**Mitigating Greenhouse Gas Emissions from Winter Production of Agricultural Greenhouses**

Lilong Chai, Chengwei Ma, Baoju Wang, Mingchi Liu and Zhanhui Wu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62271

#### **Abstract**

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tal Environment. 2011;412-413:257-264. DOI: 10.1016/j.scitotenv.2011.09.064

s11738-008-0153-7

224 Greenhouse Gases

511-523. DOI: 10.1080/01904167.2012.644371

2010;331:203–215. DOI: 10.1007/s11104-009-0246-0

Consuming conventional fossil fuel, such as coal, natural gas, and oil, to heat agricultural greenhouses has contributed to the climate change and air pollutions regionally and globally, so the clean energy sources have been increasingly applied to replace fossil energies in heating agricultural greenhouses, especially in urban area. To assess the environment performance (e.g., greenhouse gas (GHG) emissions) of the ground source heat pump system (GSHPs) for heating agricultural greenhouses in urban area, a GSHPs using the shallow geothermal energy (SGE) in groundwater was applied to heat a Chinese solar greenhouse (G1) and a multispan greenhouse (G2) in Beijing (latitude 39°40′ N), the capital city of China. Emission rates of the GSHPs for heating the G1 and G2 were quantified to be 0.257–0.879 g CO2 eq. m−2 day−1. The total GHG emissions from heating greenhouses in Beijing with the GSHPs were quantified as 1.7–2.9 Gt CO2 eq. year−1 based on the electricity from the coal-fired power plant (CFPP) and the gas-fired power plant (GFPP). Among different stages of the SGE flow, the SGE promotion contributed most GHG emissions (66%) in total due to the higher consumption of electricity in compressors. The total GHG emissions from greenhous‐ es heating with the coal-fired heating system (CFHs) and gas-fired heating system (GFHs) were quantified as 2.3–5.2 Gt CO2 eq. year−1 in Beijing. Heating the G1 and G2 with the GSHPs powered by the electricity from the CFPP, the equivalent CO2 emissions were 43% and 44% lower than directly burning coal with the CFHs but were 46% and 44% higher than the GFHs that burn natural gas. However, when using the GFPP-generated electricity to run the GSHPs, the equivalent CO2 emissions would be 84% and 47% lower than the CFHs and the GFHs, respectively.

**Keywords:** urban agriculture, greenhouse heating, greenhouse gases, fossil en‐ ergy, shallow geothermal energy

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

Agricultural buildings, such as horticultural greenhouse, usually require additional heating during winter and cold days in high latitude regions of the Northern Hemisphere [1, 2]. In northern China and many European countries, coal-fired heating system (CFHs) and the natural gas-fired heating system (GFHs) are dominant heating methods in greenhouses [3, 4]. However, conventional fossil fuels, such as coal, natural gas, and oil, which are nonrenewable and are the major greenhouse gas (GHG) contributors, may lead to the global climate change, air pollution, and energy crisis [5-7].

Renewable and clean energy, such as solar, geothermal, and shallow geothermal energy (SGE), has been increasingly applied to replace fossil energy systems in heating agricultural buildings (especially in urban area) across the world [8-11]. The SGE is mainly the stored solar energy in groundwater and soil layers less than 200 m deep from the earth soil surface [12, 13]. It can be used as heat source or sink for air conditioning in residential, industrial, and agricultural buildings with the ground source heat pump system (GSHPs), also known as geothermal heat pumps (GHPs) [4, 14].

The GSHPs has been applied to heat agricultural greenhouse in many countries [15-18]. The GSHPs could be considered with zero GHG emissions if the electricity was the only energy source that could be consumed by the system. However, producing electricity in coal-fired power plant (CFPP) or gas-fired power plant (GFPP) would emit a large quantity of GHG (e.g., CO2). Besides, the refrigerant (e.g., R22 and R134a) used by the heat pump unit has been reported with the high risk of leaking in the year-round operation [19]. Therefore, assessing GHG emissions of the GSHPs should consider both direct and indirect sources.

In northern China (the area with altitude higher than 30° in the Northern Hemisphere) [20], there was mainly two kinds of horticultural greenhouses: the Chinese solar greenhouse (denoted as G1), which may or may not require assisted heating depending on the building design and the plants be cultivated, and the multispan greenhouses (denoted as G2), which require 100% assisted heating systems (primarily in the form of coal burning or gas burning) during winter time [21, 22]. The Chinese solar greenhouse, characterized with east-west orientation, transparent camber south roof, and solid north roof and east and west walls, usually has higher heat-preserving capacity than multispan greenhouse and requires less heating [23, 24]. However, the healthy growth of thermophilic vegetables, such as cucumber and tomato, and most flowers in Chinese solar greenhouse still requires assisted heating especially during cold winter nights or consecutive days of snowing or cloudy [3].

By the end of 2007, about 19,300 ha of greenhouses and tunnels had been constructed and used in Beijing, the capital city of China [25, 26], primarily for producing vegetables, flowers, and fruits. About 6000 ha Chinese solar greenhouses (the structure similar to G1) and 1000 ha multispan greenhouses (the structure similar to G2) may require assisted heating in winter with the systems of the CFHs and GFHs [27]. Therefore, quantifying the heating rate and GHG emission rate for the primary types of agricultural greenhouses with different heating systems and energy sources is important for developing the national or regional GHG emissions inventory of the greenhouse heating and mitigation strategies.

The objectives of this chapter are to (1) address the environmental concern on agricultural production over winter; (2) quantify the heating loads and the GHG emission rates for the two primary agricultural greenhouses (the G1-Chinese solar greenhouse and the G2-multispan greenhouses) in northern China; (3) assess the annual GHG emissions inventory of the greenhouse heating with different energy sources in Beijing, the capital city of the China; and (4) identify the difference between the shallow geothermal energy and the conventional fossil energy systems in GHG emissions of agricultural greenhouses heating.

#### **2. Materials and Methods**

**1. Introduction**

226 Greenhouse Gases

pumps (GHPs) [4, 14].

air pollution, and energy crisis [5-7].

Agricultural buildings, such as horticultural greenhouse, usually require additional heating during winter and cold days in high latitude regions of the Northern Hemisphere [1, 2]. In northern China and many European countries, coal-fired heating system (CFHs) and the natural gas-fired heating system (GFHs) are dominant heating methods in greenhouses [3, 4]. However, conventional fossil fuels, such as coal, natural gas, and oil, which are nonrenewable and are the major greenhouse gas (GHG) contributors, may lead to the global climate change,

Renewable and clean energy, such as solar, geothermal, and shallow geothermal energy (SGE), has been increasingly applied to replace fossil energy systems in heating agricultural buildings (especially in urban area) across the world [8-11]. The SGE is mainly the stored solar energy in groundwater and soil layers less than 200 m deep from the earth soil surface [12, 13]. It can be used as heat source or sink for air conditioning in residential, industrial, and agricultural buildings with the ground source heat pump system (GSHPs), also known as geothermal heat

The GSHPs has been applied to heat agricultural greenhouse in many countries [15-18]. The GSHPs could be considered with zero GHG emissions if the electricity was the only energy source that could be consumed by the system. However, producing electricity in coal-fired power plant (CFPP) or gas-fired power plant (GFPP) would emit a large quantity of GHG (e.g., CO2). Besides, the refrigerant (e.g., R22 and R134a) used by the heat pump unit has been reported with the high risk of leaking in the year-round operation [19]. Therefore, assessing

In northern China (the area with altitude higher than 30° in the Northern Hemisphere) [20], there was mainly two kinds of horticultural greenhouses: the Chinese solar greenhouse (denoted as G1), which may or may not require assisted heating depending on the building design and the plants be cultivated, and the multispan greenhouses (denoted as G2), which require 100% assisted heating systems (primarily in the form of coal burning or gas burning) during winter time [21, 22]. The Chinese solar greenhouse, characterized with east-west orientation, transparent camber south roof, and solid north roof and east and west walls, usually has higher heat-preserving capacity than multispan greenhouse and requires less heating [23, 24]. However, the healthy growth of thermophilic vegetables, such as cucumber and tomato, and most flowers in Chinese solar greenhouse still requires assisted heating

GHG emissions of the GSHPs should consider both direct and indirect sources.

especially during cold winter nights or consecutive days of snowing or cloudy [3].

inventory of the greenhouse heating and mitigation strategies.

By the end of 2007, about 19,300 ha of greenhouses and tunnels had been constructed and used in Beijing, the capital city of China [25, 26], primarily for producing vegetables, flowers, and fruits. About 6000 ha Chinese solar greenhouses (the structure similar to G1) and 1000 ha multispan greenhouses (the structure similar to G2) may require assisted heating in winter with the systems of the CFHs and GFHs [27]. Therefore, quantifying the heating rate and GHG emission rate for the primary types of agricultural greenhouses with different heating systems and energy sources is important for developing the national or regional GHG emissions

#### **2.1. Selected/tested greenhouses and the GSHPs**

A Chinese solar greenhouse (G1, Figure 1 and Table 1) and a multispan greenhouse (G2, Figure 2 and Table 1), two important types of greenhouse in Northern China, were equipped with the groundwater-type GSHPs (Figure 3 and Table 2) in Beijing (latitude 39°40′N) and tested for developing heating rate and GHG emission rate. Performances of GHSPs were compared to CFHs and GFHs. In addition, different electricity generation methods (e.g., coal and gas power plant) were considered for assessing the GHG emissions of the GSHPs.

(a) Cucumbers in the G1

(b) The profile of the G1

(1) North wall 240 mm clay brick+100 mm polystyrene heat preservation layer+240 150

Surface area, m2

Chinese solar greenhouse (G1)

Table 1. Characteristics of the testing greenhouses.

Section Construction and coverage material


**Table 1.** Characteristics of the testing greenhouses.

(a) Cottons in the G2

(b) The profile of the G2 Figure 2 The multispan greenhouse. . .

Manufacturer: four Danfoss hermetic scroll compressor; rated power of electric motor driving: 16.08×4 kW; refrigerant: 58 kg R22. Rate of refrigeration capacity:

Elements Performance parameters

380 kW; rate of heating capacity: 450 kW.

(2) Condenser Horizontal shell–tube model (3) Evaporator Dry evaporator model (4) Throttle Copper capillary tube

(1) Compressor

Mitigating Greenhouse Gas Emissions from Winter Production of Agricultural Greenhouses http://dx.doi.org/10.5772/62271 229

**Section Construction and coverage material Surface area,**

240 mm clay brick+100 mm polystyrene heat preservation foam board+240 mm

(1) North wall 240 mm clay brick+100 mm polystyrene heat preservation layer+240 mm clay

(3) South roof 0.15 mm single layer transparent polyethylene (south roof was covered with 10 mm needled felt heat blanket at winter nights)

(1) North wall 5 mm coated steel sheet+100 mm polystyrene heat preservation layer+5 mm

(5) Floor Bare soil for planting with concrete walkway (floor area was covered with

(4) North roof 50 mm steel plate+100 polystyrene heat preservation layer+50 mm steel plate 108 (5) Floor Bare soil (clay) 480

(2) East and west wall 20 mm double-layer glass 110 (3) South wall 20 mm double-layer glass 98 (4) North roof 20 mm double-layer glass 108

aluminum film at horizontal height of 3.5 m at winter nights)

(a) Cottons in the G2

(b) The profile of the G2 Figure 2 The multispan greenhouse. . .

Manufacturer: four Danfoss hermetic scroll compressor; rated power of electric motor driving: 16.08×4 kW; refrigerant: 58 kg R22. Rate of refrigeration capacity:

Elements Performance parameters

380 kW; rate of heating capacity: 450 kW.

Chinese solar greenhouse (G1)

(2) East and west end

Multispan greenhouses (G2)

walls

228 Greenhouse Gases

brick

clay brick

coated steel sheet

Table 2. Characteristics of the GSHPs.

(2) Condenser Horizontal shell–tube model (3) Evaporator Dry evaporator model (4) Throttle Copper capillary tube

(1) Compressor

**Figure 2.** The multispan greenhouse.

**Table 1.** Characteristics of the testing greenhouses.

**m2**

150

36

510

126

756

(a) The heat pump and water circulating system

(b) Diagram of the GSHPs greenhouse heating system

**Figure 3.** The groundwater type of GSHPs. (I is the stage of SGE extraction, II is the stage of SGE promotion, and III is the stage of greenhouse heating. **a**–**i** are valves installed in different water pipes. Pump 1 is groundwater drawing pump; pumps 2 and 3 are circulating water pumps; and pump 4 represent the groundwater backfilling pump. TFM is the thermal flow meter installed position. 1−8 represent the thermodynamic points in each section of GSHPs [18].

The GSHPs started to heat greenhouses on October 15, 2007 and ended on February 4, 2008. Cucumbers and strawberries were grown in G1 and cotton was grown in G2 during heating tests. The fan coil units in two greenhouses were controlled by the T-type thermocouple controllers automatically. For G1, the indoor air temperature was controlled in the range of 18°C–20°C, and for G2, the indoor air temperature was controlled in the range of 18°C–22°C, considering the poor thermal stability caused by the larger volume in G2.


**Table 2.** Characteristics of the GSHPs.

#### **2.2. Power inputs and heating rate quantification**

The heat provided by the GSHPs for heating greenhouses was quantified with the thermal flow meter (TFM) (Model DN35 and DN100, Beijing Jingyuan Liquid Apparatus Company, Beijing, China). A weather station (Qingsheng Electronic Science and Technology Co. Ltd., Handan, China) installed in the agriculture station was used to monitor outdoor environmen‐ tal factors. Total electricity consumption of the GSHPs was recorded with watt-hour meter (Shanghai Huaxia Ammeters Manufactory, Shanghai, China).

#### **2.3. Quantifying the GHG emissions**

There are six kinds of gases (Table 3) highlighted as the GHG in Intergovernmental Panel on Climate Change (IPCC 2006) [5]. For the greenhouse heating with the CFHs and GFHs, carbon dioxide (CO2) is the only GHG to be considered. But for the GSHPs greenhouse heating, most electricity consumed (in Beijing area) was generated in the CFPP or the GFPP, and the process of the power generation could emit large amount of CO2 [25]. Besides, it has been reported that the leaking fraction of the refrigerant (i.e., R22(HFC-22) in this study) used in the GSHPs is around 0.02 kg−1 (2%) per year [19]. The R22 was not listed as one of the six primary GHG in the IPCC (2006), but it was reported with 1.28 times of the global warming potential (GWP) of the R134a (HFC-134a) [28]. Therefore, the GWP of R22 was estimated to be 4902, 1830, and 557 based on 20a, 100a, and 500a, respectively, based on the relationship of the GWP with the R134a.

The GHG emissions from heating G1 and G2 with the GSHPs can be quantified with Eq. 1.

$$\rm{EM}\_{\rm{CSHPs-G1-j}} = M\_{\rm{CSHPs-G1-j}} \, \* \, f\_{\rm{/-co\_2}} + \frac{HE\_{\rm{CSHPs,G2}}}{HE\_{\rm{CSHPs,G}}} M\_{\rm{R22,Gal}} \, \* \, f\_{\rm{R22-lack}} \, \* \, f\_{\rm{R22-co\_2}} \tag{1}$$

where HEGSHPs,G, total energy provided by the GSHPs for heating G1 and G2 during whole winter (monitored with TFM), MJ; HEGSHPs,G*<sup>i</sup>* , total energy provided by the GSHPs for heating


**Table 3.** GWP of different greenhouse gases

**2.2. Power inputs and heating rate quantification**

(7) Control system PLC touching screen controller

W; number: 45

**Elements Performance parameters**

(2) Condenser Horizontal shell–tube model (3) Evaporator Dry evaporator model (4) Throttle Copper capillary tube

(6) Pumps Flux: 33.2 m3

**Table 2.** Characteristics of the GSHPs.

(1) Compressor

230 Greenhouse Gases

**2.3. Quantifying the GHG emissions**

R134a.

(Shanghai Huaxia Ammeters Manufactory, Shanghai, China).

heating capacity: 450 kW.

The heat provided by the GSHPs for heating greenhouses was quantified with the thermal flow meter (TFM) (Model DN35 and DN100, Beijing Jingyuan Liquid Apparatus Company, Beijing, China). A weather station (Qingsheng Electronic Science and Technology Co. Ltd., Handan, China) installed in the agriculture station was used to monitor outdoor environmen‐ tal factors. Total electricity consumption of the GSHPs was recorded with watt-hour meter

(5) Fan-coil In G1: FP-136; rated input power: 56 W; number: 6. In G2: 42VM006; rated input power: 87

h−1; rated input power: 11 kW; number: 4

Manufacturer: four Danfoss hermetic scroll compressor; rated power of electric motor driving: 16.08×4 kW; refrigerant: 58 kg R22. Rate of refrigeration capacity: 380 kW; rate of

There are six kinds of gases (Table 3) highlighted as the GHG in Intergovernmental Panel on Climate Change (IPCC 2006) [5]. For the greenhouse heating with the CFHs and GFHs, carbon dioxide (CO2) is the only GHG to be considered. But for the GSHPs greenhouse heating, most electricity consumed (in Beijing area) was generated in the CFPP or the GFPP, and the process of the power generation could emit large amount of CO2 [25]. Besides, it has been reported that the leaking fraction of the refrigerant (i.e., R22(HFC-22) in this study) used in the GSHPs is around 0.02 kg−1 (2%) per year [19]. The R22 was not listed as one of the six primary GHG in the IPCC (2006), but it was reported with 1.28 times of the global warming potential (GWP) of the R134a (HFC-134a) [28]. Therefore, the GWP of R22 was estimated to be 4902, 1830, and 557 based on 20a, 100a, and 500a, respectively, based on the relationship of the GWP with the

The GHG emissions from heating G1 and G2 with the GSHPs can be quantified with Eq. 1.

EM \* \* \* *<sup>i</sup>*

*ij j*

winter (monitored with TFM), MJ; HEGSHPs,G*<sup>i</sup>*

GSHPs,G GSHPs Gi j GSHPs G co R22,Gi R22 leak R22 co

GSHPs,G

where HEGSHPs,G, total energy provided by the GSHPs for heating G1 and G2 during whole

2 2

, total energy provided by the GSHPs for heating

*HE M f Mf f HE* - - -- - - - = + (1)

the greenhouse of type *i* (G1 or G2) during whole winter (monitored with TFM), MJ; EMGSHPs −G*i*−*j* , equivalent CO2 emissions from the GSHPs for heating greenhouse *i* (G1 or G2) be driven by electricity generated in power plant *j* (CFPP or GFPP), kg CO2 eq.; *M*GSHPS-G*i*−*<sup>j</sup>* , the coal or natural gas consumed in power plant *j* (CFPP or GFPP) for generating the electricity that the GSHPs had used for heating greenhouse *i* (G1 or G2), kg C or kg CH4; *M*R22,G*<sup>i</sup>* , the amount of the refrigerant R22 in the GSHPs be allotted for heating greenhouse *i* (G1 or G2), kg; *fj−* co2 2, the CO2 emissions coefficient of fossil energy (coal or natural gas) in power plant *j* (CFPP or GFPP), kg CO2 eq. (kg C)−1 or kg CO2 eq. (kg CH4) −1; *f*R22*<sup>−</sup>* co2*,* the equivalent CO2 emissions coefficient of R22, kg CO2 eq. kg−1 R22; and *f*R22−leak, the leaking fraction of total R22, %.

The carbon (C) or natural gas (assumed to be 100% as CH4 in calculating CO2 emissions) consumed to produce the electricity consumed by the GSHPs was estimated for the CFPP and GFPP based on Eq. 2.

$$M\_{\text{GSHPs-Cl}-j} = \frac{\text{3600 ELE}\_{\text{CSFPs,Ci}}}{\text{CV}\_{\text{j}}} \tag{2}$$

where ELEGSHPs,G*<sup>i</sup>* , total electricity consumed by the GSHPs in heating greenhouse *i* (G1 or G2) during winter production, kWh; and CV*<sup>j</sup>* , conversion factor between heat and electricity in CFPP or GFPP, 0.27 was used for CFPP, and 0.42 was applied to GFPP in this study [29].

The GHG emissions from the GSHPs were compared with two primary greenhouse heating systems used in northern China: the CFHs and the CFHs. The equivalent quantity of CO2 emissions from the GSHPs, CFHs, and GFHs was quantified based on the same heat energy provided by the GSHPs during whole winter for the G1 and G2 (Eqs. 3–6). In northern China, the heating efficiencies of CFHs and GFHs were considered as 0.6 and 0.8, respectively [27, 29].

$$M\_{\text{carbon-Cl}} = \frac{\text{HE}\_{\text{GSHPs,Ci}}}{F\_{\text{carbon-heat}} \ast f\_{\text{C-HE}}} \tag{3}$$

$$M\_{\text{Gas}-\text{Gl}} = \frac{HE\_{\text{GSHPs,Gl}}}{F\_{\text{gas}-\text{heat}^\*} f\_{\text{G-mt}}} \tag{4}$$

$$\text{EM}\_{\text{CFFs},\text{GI}} = \text{M}\_{\text{Carbon}\to\text{Cl}^{\cdot}} \text{\* } f\_{\text{C}-\text{co}\_2} \tag{5}$$

$$\text{EM}\_{\text{GFAs},\text{G}\prime} = \text{M}\_{\text{Cas}\sim\text{G}\prime} \, ^\ast f\_{\text{G}\sim\text{co}\_2} \tag{6}$$

where EMCFHs, CO2 emissions from greenhouse CFHs, kg CO2; EMGFHs, CO2 emissions from greenhouse GFHs, kg CO2; *f*C*<sup>−</sup>*CO2, CO2 emissions coefficient of the CFHs, kg CO2 (kg C)−1; *f*G−co2, CO2 emissions coefficient of the natural GFHs, kg CO2 (kg CH4) −1; *f*C*<sup>−</sup>*HE, the heating efficiency of the CFHs, dimensionless; *f*G*<sup>−</sup>*HE, the heating efficiency of the GFHs, dimensionless; *F*carbon*<sup>−</sup>*heat, specific calorific value of burning per kg carbon, MJ (kg C)−1; *F*carbon−heat, specific calorific value of burning per kg natural gas, MJ (kg CH4) −1; *M*carbon−G*<sup>i</sup>* , the carbon consumed in heating greenhouse *i* (G1 or G2) during whole winter, kg C; *M*gas−G*<sup>i</sup>* , the natural gas consumed in heating greenhouse *i* (G1 or G2) during whole winter, kg CH4; and *M*R22−G*<sup>i</sup>* , the R22 used in the GSHPs be attributed to greenhouse *i* (G1 or G2) based on the proportion of heat energy received by G1 and G2, kg.

Under the normal temperature and atmospheric pressure (288 K and 1 atm), burning a kilogram of the standard coal (C) and natural gas (CH4) in oxygen (O2) completely has potential to emit 3.67 and 2.75 kg CO2, respectively, with Eqs. 7 and 8 [30]. Meanwhile, the *F*carbon−heat and *F*gas−heat were quantified as 29.3 and 52.6 MJ for burning each kilogram of the C and CH4, respectively.

$$2\text{ kg C} + 2.67\text{ kg O}\_2 \rightarrow 3.67\text{ kg CO}\_2 + 29.3\text{M/energy}\tag{7}$$

$$2\text{ kg CH}\_4 + 4\text{ kg O}\_2 \rightarrow 2.75\text{ kg CO}\_2 + 2.25\text{ kg H}\_2\text{O} + 52.6\text{ MJ energy}\tag{8}$$

Total GHG emissions from greenhouses heating in Beijing can be estimated based on the total area of greenhouses (similar to G1 or G2) and heat rate per square greenhouse floor (Eqs. 9– 11):

$$\text{EMC}\_{\text{Rejings, CSHPs,}} = \frac{A\_{\text{G1, Beiling}} \times f\_{\text{G1,heating}} \text{EM}\_{\text{CSHPs,}\text{G1-olej}}}{480} + \frac{A\_{\text{G2, Beiling}} \times f\_{\text{G2,heating}} \text{EM}\_{\text{CSHPs,}\text{G2-olej}}}{756} \tag{9}$$

$$\text{EMG}\_{\text{Re/lag, Carbon}} = \frac{A\_{\text{G1, Beiling}} \times f\_{\text{G2,heating}} \text{EM}\_{\text{CFHs,C1}}}{480} + \frac{A\_{\text{G2, Beiling}} \times f\_{\text{G2,heating}} \text{EM}\_{\text{CFHs,C2}}}{756} \tag{10}$$

$$\text{EMG}\_{\text{Beljng,gas}} = \frac{A\_{\text{G1, Beljng}} \times f\_{\text{G2,heating}} \text{EM}\_{\text{GFik,G1}}}{480} + \frac{A\_{\text{G2, Beljng}} \times f\_{\text{G2,heating}} \text{EM}\_{\text{GFik,G2}}}{756} \tag{11}$$

Where, EMGBeijing,GSHPs, *<sup>j</sup>* , equivalent CO2 emissions from the GSHPs heating for greenhouses similar to G1 and G2 be driven electricity generated in the power plant *j* (CFPP or GFPP), kg CO2 eq. in Beijing; EMGBeijing,carbon, equivalent CO2 emissions from the CFHs heating for greenhouses similar to G1 and G2 in Beijing, kg CO2 eq; and EMGBeijing,gas, equivalent CO2 emissions from the GFHs heating for greenhouses similar to G1 and G2 in Beijing, kg CO2 eq.

#### **3. Results and Discussion**

G HE

*i*

= (4)

−1; *f*C*<sup>−</sup>*HE, the heating efficiency

, the R22 used in the GSHPs

, the carbon consumed in heating

, the natural gas consumed in heating

EM \* *i i M f* - - = (5)

EM \* *i i M f* - - = (6)

GSHPs,G

*f*

gas heat\*


CFHs,G Carbon G C co2

GFHs,G Gas G G co2

where EMCFHs, CO2 emissions from greenhouse CFHs, kg CO2; EMGFHs, CO2 emissions from greenhouse GFHs, kg CO2; *f*C*<sup>−</sup>*CO2, CO2 emissions coefficient of the CFHs, kg CO2 (kg C)−1; *f*G−co2,

of the CFHs, dimensionless; *f*G*<sup>−</sup>*HE, the heating efficiency of the GFHs, dimensionless; *F*carbon*<sup>−</sup>*heat, specific calorific value of burning per kg carbon, MJ (kg C)−1; *F*carbon−heat, specific calorific value

be attributed to greenhouse *i* (G1 or G2) based on the proportion of heat energy received by

Under the normal temperature and atmospheric pressure (288 K and 1 atm), burning a kilogram of the standard coal (C) and natural gas (CH4) in oxygen (O2) completely has potential to emit 3.67 and 2.75 kg CO2, respectively, with Eqs. 7 and 8 [30]. Meanwhile, the *F*carbon−heat and *F*gas−heat were quantified as 29.3 and 52.6 MJ for burning each kilogram of the C and CH4,

−1; *M*carbon−G*<sup>i</sup>*

2 2 1 kg C 2.67 kg O 3.67 kg CO 29.3 MJ energy +® + (7)

42 2 2 1 kg CH 4 kg O 2.75 kg CO 2.25 kg H O 52.6 MJ energy +® + + (8)

G1, Beijing G1,heating GSHPs,G1 elej G2, Beijing G2,heating GSHPs,G2 elej

G1, Beijing G2,heating CFHs,G1 G2, Beijing G2,heating CFHs,G2

480 756

EM A EM

EM A EM

= + (10)

*A f <sup>f</sup>* - - ´ ´ = + (9)

Total GHG emissions from greenhouses heating in Beijing can be estimated based on the total area of greenhouses (similar to G1 or G2) and heat rate per square greenhouse floor (Eqs. 9–

480 756 *<sup>j</sup>*

*A f* ´ ´ *f*

*F* -

*HE*

Gas G


*M*

CO2 emissions coefficient of the natural GFHs, kg CO2 (kg CH4)

greenhouse *i* (G1 or G2) during whole winter, kg CH4; and *M*R22−G*<sup>i</sup>*

of burning per kg natural gas, MJ (kg CH4)

G1 and G2, kg.

232 Greenhouse Gases

respectively.

11):

EMG

Beijing,GSHPs,

EMG

Beijing,Carbon

greenhouse *i* (G1 or G2) during whole winter, kg C; *M*gas−G*<sup>i</sup>*

*i*

#### **3.1. Emissions from the GSHPs**

Total heat energy provided by the GSHPs for G1 and G2 were 149270.4 and 640659.1 MJ during 2007–2008 winter at the electricity consumptions of 10826.1 and 44372.2 kWh, respectively. The electricity consumed by the GSHPs usually came from the power plants of CFPP and GFPP in northern China. Therefore, the difference between the CFPP and GFPP in producing GHG at different stages of the SGE in the GSHPs was compared (Table 4).


Note: (1) G1=480m2 , heated with GSHPs for 146 days; G2=756 m2 , heated with GSHPs for 111 days. (2) In calculating the CO2 emissions, the CH4 was assumed to be 100% chemical component of the natural gas.

**Table 4.** GHG emission from G1 and G2 heating with GSHPs.

In producing the amount of the electricity consumed by the GSHPs for G1 and G2, about 4.9 and 20 t of coal (C) were consumed in CFPP. If use the GFPP produced electricity, the total natural gas burned could be 1.8 and 7.2 t (CH4). During 2007–2008 winter heating, the GWP of the GHG emissions from G1 and G2 (Figure 4) was estimated to be 18.3 and 74.9 t CO2 eq. with the CFPP and 5.3 and 21.7 t CO2 eq. with the GFPP, respectively, over a 20-year time horizon. The GWP of 100a was 1.5%–1.6% lower than 20a for G1 and 5.3%–5.4 % lower than 20a for G2 due to the reduced GWP on R22. Similar to HCFC-22, R22 has shorter atmospheric lifetime [5]. Generally, the CO2 eq. contributed by the leak of R22 accounted for 2.4% and 8.4% in the scenarios of CFPP and GFPP, respectively.

**Figure 4.** GWP of the GHG emissions derived from the GHSPs heating in G1 and G2.

Among different stages of the SGE flow, most GHG emissions (66%) happened at the stage of SGE promotion due to the higher consumption of electricity in compressors. Therefore, improving the efficiency of the compressors has the potential to reduce the GHG emissions from the GHSPs heating.

#### **3.2. Greenhouse gas emissions from fossil energy systems**

Providing G1 and G2 with the same quantity of heat that the GSHPs has provided (i.e., 149270.4 and 640659.1 MJ) requires the CFHs to consume 8.49 and 36.40 t of standard coal and the GFHs to consume 3.55 mg (4964 m3 at 288 K and 1 atm) and 15.22 t (21,304 m3 at 288 K and 1 atm) of natural gas (CH4), respectively. Accordingly, the GHG emissions from heating G1 and G2 (Figure 5) were estimated to be 32.7 and 133.7 t CO2 eq. for the CFHs system and 9.8 and 41.8 t CO2 eq. for the GFHs system.

#### **3.3. Standardized GHG emissions from greenhouse heating**

The unit electricity consuming rate of the GSHPs was 0.15 and 0.53 kWh m−2 d−1 for heating G1 and G2 and which can be standardized as 1500 and 5300 kWh ha−1 d−1 in Beijing during

**Figure 5.** The GHG emissions from the GHSPs and the fossil energy systems.

0.464

0.0

0.2

0.4

The GHG emision rate,

kg CO2 eq m-2 d-1

0.6

0.8

1.0

1.2

1.4

0.464

In producing the amount of the electricity consumed by the GSHPs for G1 and G2, about 4.9 and 20 t of coal (C) were consumed in CFPP. If use the GFPP produced electricity, the total natural gas burned could be 1.8 and 7.2 t (CH4). During 2007–2008 winter heating, the GWP of the GHG emissions from G1 and G2 (Figure 4) was estimated to be 18.3 and 74.9 t CO2 eq. with the CFPP and 5.3 and 21.7 t CO2 eq. with the GFPP, respectively, over a 20-year time horizon. The GWP of 100a was 1.5%–1.6% lower than 20a for G1 and 5.3%–5.4 % lower than 20a for G2 due to the reduced GWP on R22. Similar to HCFC-22, R22 has shorter atmospheric lifetime [5]. Generally, the CO2 eq. contributed by the leak of R22 accounted for 2.4% and 8.4%

74.9

73.8

5.3

G1 CFPP G2 CFPP G1 GFPP G2 GFPP

Power Sources

Among different stages of the SGE flow, most GHG emissions (66%) happened at the stage of SGE promotion due to the higher consumption of electricity in compressors. Therefore, improving the efficiency of the compressors has the potential to reduce the GHG emissions

Providing G1 and G2 with the same quantity of heat that the GSHPs has provided (i.e., 149270.4 and 640659.1 MJ) requires the CFHs to consume 8.49 and 36.40 t of standard coal and the GFHs to consume 3.55 mg (4964 m3 at 288 K and 1 atm) and 15.22 t (21,304 m3 at 288 K and 1 atm) of natural gas (CH4), respectively. Accordingly, the GHG emissions from heating G1 and G2 (Figure 5) were estimated to be 32.7 and 133.7 t CO2 eq. for the CFHs system and 9.8 and 41.8

The unit electricity consuming rate of the GSHPs was 0.15 and 0.53 kWh m−2 d−1 for heating G1 and G2 and which can be standardized as 1500 and 5300 kWh ha−1 d−1 in Beijing during

5.0

21.7

20a GWP 100a GWP

20.6

in the scenarios of CFPP and GFPP, respectively.

234 Greenhouse Gases

18.3

Global warming potential, Mg CO2 eq.

from the GHSPs heating.

t CO2 eq. for the GFHs system.

18.0

**Figure 4.** GWP of the GHG emissions derived from the GHSPs heating in G1 and G2.

**3.2. Greenhouse gas emissions from fossil energy systems**

**3.3. Standardized GHG emissions from greenhouse heating**

2007–2008 winter. The 20a and 100a GWPs of the GHG emissions from the GSHPs heating for G1 and G2 (Figure 6a) were 0.076–0.893 and 0.072–0.879 g CO2 eq. m−2 d−1, respectively. The GHG emission rate of G2 is 3.42 times of G1 because Chinese solar greenhouse has better heatpreserving capacity than multispan greenhouses.

Regarding to the CFHs and GFHs, the standardized GWPs of the GHG emissions (Figure 6b) were 0.142–1.214 CO2 eq. m−2 day−1, and there were no difference between 20a and 100a because the GWP of the CO2 will not change with the time.

(a) The shallow geothermal energy

1.214

1.214

(b) The fossil energy

G1 CFHs G2 CFHs G1 GFHs G2 GFHs

Greenhouses and energy sources

0.142

0.142

0.379

20a GWP 100a GWP

0.379

(a) The shallow geothermal energy

G1 CFPP G2 CFPP G1 GFPP G2 GFPP

Greenhouses and energy sources

0.076

0.072

0.245

20a GWP 100a GWP

0.257 0.259

**Figure 6.** Standardized GHG emissions (GWP) for different greenhouse types and energy sources.

0.261

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

The

GHG emission rate,

kg CO2 eq m-2 d-1

0.893

0.879

#### **3.4. Emissions inventory in Beijing, China**

According to the areas of the G1 and G2 in Beijing that require assisted heating (6000 ha Chinese solar greenhouses-G1 and 1000 ha multispan greenhouses-G2), the total GHG emissions from greenhouses heating with the CFHs or GFHs were quantified as 5238 or 2294 Mt CO2 eq. in Beijing, and there is no difference between 20a and 100a GWP (Figure 7).

**Figure 7.** Total GHG emissions from heating greenhouses with fossil energies in Beijing.(G1-CFHs, heating all the G1 type greenhouses in Beijing with CFHs; G2-CFHs, heating all the G2-type greenhouses in Beijing with CFHs; G1-GFHs, heating all the G1-type greenhouses in Beijing with GFHs; G2-GFHs, heating all the G2-type greenhouses in Beijing with GFHs).

The total GHG emissions from heating greenhouses in Beijing with the GSHPs were quantified as 1658 and 2909 Mt CO2 eq., based on 20a GWP or 1619 and 2839 Mt CO2 eq. based on 100a GWP (Figure 8). The GHG emissions from heating G1-type greenhouses are higher than heating G2-type of greenhouses due to the large area of the G1 built and used in Beijing during 2007–2008 winter.

**Figure 8.** Total GHG emissions from heating greenhouses with the GSHPs in Beijing (G1-CFPP, heating all the G1-type greenhouses in Beijing with the GSHPs be powered with the electricity generated in CFHs; G2-CFPP, heating all the G2-type greenhouses in Beijing with the GSHPs be powered with the electricity generated in CFHs; G1-GFPP, heating all the G1-type greenhouses in Beijing with the GSHPs be powered with the electricity generated in GFHs; G2-GFPP, heating all the G2-type greenhouses in Beijing with the GSHPs be powered with the electricity generated in GFHs).

Applying the GSHPs to heat G1 and G2 with the electricity from the CFPP, the equivalent CO2 emissions were 43% and 44% lower than directly burning coal with the CFHs but were 46.4% and 44.2% higher than the GFHs that burning natural gas. However, when using the GFPP generated electricity to run the GSHPs, the equivalent CO2 emissions would be 83.5% and 83.8% lower than directly burning coal with the CFHs and were 45.9% and 48.1% lower than the GFHs that burning natural gas.

#### **3.5. Uncertainty evaluation**

(a) The shallow geothermal energy

1.214

1.214

G1 CFPP G2 CFPP G1 GFPP G2 GFPP

Greenhouses and energy sources

0.076

0.072

0.245

0.379

20a GWP 100a GWP

0.379

20a GWP 100a GWP

0.257 0.259

0.261

0.464

0.0

**3.4. Emissions inventory in Beijing, China**

0.2

0.4

The GHG emision rate,

kg CO2 eq m-2 d-1

0.6

0.8

1.0

1.2

1.4

0.464

**Figure 6.** Standardized GHG emissions (GWP) for different greenhouse types and energy sources.

Beijing, and there is no difference between 20a and 100a GWP (Figure 7).

20a GWP 100a GWP

1748

1225

**Figure 7.** Total GHG emissions from heating greenhouses with fossil energies in Beijing.(G1-CFHs, heating all the G1 type greenhouses in Beijing with CFHs; G2-CFHs, heating all the G2-type greenhouses in Beijing with CFHs; G1-GFHs, heating all the G1-type greenhouses in Beijing with GFHs; G2-GFHs, heating all the G2-type greenhouses in Beijing

G1-CFHs G2-CFHs G1-GFHs G2-GFHs CFHs GFHs

Greenhouse types and energy sources

546

5238

2294

4013

0

1000

2000

The GHG emissions,

with GFHs).

 Mt

CO2 eq yr-1

3000

4000

5000

6000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

The

236 Greenhouse Gases

GHG emission rate,

kg CO2 eq m-2 d-1

0.893

0.879

(b) The fossil energy

According to the areas of the G1 and G2 in Beijing that require assisted heating (6000 ha Chinese solar greenhouses-G1 and 1000 ha multispan greenhouses-G2), the total GHG emissions from greenhouses heating with the CFHs or GFHs were quantified as 5238 or 2294 Mt CO2 eq. in

G1 CFHs G2 CFHs G1 GFHs G2 GFHs

Greenhouses and energy sources

0.142

0.142

It was assumed that all the solar greenhouse and multispan greenhouses with the same heating rate of G1 and G2 in this study, which would lead to errors due to the varying structures and materials in different greenhouses. For the solar greenhouses with improved wall materials and structures, the heat loss and heating rate would be lower [31]. Besides, heat-preserving technologies, such as multilayer aluminum foil heat reflecting materials, would have lower heat loss and heating rate than G2 in this study [3].

The shallow geothermal heat used in the GSHPs came from the groundwater (14°C), which has different GHG emissions from the borehole or U-tube-based HPs [19, 32]. Besides, the leaking factor of R22 was assumed to be 2% per year based on European studies, which may be changing with the change of the pump unit and maintenance of the system.

The GHG emissions calculated in this study are based on the real heating quantity required by G1 and G2 during winter heating, and the cycle of the SGE from extraction, enhance, and greenhouse heating was considered, the analysis can be considered as a partial life cycle assessment (LCA). However, a full LCA analysis could be applied to account the GHG emissions from greenhouse constructing with different materials, the transportation of the coal or natural gas for the location of the greenhouses, and the plants cultivated in the greenhouses [33-35].

In this study, we assumed that all the G1-type Chinese solar greenhouses would need additional heating in calculating the GHG emissions. However, novel structures and materials were applied for building Chinese solar greenhouses in Beijing in recent years [36], which improved the heat-preserving capacity of the greenhouse so that heating was not required in winter time. Therefore, the GHG emissions from heating Chinese solar greenhouse could be lower than the amount calculated in this study.

#### **4. Summary**

The unit electricity consuming rate of the GSHPs were 0.15 and 0.53 kWh m−2 d−1 for heating the Chinese solar greenhouse (G1) and multispan greenhouse (G2) or expressed as 1500 and 5300 kWh ha−1 d−1 in Beijing. The 20a and 100a GWPs of the GHG emissions from the GSHPs heating for G1 and G2 were 0.076–0.893 and 0.072–0.879 g CO2 eq. m−2 d–1, respectively.

The total GHG emissions from heating greenhouses in Beijing with the GSHPs were quantified as 1658–2909 Mt CO2 eq. Among different stages of the SGE flow, most GHG emissions (66%) happened at the stage of SGE promotion due to the higher consumption of electricity in compressors.

The total GHG emissions from greenhouses heating with the CFHs or GFHs were quantified as 5238 and 2294 Mt CO2 eq. in Beijing, respectively. Applying the GSHPs to heat G1 and G2 with the electricity from the CFPP, the equivalent CO2 emissions were 43% and 44% lower than directly burning coal with the CFHs but were 46.4% and 44.2% higher than the GFHs that burning natural gas. However, when using the GFPP-generated electricity to run the GSHPs, the equivalent CO2 emissions would be 83.5% and 83.8% lower than directly burning coal with the CFHs and were 45.9% and 48.1% lower than the GFHs that burning natural gas.

The glass-covered G2 consumed more heating energy than G1 during the heating period. This demonstrated that the Chinese solar greenhouse design had better heat preservation than the glass greenhouse. Besides, novel structures and materials applied for building Chinese solar greenhouses in Beijing could further reduce the GHG emissions from heating.

#### **Acknowledgements**

leaking factor of R22 was assumed to be 2% per year based on European studies, which may

The GHG emissions calculated in this study are based on the real heating quantity required by G1 and G2 during winter heating, and the cycle of the SGE from extraction, enhance, and greenhouse heating was considered, the analysis can be considered as a partial life cycle assessment (LCA). However, a full LCA analysis could be applied to account the GHG emissions from greenhouse constructing with different materials, the transportation of the coal or natural gas for the location of the greenhouses, and the plants cultivated in the greenhouses

In this study, we assumed that all the G1-type Chinese solar greenhouses would need additional heating in calculating the GHG emissions. However, novel structures and materials were applied for building Chinese solar greenhouses in Beijing in recent years [36], which improved the heat-preserving capacity of the greenhouse so that heating was not required in winter time. Therefore, the GHG emissions from heating Chinese solar greenhouse could be

The unit electricity consuming rate of the GSHPs were 0.15 and 0.53 kWh m−2 d−1 for heating the Chinese solar greenhouse (G1) and multispan greenhouse (G2) or expressed as 1500 and 5300 kWh ha−1 d−1 in Beijing. The 20a and 100a GWPs of the GHG emissions from the GSHPs heating for G1 and G2 were 0.076–0.893 and 0.072–0.879 g CO2 eq. m−2 d–1, respectively.

The total GHG emissions from heating greenhouses in Beijing with the GSHPs were quantified as 1658–2909 Mt CO2 eq. Among different stages of the SGE flow, most GHG emissions (66%) happened at the stage of SGE promotion due to the higher consumption of electricity in

The total GHG emissions from greenhouses heating with the CFHs or GFHs were quantified as 5238 and 2294 Mt CO2 eq. in Beijing, respectively. Applying the GSHPs to heat G1 and G2 with the electricity from the CFPP, the equivalent CO2 emissions were 43% and 44% lower than directly burning coal with the CFHs but were 46.4% and 44.2% higher than the GFHs that burning natural gas. However, when using the GFPP-generated electricity to run the GSHPs, the equivalent CO2 emissions would be 83.5% and 83.8% lower than directly burning coal with

The glass-covered G2 consumed more heating energy than G1 during the heating period. This demonstrated that the Chinese solar greenhouse design had better heat preservation than the glass greenhouse. Besides, novel structures and materials applied for building Chinese solar

the CFHs and were 45.9% and 48.1% lower than the GFHs that burning natural gas.

greenhouses in Beijing could further reduce the GHG emissions from heating.

be changing with the change of the pump unit and maintenance of the system.

[33-35].

238 Greenhouse Gases

**4. Summary**

compressors.

lower than the amount calculated in this study.

The study was sponsored by the "Beijing Natural Science Foundation (6132011)," "Young Researcher Foundation (QNJJ201212)" in Beijing Academy of Agricultural and Forestry Sciences, "Earmarked Fund for Modern Agro-industry Technology Research System (CARS-25-D-04)," and "Twelve-Five-Year National Science and Technology Support Program (2011BAD12B01)."

We also thank the support from the program of "Promotion network of research and applica‐ tion services on vegetables varieties (KJCX20140416)".

### **Author details**

Lilong Chai1,2\*, Chengwei Ma3 , Baoju Wang1,4, Mingchi Liu3,4 and Zhanhui Wu3,4

\*Address all correspondence to: lchaipurdue@gmail.com

1 National Engineering Research Centre for Vegetables, Beijing Academy of Agriculture and Forestry Sciences, Beijing, P.R. China

2 Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, USA

3 College of Water Resource and Civil Engineering, China Agricultural University, P.R. China, Beijing, P.R. China

4 Key Laboratory of Urban Agriculture (North), Ministry of Agriculture, P.R. China, Beijing, P.R. China

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