Active solar collector located on roof ridge assisted Chinese solar greenhouse for thermal environment enhancement

ABSTRACT

Maximizing the use of solar energy to increase the indoor temperature of greenhouse is of great significance to the green and energy-saving intensive greenhouse production in China. In this study, an active solar heating system (ASHS) was developed and analyzed, consisting of five solar water heaters equipped with vacuum tube solar collectors, one heat storage tank and heating system including booster pump and heating pipes. During the day, the water was heated in the solar collectors and stored in tanks before being placed into circulation in the heating pipes to distribute the heat to the aerial and root zones of plants. To evaluate the performance of ASHS and the feasibility of practical application, climate and plant growth parameters were monitored in two Chinese Solar Greenhouses (CSGs) located in Yangling, China. Results revealed that under the operating condition with the flow rate of 5.4 L/min, the effective heat collection, average heat collection efficiency and energy-saving efficiency of ASHS were 1.9 MJ/(m²•h), 64% and 84%, respectively. The coefficient of performance (COP) was between 4.5 and 6.2, and the payback period was 12.0 years. Furthermore, the results revealed that the ASHS successfully provided 76% of its annual heating needs, heated the CSG for 4 months and maintained the nocturnal temperature above 8 ℃. The actual application effect proved that it increased average indoor air temperature by 2.2 ℃ and substrate temperature by 3.3 ℃ in heated greenhouse. The growth rate of tomato plant height in the heated greenhouse was 0.7427 cm/d, greater than that in the unheated greenhouse (0.6 cm/d). The novel system proposed in this study adopting particular location enables the rise of the ambient and the substrate temperatures with lower energy consumption, improvement of the environmental conditions in CSG, and a more suitable growing environment for plants in the cold season.

KEYWORDS:

• Chinese solar greenhouse
• active solar heating system
• heat collection efficiency
• energy saving efficiency

Nomenclature

$A_c$	=	heat collection area of ASHS, 7.9 m2
$A_{gc}$	=	surface area of CSG, 117 m2
$A_{gg}$	=	area of indoor ground, 64 m2
$A_p$	=	area of heating pipes, 3.1 m2
$C_{p,a}$	=	special heat of indoor air, 1.0044 kJ/(kg·℃)
$C_{p,w}$	=	special heat of water, 4.2 kJ/(kg·℃)
$COP$	=	coefficient of performance
$D{H}_m$	=	number of days per month, day/ month
$E_i$	=	average solar radiation received by solar heat collection system at time $i$, W/m2
$E_{wp}$	=	actual power consumption of 1d pump operated by ASHS, MJ
$F_R$	=	heat remove factor
$H{H}_d$	=	daily heating hour, 10 h/day
$K_c$	=	heat transfer coefficient of cover material 0.60 W/((m2·℃)
$K_g$	=	heat transfer coefficient of indoor ground, 0.36 W/(m2·℃)
$K_p$	=	(heat transfer coefficient of heating pipes, W/m2·K)
$N$	=	infiltration rate, 0.35 s−1
$Q_c$	=	heat collection of collector, MJ
$Q_{c,i}$	=	heat collection rate of collector at time $i$, W
$Q_{lga}$	=	CSG heat loss by air infiltration
$Q_{lgc}$	=	CSG heat loss through the cover material by conduction, W
$Q_{lm}$	=	CSG monthly heating requirement, kJ
$Q_{lt}$	=	heat loss of the tank during non-operating hours, MJ
$Q_r$	=	heat released from heating system, MJ
$Q_{r,i}$	=	heat release rate of heating system at time $i$,W
$Q_s$	=	the cumulative solar radiation,MJ
$q$	=	heat collection per unit area of collector, MJ/m2
$R_s$	=	energy saving rate of ASHS
$R_e$	=	energy utilization ratio of ASHS
$T_{ai}$	=	indoor ambient temperature℃
$T_{ao}$	=	outdoor ambient temperature℃
$T_{a,i}$	=	indoor ambient temperature at time $i$, ℃
$T_{i,i}$	=	inlet temperature of collector at time $i$, ℃
$T_{o,i}$	=	outlet temperature of collector at time $i$, ℃
$T_{p,i}$	=	water temperature inside the heating pipe at time$i$, ℃
$T_{c,end}$	=	water temperature in the tank when collector was stopped,℃
$T_{r,start}$	=	water temperature in the tank when heating system started running,℃
$T_{r,end}$	=	water temperature in the tank when collector was stopped, ℃
$\mathrm{\Delta }{t}_i$	=	data collection interval, 10 min (600s)
$U_L$	=	heat loss efficient of the collector
$V_g$	=	volume of CSG, 345.92 m3
$V_{tank}$	=	volume of heat storage tank, 0.7 m3
$v_w$	=	volumetric flow rate of the working fluid,m3/s

Greek symbols

$\alpha$	=	absorptivity
$\tau$	=	transmissivity
${\eta }_c$	=	instantaneous heat collection efficiency of collector
${\eta }_{c,ave}$	=	average heat collection efficiency of collector
${\rho }_a$	=	density of indoor air, 1.342 kg/m3
${\rho }_w$	=	density of water, 1.0 × 103kg/m3

Acknowledgments

This work was supported by the earmarked fund for Technical System Project of National Bulk Vegetable Industry (CARS-23-C05), Ministry of Agriculture, PR China, and the Regional Innovation Capability Guidance Program of Shaanxi Province Science and Technology Department (2021QFY08‐04), and Shaanxi Province Science and Technology Innovation Drive Project - Advantageous Industrial Technology Research and Development (NYKJ-2020-YL-08)

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Additional information

Funding

The work was supported by the Regional Innovation Capability Guidance Program of Shaanxi Province Science and Technology Department [2021QFY08‐04]; Technical System Project of National Bulk Vegetable Industry , Ministry of Agriculture, PR China [CARS-23-C05]; Shaanxi Province Science and Technology Innovation Drive Project - Advantageous Industrial Technology Research and Development [NYKJ-2020-YL-08]