tank. The soil accumulator comprises a number of perforated coils located in the soil. Ventilator capacity is regulated by setting the number of rotor revolutions in the power transmission system installed in the A/C current frequency inverter. The heated air was directed to the soil or water accumulator depending on the position of the ball valve. The change of this position was connected with the assumed temperature in the soil accumulator. When the assumed temperature in this accumulator was achieved, air was pumped into the water accumulator. The standard i.e. the horticultural basis of greenhouse production (peat, tree bark and perlite) was used as a deposit for the soil accumulator. In this research use was made of an electrically-powered (2.2 kW) ventilator with maximum airflow of 0.4 m3 s1 and with maximum static pressure of 4.5 k Pa. The soil accumulator comprised perforated tubes with diameter of 50 mm. These tubes were located in the deposit at a depth of 0.3 m and at a distance of 0.6 m from one other. Average temperature and humidity in the soil collector was determined on the basis of gauge point values located at the following depths: 0.05 m, 0.15 m and 0.25 m and at a distance from the axis of the perforated tube equal to: 0.0 m, 0.1 m, 0.2 m and 0.3 m. By this virtue the isolated space constituted a repeatable element of the analysed accumulator. This kind of solution was employed on the basis of an analysis carried out on the horticultural base with the use of warm air [16]. The liquid accumulator was a reservoir with known cubic capacity. Average temperature in this accumulator was determined on the basis of sensor indicators located at a distance of 0.1 m from the bottom and upper levels of the liquid. The liquideliquid exchanger was placed inside the liquid accumulator. Heated air was sucked up from the upper part of the tunnel and pumped to a liquideair heat exchanger where the circulating water was heated.
energizing perforated pipe
The unitary decrease of humidity soil, cm3·cm–3·hour –1
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0,0001 0,00008 0,00006 0,00004 0,00002 0 0
50
100
150
200
250
300
A quantity of the air pumped into the soil, l·s–1 Fig. 2. Dependency of unit change of base humidity on the amount of air pumped into the soil accumulator.
The following levels were recorded at regular intervals of 1 min: a) the liquid stream flowing through the impulse flow meter, b) water and air temperature measured with the use of copperconstantan thermocouples, c) heated air and inside air humidity measured by a capacitive sensor, d) collector deposit temperature and humidity measured with the use of a Time Domain Reflectometry device, e) wind speed measured with the use of a wind anemometer, f) electricity consumed by the circulating pumps and ventilator measured with the use of a wattmeter. An analysis was performed on structures where plants had not been cultivated, however, both the humidity and the temperature of the soil accumulator deposit were determined at the level recommended for potentially cultivated plants; the heat storage process in the liquid accumulator took place once the set temperature value in the soil accumulator had been exceeded. 2.2. Analysis In formulating the presented dependencies, it was accepted that the processes taking place in the accumulators may be defined by means of macroscopic heat and mass exchange in soil accumulators. The following simplification assumptions were drawn up: a) Referring to humid air
heat screen impulse flowmeter circulating pump
B physical parameters of humid air are solely a function of its humidity and temperature, B the process of mixing air inside the structure with out-flowing air or air pumped
ventilator
W
manual valve I
water tank
into the soil accumulator takes place under stable surrounding pressure conditions, B heat proper to dry air remains stable during flow processes,
shut-off valve
b) Referring to heat accumulators B soil air humidity may be calculated on the basis of the generally accessible Kelvin
II air-liquid exchanger
liqiud-liquid exchanger
formula, B thermo-physical properties of bodies storing heat are stable during each cycle, B processes taking place in bodies (heat, soil water movement) are described through the macroscopic model, B convection heat exchange takes place on the surface of the soil, whilst surface
soil collector Fig. 1. Plastic tunnel energy storage system.
evaporation is not noted, B the decided upon heat exchange process in the chosen accumulator is subject to analysis.
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Heat stored in the soil collector, MJ
2658
140 120 100 80 60 40 20 0 1
6
11
16
21
26
31
36
41
46
51
56
61
66
71
Cycles of the experiment, units Fig. 3. Heat stored in the soil accumulator during the experiment.
2.2.1. Energy storage in the soil accumulator Heat storage for established heat exchange may be defined as follows:
the surrounding air inside the structure. This relationship, following conversion was as follows:
Q_ wej ¼ Q_ gl þ Q_ wyj þ Q_ teta Q_ wn þ Q_ str
twyj ¼
(1)
Energy reaching the soil (Q_ wej ) is the product of the stream of air pumped into the soil, the density, temperature and heat of the air proper and time:
ahtwew þ ltgl l þ ah
(4)
a e heat transfer coefficient in free convections flow (for horizontal
where: mpow e mass stream of air, kg s1; rpow e air density, kg m3; Cp-wej e heat specific of air entering the soil, J kg1K1; twej e entry air temperature from the soil, K; s e time, s. The stream of heat transferred from the soil together with outlet air (wyj) is directly proportional to the stream of air, its density and proper heat, as well as the temperature of outlet air from the soil in a given period:
surface), W m2 K1; h e depth of analysed layer, m; twew e air temperature inside in the tunnel, K; tgl e soil temperature, K; l e thermal conductivity of the soil, W m1 K1. Specific heat of outlet air from the soil (Cp-wyj) was calculated according to the previously shown models as heat proper of air entering the soil (Cp-wej). Calculations indicated that relative humidity of outlet air from the soil is equal to the state of saturated air. The only difference during calculation was that rather than entry temperature (twej) it was outlet air temperature that was considered (twyj). Increase in stored energy in the soil (Q_ wej ) was calculated as follows:
dtwyj Q_ wyj ¼ mpow rpow Cpwyj ds
dtgl Q_ gl ¼ abhrgl Cpgl ds
dtwej Q_ wej ¼ mpow rpow Cpwej ds
(2)
(3) 1 1
Heat from balance equation, MJ
where: Cp-wyj e heat specific of outlet air from the soil, J kg K ; twyj e outlet air temperature from the soil, K. In defining the temperature of air leaving the soil (twyj) use was made of the relationship being the result of boundary conditions (type III) of heat exchange between the surface layer of the soil and 160 140
where: a e distance between pipes in the soil collector, m; b e unit length of pipe in the soil, m; rgl e soil density, kg m3; Cp-gl e specific heat of the soil, J kg1K1; tgl e soil temperature, K. Heat entering the tunnel from the soil (Q_ wn ) is the product of experimental tunnel surface, the coefficient of heat takeover between the top layer of the soil and the surrounding air, and the difference in soil outlet air temperature and air temperature in the tunnel:
d twyj twew Q_ wn ¼ aFtun ds
120 100 80
emax=81%; σ=16.9 MJ
20 0 0
20
40
60
80
100
120
(6)
where: a e heat transfer coefficient in free convections flow (for horizontal surface), W m2 K1; Ftun e area of soil surface in the tunnel, m2. Heat used on water evaporation in the soil (Q_ teta ):
60 40
(5)
140
160
Heat established through experimentation, MJ Fig. 4. Comparison of calculated and measured values for heat storage in the soil accumulator.
dmsw Q_ teta ¼ L ds
(7)
where: msw e soil water mass change, kg; L e latent heat of vaporisation of water, J kg1 K1.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665 10 9
700
COPgl coefficient, [-]
Solar radiation intensity , W m–2
800
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600 500 400
W m–2
300
100 24
30
36
42
48
7 6 5 4 3 2 1
15 45 80 110 145
200
8
0 0
2
4
6
54
The impact of air temperature, oC
8
10
Time storage, h Fig. 6. Temporary changing of the COP during cycle one of the experiment.
Fig. 5. The impact of air temperature and solar radiation intensity on the density of stored heat stream in the soil accumulator for air streams above 210 l s1.
relationship of stored energy in the soil to energy used during storage: In analysing the process of discharging the soil accumulator air temperature was marked out within the structure which would have existed had the interior of the structure not been supplied with heat. Temperature was defined by balancing the heat gathered inside the structure with losses which take into account all mechanisms of heat loss. This balance may be expressed as follows:
Cpwew rpow Vðt0 t0þDs Þ ¼ Fosl kosl
d t0 t20þDs tot ds
(8)
where: Cp-wew e heat specific of air inside, J kg1K1; V e volume of the tunnel, m3; t0 e initial air temperature inside the tunnel, K; Fosł e area of cover surface of the tunnel, m2; kosł e heat transfer coefficient of the tunnel cover, W m2 K1; tot e outside air temperature, K. After converting this relationship, air temperature inside the structure at the end of the given time period (t0þDs) is expressed by the following formula:
t0þDs ¼
2Cpwew V rpow t0 Fosl kosl Dsðt0 tot Þ 2Cpwew V rpow þ Fosl kosl Ds
(9)
where: t0þDs e air temperature inside the structure at the end of the given time, K; t0 e initial air temperature inside the tunnel, K; Cp-wew e specific heat of air in the tunnel, J kg1 K1; Ds e time period, s. In order to establish the correctness of this relationship, theoretical air temperature during accumulator discharge was defined as follows: in addition to calculated temperature value (t0þDs) thermal gain resulting from soil accumulator discharge was added. Air temperature resulting from soil accumulator discharge (t0þDs-gl) is expressed as follows:
t0þDsgl ¼
abhrgl Cpgl tgl0 tgl0þDs Cpwew V rpow
DQgl Pw s
(10)
(11)
The effectiveness of storing heat in the soil base (COPgl) is the
(12)
COPgl e coefficient of effectiveness of storing heat in the soil accumulator, e; DQgl e change of heat storage in the soil accumulator, J; Pw e power of engine to running the fan, W; sw e fan operating time, s. 2.2.2. Heat storage in the water accumulator In the analysis it was accepted that energy stored in the tank (Q_ zb ) is an additive heat function supplied by the coils (Q_ wez ) and heat penetrating the tank walls (Q_ wn ). The assessment may be expressed as follows:
Q_ zb ¼ Q_ wez þ Q_ wn
(13)
With time the tank walls (s) supply heat to the inside of the structure in proportion to their surfaces (Fzb), the heat penetration coefficient (kzb) and differences in temperature between average w2 water temperature in the tank (tw ¼ tw1 þt 2 ) and temperature inside the tunnel (twew) in the form of:
dðtw twew Þ Q_ wn ¼ Fzb kzb ds
(14)
where: Fzb e area of the tank walls, m2; kzb e heat transfer coefficient of the tank walls, W m2 K1; tw e average water temperature in the tank, K; twew e air temperature inside in the tunnel, K; tw1 e water temperature at the upper part of the tank, K; tw2 e water temperature at the bottom of the tank, K. The tank wall heat penetration coefficient is calculated as follows:
kzb ¼
where tgl0, tgl0þDs e initial (tgl0) and final (tgl0þDs) temperature of soil accumulator discharge in the analysed time period (Ds), K. Coefficient kosł takes into account heat loss mechanisms in the structure according to Kurpaska [14]. Once the balance constituents have been calculated (equal to 1), the heat loss stream (Qstr) may finally be calculated as follows:
Q_ str ¼ Q_ wej DQ_ gl Q_ wyj DQ_ teta Q_ wn
COPgl ¼
Fzb
mw Cw ðtw0 twk Þ tw0 þ twk twew0 þ twewk s 2 2
(15)
where: mw e mass water in the tank, kg; Cw e specific heat of water, J kg1K1; tw0 e initial water temperature, K; tw-k e final water temperature, K; Fzb e area of the tank walls, m2; twewe0 e initial air temperature inside the tunnel, K; twewek e final air temperature inside the tunnel, K. Starting from the basic balance, heat stored in the tank (Q_ zb ) is the product of water mass in the storage tank (mw), specific heat of water (Cw) and the difference in water temperatures at the end (twk) and at the beginning (tw0) of the considered period of time. The following description may be used:
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18
COPgl coefficient, [-]
16 14 12 10 8 6 4 2 0 1
4
7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70
Cycles of the experiment Fig. 7. Values of the COP during the cycles of the experiment.
dðt tw0 Þ Q_ zb ¼ mw Cw wk ds
(16)
Heat taken from the coil (wez) is directly proportionate to the intensity of liquid flow in the coil (Vp), proper water heat (cw) and the difference in supply and return temperature of water in the coil (tz, tp) in the given period of time:
d tzas tpow Q_ wez ¼ Vp Cw ds
(17)
0 Heat taken from the coil (Q_ wez ) may also be calculated from the coefficient product of water supply through the coil (kw), coil surface (Fw) and the difference between the average temperature of t þt the water feeding the coil ð zas 2 pow Þ and the average temperature of wk Þ during a given period of time: the water in the tank ðtw0 þt 2
0 Q_ wez
tzas þ tpow tw0 þ twk d 2 2 ¼ kw Fw ds
kw
COPw ¼
Qzb Pw sw þ Pp sp
(18)
(19)
(20)
where: Pp e power of the circulation pump, W; sp e pump operating time, s. The total amount of stored heat in the analysed accumulators, the amount of heat supplied to the inside of the structure during soil accumulator discharge and heat demand by the analysed structure was calculated on the basis of momentary values of specific constituents stemming from the work of the systems. The following model demonstrates this relationship:
Zs
where: Fw earea of the coil, m2; kw e heat transfer coefficient of the coil, W m2 K1; tzas e inlet water temperature, K; tpow e outlet water temperature, K; tw0 e initial water temperature, K; twk e final water temperature, K. After comparing models 17 and 18 and after conversion the following relationship of the coil heat supply coefficient was obtained:
2mw cw tzas tpow ¼ tzas þ tpow Fw ðtw0 þ twk ÞFw
The effectiveness of the heat storage process in the water tank (COPw) is defined in a similar manner as in the case of soil heat storage analysis, as the relationship of energy stored in the tank to energy used for driving the ventilator and pumps:
Qtot ¼
qðtðsÞÞds
(21)
0
where: q(t(s)) e heat stream, W. These errors were calculated on the basis of the following standard formulae relative error as:
e ¼
jEzm Eobl j 100 Ezm
(22)
root mean-square error:
s¼
n X ðEzm E
2 obl Þ
i¼1
!0;5
n
(23)
where: Ezm e measured energy, J; Eobl e calculated energy, J; n e number of comparisons. At the same time regression analysis was carried out by calculating the parameters of the line regression.
3. Results and discussion
Fig. 8. The quantity of stored heat in the water tank during the experiment.
The experimental research was carried on a 54 m2 plastic tunnel, a 27 m2 soil accumulator and a 0.7 m3 liquid accumulator. During accumulator charging the level of soil water decreases, as air passing through soil pores and capillaries causes soil water to evaporate, thus decreasing the level of soil humidity. As soil humidity decreases heat is gathered from passing hot air and this causes its temperature to decrease. In turn, decrease in the
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50
Heat from balance equation, MJ
40
30
20
e max =77%
σ =5.5MJ 10 Fig. 11. The COP on the given day of the experiment.
0 0
10
20
30
40
50
Measured heat, MJ Fig. 9. Comparison of calculated and measured values for heat storage in the water tank.
temperature of passing air brings about a decrease in the temperature gradient between the air and soil particles, which in turn influences the intensity of receiving air, which influences the effectiveness of the storing process. For this reason Fig. 2 presents the dependency of unit change of base humidity on the amount of air pumped into the soil. Change in base humidity in relation to the amount of pumped air was as follows:
Qjedn ¼ 3 107 Vpow 1 106 R2 ¼ 0:77
; with use range : 50 Vpow 240; ls1
During research, soil humidity was kept at the same level through irrigation.
this comparison demonstrates considerable similarity which is evidence that the chosen method and adapted simplifications were appropriate. In order to define the amount of heat obtained in the accumulator on the basis of varying experimental factors for further purposes energy effects were calculated in terms of heat stream units. As it is not necessary to present all of the possible relationships, Fig. 5 has been used to offer an example of the impact of pumped air temperature and the sum of solar radiation intensity on changes in the stream of stored heat in the soil accumulator for maximum values of air volume streams pumped into the accumulator. In order to make use of the findings a multiple regression model was prepared; this model defines the density of the heat stream stored in the base as variable experiment factors. This relationship is expressed as follows: 1;31 1;07 qakum ¼ 0; 18R1;05 þ 2; 5 zew þ 0; 057Vpow 1396; 08tzas
R2 ¼ 0; 81; with use range : 0 Rzew 709 Wm2 ; 20 tzas 51; 7 C; 50 Vpow 239 l s1
3.1. Heat stored in the soil accumulator Fig. 3 shows the amount of heat collected during the experiment. The differences in the amount of stored heat in the accumulator stem from different experimental conditions and the time of storage. This figure presents average values of various heat constituents when charging the soil accumulator. Calculated heat losses (constituting approximately 18% of stored heat in the soil accumulator) are the result of gauge procedure (depth of accumulator layer was limited to 0.5 m) and the method applied on temperature of air leaving the soil accumulator. Fig. 4 presents a comparison between heat stored in the accumulator (gauged) and heat from assessment. As can be noted,
50
Temperature of pumped air, o C
COPw coefficient, [-]
3,5
The energy effectiveness coefficient in the store process determines whether it is worthwhile to adopt the analysed accumulator. Fig. 6 presents the COP time variable coefficient for the one in the experiment. It must be noted that the average value of this coefficient throughout the charge cycle stands at 3.5. In turn, Fig. 7 presents calculated average coefficient values obtained for the entire research cycle.
3 2,5 2 1,5 1
46
42
38
COPw 0,5 1 1,5 2,5 3,5
34
0,5 0 0
1
2
3
4
5
6
30 80
100
120
140
160
180
200
220
240
260
280
300
Air volume stream, l·s–1
One cycle charging time, h Fig. 10. Actual change of the COP during water accumulator charging.
Fig. 12. Dependence of the COP on air volume stream and temperature of pumped air for minimum sum of solar radiation.
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without discharging
Air temperature inside the tunnel, oC
discharging 30 25 20 15 10 5 Fig. 15. Amount of heat supplied to the inside of the tunnel during the experiment.
0 0
2
4
6
8
10
12
14
Time of discharging, h Fig. 13. Temporary change in air temperature inside the tunnel with and without discharging the soil accumulator.
Statistical analysis has shown that the value of the COP significantly correlates with the introduced research factors. On the basis of the COP one may state that together with the increase in temperature of pumped air and the sum of solar radiation the effectiveness of energy grows, whilst with the increase in pumped air stream volume, effectiveness decreases.
3.2. Heat stored in the water accumulator Fig. 8 presents the quantity of stored heat in the water tank during the experiment. The difference in the quantity of collected heat stems not only from the conditions of the surroundings, but also from the set temperature of the soil, which the soil accumulator e operating with this system e had to attain. This is because the steering system charged the liquid accumulator only after the arbitrary temperature had been reached in the soil accumulator. The amount of collected heat in the water accumulator is defined as follows: 3:76 0:77 qzb ¼ 0:578R0:706 þ 0:2 zew þ 14:11Vpow 4:41DT
R2 ¼ 0:82
kw ¼ 13:35m233:1 þ 2:9 109 w
tzas þ tpow 2
5:64 þ0:24
R2 ¼ 0:84 with use range: 0.019 mw 0.3 kg s1; 24.44 48.63 C. Much the same as in the case of the soil accumulator the utility of this method of heat storage is decided by the COP. The time sequence of the averaged COP for a given storage cycle is illustrated in Fig. 10. Concerning the presented cycle of charging, the coefficient value clearly demonstrates energy cost-effectiveness of storage, as the average value of the COP is 1.2. This cycle was carried out with the following parameter values: the sum of solar radiation 4.03 kWh, average temperature inside the tunnel equivalent to 43.8 C. Fig. 11 presents the sequence of the COP throughout the experiment. Heat storage effectiveness was between 0.12 and 7.5. Presuming it unnecessary to present all of the possible cases, the analysis was limited to establishing the impact of the following decision variables: solar radiation amounts, stream volume of air
Heat calculated from the model, MJ
with the use range: 2.3 DT 12,1 K; 89 Vpow 290 l/min; 170 Rzew 786 W m2. where: Vpow- stream of air volume, l min1 whilst DT is the difference in air temperature between air pumped into the exchanger and air inside the tunnel, K.
Confirmation of the correctness of the chosen model is the graphic depiction of obtained experimental data presented in Fig. 9. The x-axis presents the amount of heat stored in the liquid accumulator (calculated with a relationship of 16), whilst the y-axis presents the amount calculated from the equation of 13. From a cognitive point of view it is undoubtedly interesting to assess heat penetration coefficient value for the liquideliquid coil located in the water accumulator. For the obtained gauge and calculation results (relationships 18 and 19), a multiple regression model was found which permits the value of the analysed coefficient to be calculated in the form of easily measured parameters. This model is as follows:
50 40 30 20
emin=0.3%; emax=61%
10
σ=3.32 MJ 0 0
10
20
30
40
50
Heat established through experimentation, MJ Fig. 14. Comparison between measured and calculated air temperature in the tunnel during soil accumulator discharge.
Fig. 16. Comparison between measured and calculated amount of heat supplied to the inside of the tunnel during soil accumulator discharge.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665
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COPgl coefficient, [-]
7 6 5 4 3 2 1 0 0
2
4
6
8
10
12
14
Time of discharging, h Fig. 17. Time variability of the COP in the analysed discharge cycle.
pumped through the ventilator and temperature of pumping heated air. The above decision variables were chosen purposefully, as pumping temperature is strictly correlated with temperature inside the structure. Furthermore, detailed analysis indicated that heat transferred through penetration is several times smaller than heat transferred from the liquideliquid exchanger. Fig. 12 gives an example of the impact of air volume stream and temperature differences on COP value in the case of low levels of solar radiation: 0.7 kWhe1.5 kWh. Statistical analysis indicates that the COP value significantly correlates with experimental factors, whilst the tendency of change in the COP indicates that together with increase in pumped air temperature and radiation value energy effectiveness increases, whilst with increase in pumped air volume stream, effectiveness decreases.
3.3. Discharging the soil accumulator Fig. 13 presents time change in air temperature inside the structure with and without supplying heat to the soil accumulator. Temperature without discharging was calculated through model (10), whilst temperature with discharging was a gauged value. Air temperature in the tunnel with discharge ranges was between 18 C and 24 C, whilst without discharge between 16 C and 24 C. One may also note that, as a result of soil accumulator discharge, air temperature in the tunnel increased by about 4 C in comparison to the same situation without discharge. During discharge the experiment was carried out for the following
22
COPgl Temperature of pumped air, o C
21
0,1 0,4 1 1,8 3
20 19 18 17 16 15 14 50
100
150
200
250
300
Air volume stream, l·s–1 Fig. 18. COP dependence on stream volume and temperature of charge air.
Fig. 19. Time sequence of gauged parameters during an exemplary cycle (favourable weather conditions).
21 twyj 23.6 C; parameter values: 10.7 tot 19.8 C; 1 1 0.1 Vw 1.15 m s ; mpow ¼ 167.7 kg s . Fig. 14 presents a comparison (for one 24-hour cycle) between measured and calculated temperature inside the structure during soil accumulator discharging (calculated from models 9 and 10). This comparison demonstrates considerable similarity, whilst differences occur as the result of the not so fully reliable measuring equipment and the use of simplifications in defining matters occurring in the soil (e.g. use of the macroscopic model). Fig. 15 presents the amount of heat (calculated on the basis of relationship 9) for the entire experimental cycle supplied from the soil accumulator to the inside of the structure. The amount of heat supplied from the soil energy accumulator is approximately between 6 MJ to almost 45 MJ of heat. One may state, therefore, that thanks to discharge of the soil accumulator it is possible to save in terms of heat demand in the tunnel between 6 MJ and 45 MJ depending on the day. In order to check whether the proposed accumulator discharge model defines this process correctly (relationships 8 to 12), a comparison was made between the measured amount of heat and the calculated amount of heat from the model. The results of this analysis are presented in Fig. 16. When analysing the values of compatibility meters, one may unequivocally state that the proposed discharge model displays a high level of convergence. Change in the COP heat discharge effectiveness coefficient (calculated on the basis of relationship 12) from the soil accumulator in terms of time for the given day of the experiment, is presented in Fig. 17. By analysing values introduced in order to assess the effectiveness of the COP it was found that during the analysed discharge cycle value unity (in other words the kind which e from the energy point of view e disqualifies the discharge process) was only attainable at the final stage of discharge. On the basis of analyses carried out for other cycles this tendency was observed in all cases of discharge. For this reason one may decide to include an additional criterion in the soil accumulator discharge process (not only heat requirement by the greenhouse) i.e. the difference in temperature between the pumped-in air and the soil accumulator. Fig. 18 illustrates the impact of the volume stream and the temperature of charge air on the value of the heat receipt coefficient (COP) from the soil accumulator. The highest COP values are attained with charge air temperature of about 14.5 C.
S. Kurpaska, H. Latala / Renewable Energy 35 (2010) 2656e2665
50
40
40
32
30
24
20
16
10
8
0
5 0
4
8
12
16
20
24
28
32
36
Heat provided from the soil collector, MJ
Heat storaged in the water tank, MJ Heat storaged in the soil collector, MJ
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Duration of experiment, h Heat provided from the soil collector Heat storaged in the soil collector Heat storaged in the water tank Fig. 20. Time sequence of analysed components of heat during the experiment (unfavourable weather conditions).
For the purpose of the presentation two typical days were chosen with high radiation and high temperature of surroundings, as well as low radiation and surrounding temperature. Figs. 19 and 20 illustrate the time sequence of gauged parameters and specific constituent parts of the heat storage system (calculated on the basis of model 21) during an exemplary cycle with favourable weather conditions, whilst Figs. 21 and 22 are for low temperature values within the structure. Heat demand by the structure was calculated for internal temperature of 12 C. Based on an analysis of total work cycles of the analysed accumulators one may explicitly state that heat storage in the water accumulator is justified, as the heated water may be used both for technological and welfare purposes. In establishing work parameters it is necessary to find the best stream volume of air pumped by the heat exchanger, in order to ensure that the value of the COP is the highest. In turn, charging and discharging (and by the same virtue supplying heat to the inside of the structure) is only justified when night temperature inside the structure is lower than that assumed for plants. In analysing the presented research findings (in both work cycles of the considered system i.e. under favourable and unfavourable conditions), one may
Fig. 21. Time sequence of gauged parameters during an exemplary cycle (unfavourable weather conditions).
Fig. 22. Time sequence of analysed components of heat during the experiment (unfavourable weather conditions).
state that during favourable climate conditions there is no justification to store heat in the soil accumulator (it would not be logical to do this) as there is no demand for heat by the structure. In order to sensibly implement this process it is necessary to make use of surrounding temperature values and thus foresee the need to supply previously stored heat in the soil accumulator. During unfavourable weather conditions it is sensible to store heat and discharge the soil accumulator. In the presented case heat supplied from the soil accumulator covers about 20% of heat demand by the examined structure.
4. Conclusions 1. Constituent parts of heat balance occurring during the heat surplus storage process in the plastic tunnel may be defined by means of the macroscopic model. On the basis of research maximum relative errors of gauged and calculated values are between 61% (discharging of the soil accumulator) and 181% (liquid accumulator storage), whilst the average square error stands between 3.32 (discharging) and 16.9 (soil accumulator storage) of MJ heat. 2. During storage the amount of stored heat in the heat accumulator under consideration stood at between 6 MJ and 45 MJ (liquid accumulator) and between 9 MJ and 130 MJ (soil accumulator), whilst during soil accumulator discharging heat was supplied at between 0.6 MJ and 46 MJ. 3. The change tendency of the heat storage effectiveness coefficient (COP) both in the soil and the liquid tank indicates that together with the increase in pumped air temperature and radiation value the effectiveness of energy grows, whereas with growth in stream volume of pumped air effectiveness decreases. 4. In the soil accumulator discharge process the highest value of the COP was attained with an air stream of between 130 l s1 and 190 l s1 and temperature of air pumped into the soil of about 14.5 C. 5. When discharging the soil accumulator it is necessary to take into consideration air temperature inside the structure, whilst the discharge process should take place when the value of this temperature is lower than that recommended for plants.
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