reductions in energy consumption and co2 emissions for greenhouses ...

REDUCTIONS IN ENERGY CONSUMPTION AND CO2 EMISSIONS FOR GREENHOUSES HEATED WITH HEAT PUMPS Y. Tong, T. Kozai, N. Nishioka, K. Ohyama

ABSTRACT. In the greenhouse industry, it is essential to use energy efficiently by operating economically feasible heating systems instead of the traditional combustion-based heating systems. In our experiment, a heating system with household heat pumps was operated at nighttime. Another heating system with a kerosene heater was used as a comparison. We compared the energy consumption, energy utilization efficiency, CO2 emissions and energy costs of two heating systems. When the inside air temperature was maintained at around 16°C while the outside air temperature ranged between -5°C and 6°C, the results showed that 1) the energy consumption was 25% to 65% lower in the greenhouse with heat pumps (Ghp) when compared to the greenhouse with a kerosene heater (Gkh), 2) the energy utilization efficiency of the heat pumps was 1.3 to 2.6 times higher than that of the kerosene heater, 3) CO2 emissions were reduced by 56% to 79% in the Ghp when compared to the Gkh, and 4) energy cost was lower in the Ghp when compared to the Gkh. These results indicate that the heat pump system is found more efficient than the kerosene heating system and economically feasible for greenhouse heating. Keywords. Coefficient of performance, Energy utilization efficiency, Renewable energy.

T

here has been much research on using energy efficiently since the first oil crisis in the early 1970s when insufficient oil supply caused a significant increase in energy prices (Byun et al., 2006; Bakker et al., 2008). Energy costs peaked again in 2008 and in 2011 (Petroleum Association of Japan, 2011). Furthermore, the global effort to reduce CO2 emissions has led to innovative technologies to improve energy utilization efficiency (Bakker, 2009). It is essential for the greenhouse industry to reduce energy consumption and CO2 emissions since fossil fuel energy is still used for heating during the cold winter season, and energy accounts for a substantial fraction of total production costs (Diez et al., 2009; Aye et al., 2010). There are two main ways of reducing energy consumption and CO2 emissions in greenhouses. One is to

Submitted for review in June 2011 as manuscript number SE 9227; approved for publication by the Structures & Environment Division of ASABE in January 2012. The authors are Yuxin Tong, Assistant Professor, (1) Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China and (2) Key Lab of Energy Conservation and Water Treatment of Agricutural Structures, Ministry of Agriculture, Beijing 100081, China; (3) Center for Environment, Health and Field Sciences, Chiba University, Kashiwa-noha 6-2-1, Kashiwa, Chiba 277-0882, Japan; Toyoki Kozai, ASABE Member, Professor Emeritus; Center for Environment, Health and Field Sciences, Chiba University, Kashiwa-no-ha 6-2-1, Kashiwa, Chiba 2770882, Japan; Naoko Nishioka, Staff Member, National Institute of Advance Industrial Science and Technology (AIST), Umezono, Tsukuba, Ibaraki, Japan; and Katsumi Ohyama, ASABE Member, Associate Professor, Center for Environment, Health and Field Sciences, Chiba University, Kashiwa-no-ha 6-2-1, Kashiwa, Chiba 277-0882, Japan. Corresponding author: Yuxin Tong, (1) Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China; (2) Key Lab of Energy Conservation and Water Treatment of Agricultural Structures, Ministry of Agriculture, Beijing 100081, China 010-82106021; phone: +86-4-1082105983; e-mail: [email protected].

design energy-saving greenhouses with less energy demand and/or less energy losses. Another way is to improve the energy utilization efficiency and/or use renewable energy sources instead of fossil fuels that are directly related to CO2 emissions. One method of improving energy utilization efficiency and employing renewable energy sources is to use electricity-driven heat pumps instead of traditional combustion-based heating systems (Sami and Tulej, 1994; Hardin et al., 2008). Heat pumps are widely recognized as highly energy efficient systems for many applications such as heating, cooling, and dehumidification (Ozgener and Hepbasli, 2007). Heat pumps perform heating by moving heat energy efficiently from one heat energy source to another (Kulcar et al., 2008). Energy sources can be renewable energy sources including air, ground water, ground soil, and solar energy, which are clean, inexpensive, ever present in large quantities, and easy to access by using heat pump technologies (Chen and Lan, 2009). Therefore, heat pump technologies are economical, sustainable, and without producing harmful emissions on site. The heat pump technologies are being improved every year. Coefficient of performance (COP), indicating how much heat energy is gained or removed by the heat pumps compared with that consumed by their compressors, is an important performance index of heat pumps. In Japan, the COP for heating of household heat pumps increased from around 4.3 in 1998 to 7.1 in 2011 under the Japanese industrial standard conditions of 20°C indoors and 7°C outdoors, reported by Mitsubishi Company (2011). The COP is expected to increase to around 8 as heat pump technology further improves (Kozai, 2009). Thus, great potential reduction in energy consumption and CO2 emissions exists when employing the recently developed heat pumps for heating.

Applied Engineering in Agriculture Vol. 28(3): 401-406

© 2012 American Society of Agricultural and Biological Engineers ISSN 0883-8542

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In the recent years, there are many reports about greenhouse heating by using heat pumps, such as ground-source heat pump (e.g., Benli and Durmus, 2009; Cai et al., 2010; Benli, 2011), solar assisted ground-source heat pump (e.g., Ozgener and Hepbasli, 2005; Ozgener, 2010), and airsource heat pump (e.g., Aye et al., 2010; Tong et al., 2010). However, few studies have been published on the economic analysis and potential reduction in energy consumption and CO2 emissions when using household heat pumps with high COP for greenhouse heating. Therefore, the objectives of our experiment were to: 1) analyze the energy consumption and the energy utilization efficiency of the heat pumps and a kerosene heater, 2) analyze the CO2 emissions of the heat pumps and the kerosene heater, and 3) investigate the energy costs for the heat pumps and the kerosene heater.

MATERIALS AND METHODS EXPERIMENT SETUP Two North-South oriented, single-span, saddle-roof greenhouses built of steel pipes and located in Kashiwa, Japan (35°87′ N and 139°58′ E) were used during this experiment. Both greenhouses were 21 m long, 7.2 m wide and 3.7 m high. The roofs and side walls were covered with polyethylene terephthalate (PET) and polyvinyl chloride (PVC) films, respectively. The internal thermal screens of the roofs and side walls were made of polyester-woven cloth and polyolefin film, respectively. The thermal screens of the side walls were closed during the experiment, while those of the roofs were opened during the daytime (08:0017:00) and closed during the nighttime (17:00-08:00) by using a timing device. Ten air-to-air heat pumps (MSZ-SV288-W, Mitsubishi, Co., Tokyo, Japan; heating capacity: 2.8 kW each; COP: 5.4, for heating at the Japanese Industrial Standard conditions of 20°C indoors and 7°C outdoors), were installed along the walls of one greenhouse (Ghp). The internal units of the heat pumps were installed on supporting poles at the height of 1.5 m above the ground, while the external units were installed on supporting poles of 0.5 m above the ground and near to their internal coils. In another greenhouse (Gkh), one kerosene heater (KA-205, Nepon, Co., Tokyo, Japan; heating capacity: 23.3 kW), which has been widely used for greenhouse heating in Japan, was installed at the northern end, and supplied hot air through two plastic ducts laid on the ground along both side walls. The experiment was conducted from 1 January to 1 March 2009 during the nighttime (18:00-07:00) and the inside air temperature was maintained at 16°C in both greenhouses. Tomato (Solanum lycopersicum cv. Momotaro York) plants were grown at a density of 2.0 plants·m-2 during the experiment. In this experiment, the tomato plants and its culture conditions were identical to those described by Tong et al. (2010). MEASUREMENTS The air temperature and relative humidity inside the greenhouses were measured by sensors (SHT-71, Sensirion AG, Switzerland; precision: temperature ±0.4°C, relative

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humidity ±3%). The outside air temperature (Tout) and relative humidity (MT-060, EKO Instruments, Co., Ltd., Tokyo, Japan; precision: temperature ±0.3°C, relative humidity ±2%) were recorded in a small weather station 30 m from the experimental greenhouses. The electric-energy consumption of each heat pump (Whp) and the kerosene heater (Wkh) were measured by wattmeters (Clamp on Power Hitester 3169/3168, Hioki, Co., Nagano, Japan; resolution: 0.01Wh). The kerosene consumption of the kerosene heater (Vk) was measured by weighting the oil tank continuously during the experiment using an electric balance (LDS-60H, Shimadzu, Co., Tokyo, Japan; resolution: 0.01 kg). The heat transfer at the ground surface in the Ghp and Gkh were measured at two points with heat flux plates (MF-180M, EKO Instruments, Co., Ltd., Tokyo, Japan; resolution: 0.001 mW) in each greenhouse. All the data were recorded every minute. SENSOR CALIBRATIONS The air temperature and relative humidity sensors were calibrated by using Assmann aspiration psychrometer (Y-5011, Yoshino, Co., Tokyo, Japan) at air temperatures of -10°C, 0°C, 10°C, 20°C. The wattmeters were calibrated by using a standard calibration unit (Clamp on Power Hitester 9796, Hioki, Co., Nagano, Japan). The electric balance was calibrated by using its standard weight (60 kg). The heat flux plates were calibrated by the standard heat flux plates (MF-180M, EKO Instruments, Co., Ltd., Tokyo, Japan). PERFORMANCE CALCULATIONS Energy Consumption The hourly energy consumption of both greenhouses was estimated by: Qhp = Whp λe

(1)

Qkh = Wkh λe + Vk λk

(2)

where Qhp and Qkh = the hourly energy consumed by the heat pumps and the kerosene heater, respectively, (MJ); λk = the heat energy generation coefficient of kerosene, 36.7 MJ L-1 (Kozai, 2009); Vk = the hourly kerosene consumed by the kerosene heater (L); Whp and Wkh = the hourly electric-energy consumed by the heat pumps and the kerosene heater, respectively, (kWh); λe = the energy consumption rate of the power generation system of Tokyo Electric Power Co. Inc., 9.97 MJ kWh-1 (Ministry of Environment Japan, 2007). Energy Utilization Efficiency The hourly heat energy generated in the Ghp and Gkh (Qh-hp and Qh-kh, respectively) was estimated by the method described by Tong et al. (2010). The energy utilization efficiency (EUE) of the heat pump and the kerosene heater (EUEhp and EUEkh, respectively) was calculated by:

APPLIED ENGINEERING IN AGRICULTURE

EUEhp = EUEkh =

RESULTS AND DISCUSSION

Qh−hp Qh

(3)

Qh−kh Qkh

(4)

CO2 Emissions The hourly amount of CO2 emissions of both greenhouses was estimated by:

Mhp = Whp me

(5)

Mkh = Wkh me + Vk mk

(6)

where Mhp and Mkh = the hourly amount of CO2 emissions generated in the Ghp and Gkh, respectively, (kg); me = the CO2 generation rate of electricity, 0.425 kg kWh-1 (The Tokyo Electric Power Co., Inc., 2007); mk = the CO2 generation rate of kerosene, 2.49 kg L-1 (Ministry of Environment Japan, 2007). The Energy Consumption and CO2 Emissions Under Different Cops According to the COP values reported by Tong et al., (2010), the average hourly COP of 3, 4, 5, or 6 were assumed under the same air temperature conditions as the present experiment. Since the COP is defined as the ratio of the heat energy generated in the greenhouse (Qh-hp) to the electric-energy consumed by the compressors of the heat pumps, the Whp can be calculated by dividing the Qh-hp by the COP. The hourly energy consumption and CO2 emissions under different COP of 3, 5, or 6 were calculated by using equations 1 and 5, respectively. The Costs and the Associated CO2 Emissions per Unit Heat Energy Generated The costs and the associated CO2 emissions per unit heat energy generated were calculated by:

Ps =

Ps λs

(7)

Ms =

ms λs

(8)

where Ps = the cost per unit heat energy generated ($ MJ-1); ps = energy cost ($ unit-1); λs = heat energy generation coefficient per unit energy source [electricity, 3.6 MJ kWh-1; natural gas, 41.1MJ (Nm3)-1 (Kozai, 2009)]; Ms = associated CO2 emissions per unit heat energy generated (kg MJ-1); ms = CO2 generation rate per unit energy source (kg unit-1).

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THE ENERGY CONSUMPTION AND ENERGY UTILIZATION EFFICIENCY OF TWO HEATING SYSTEMS AS AFFECTED BY OUTSIDE AIR CONDITIONS The hourly energy consumption in both Ghp and Gkh decreased with increasing outside air temperature, when the inside air temperature was maintained at around 16°C (fig. 1). When the outside air temperature ranged between -5 and 6ºC, the hourly energy consumption per floor area in the Ghp was in the range of 0.22-0.56 MJ m-2, while that in the Gkh was in the range of 0.42-0.76 MJ m-2. The energy consumption was reduced by 25-65% in the Ghp compared with the Gkh. This reduction in energy consumption in both Ghp and Gkh increased with increasing outside air temperature. The reduction was higher than the previously reported energy saving of 42% by comparing the ground-source heat pump with coal-fired heating system (Cai et al., 2010) and 16% by comparing the air-to-water heat pump with LPG (liquefied petroleum gas)-fired heating system (Aye et al., 2010). The energy consumption of the heat pumps and kerosene heater is related to the difference between inside and outside greenhouse air temperature and/or the heating load of the greenhouses. Energy consumption increases with increasing inside air temperature set point. To achieve a particular inside air temperature, the heating load increases with decreasing outside air temperature. Accordingly, the energy consumption increased as outside air temperature decreased, especially in the Ghp during defrost mode (Hardin et al., 2008). Figure 1 shows scattered data for energy consumption in both Ghp and Gkh. This was because the heating load of the greenhouse was also affected by the outside wind speed, condensation on the cover and/or clouds. A strong outside wind speed increased the heat energy transmission through the cover material and heat transfer by ventilation (Bailey, 1994). Condensation on the cover reduced the transmittance and increased the emissivity of long-wave radiation (Walker and Walton, 1971; Pieters et al., 1995). The heat loss by long-wave radiation from the cover materials was lower on a cloudy day compared to a clear day (Nijskens et al., 1984). Larger scatter data of energy consumption were observed in the Gkh than that in the Ghp probably due to the difference in the control method of air temperature. The heat pump employed proportionalintegral-derivative (PID) control, while the kerosene heater employed a simple on/off control that can result in larger fluctuation of heat load as well as air temperature. The larger fluctuation of heat load may lead to the larger variation of energy consumption in the Gkh. The energy consumption of the heat pumps is related to their COP which is mainly affected by their working conditions and the heat pump technology. To achieve a certain heating requirement, the energy consumption can be reduced by using heat pumps with high COP, especially under a high outside air temperature and/or a low inside set point temperature (fig. 2). The maximum COP can be obtained if the ratio of the heating load of the greenhouse to the total heating capacity of the heat pumps is in a range of 0.6 to 0.8 (Kozai et al., 2009). When the ratio is lower than

403

80

Tin : 16.8±0.4 (ºC)

COP : 3

Hourly CO2 emissions (g m-2)

Hourly energy consumption (MJ m-2 )

0.9

heater

0.7

COP : 4 0.5

COP :5 COP : 6

0.3

0.1 -8

-6

-4

-2

0

2

4

40

G kh 2

y = 0.037 x − 1.9 x + 17 R 2 = 0.85

20

G hp -6

-4

-2

0

2

4

6

8

Tout (ºC)

the above range, the increase in COP with increasing outside air temperature and/or decreasing inside set point temperature is slowed down by the decreasing heating load. Thus, the reduction in energy consumption of the greenhouses may decrease with decreasing heating load of the greenhouse. The energy utilization efficiency of the heat pumps was 1.3 to 2.6 times higher than that of the kerosene heater (fig. 3). The energy utilization efficiency of the heat pumps increased with increasing COP, while that of the kerosene heater was almost constant. Thus, compared with combustion-based heating systems (such as kerosene heaters), technology improvements are making heat pumps increasingly competitive in terms of energy saving (Kozai, 2009).

Figure 4. Hourly CO2 emissions affected by the outside air temperature (Tout) in the greenhouse with heat pumps (Ghp) and the greenhouse with a kerosene heater (Gkh). Data collected on 7-11 January, 13-16 January, 25-26 January, 8-9 February, 19-20 February, and 26-28 February, 2009 are shown as examples. Each data point represents the hourly average.

In the Ghp, the heat pumps were driven by electricity (no CO2 was generated on site), so the CO2 emissions depended on the following 1) The efficiency of the power generation system: the higher the efficiency of producing the same amount of electricity, the lower the CO2 emissions, 2) The energy sources employed in the power generation system: the higher the percentage of renewable energy sources employed, the lower the CO2 emissions. For example, CO2 emissions in Norway, where renewable sources such as hydroelectric power are widely used, are significantly lower than in other European countries (Jenkins et al., 2008; Mancarella and Chicco, 2008). Similarly, France, where much power is generated by nuclear power plants, has relatively low CO2 emissions (Mancarella and Chicco, 2008; Omer, 2008), and 3) The COP of the heat pumps: the higher the COP, the lower the CO2 emissions (fig. 5). In the Gkh, the heater with the energy utilization efficiency of 0.8 on average was driven by kerosene (CO2 was generated on site), so the CO2 emissions were directly related to kerosene consumption. In figure 4, the spread of the data is due to the fluctuating energy consumption, as shown in figure 1. 80 Hourly CO2 emissions (g m-2 )

CO2 EMISSIONS DUE TO OPERATION OF THE HEAT PUMPS AND THE KEROSENE HEATER AS AFFECTED BY OUTSIDE AIR CONDITIONS Similar to the trend of hourly energy consumption, the hourly CO2 emissions in both Ghp and Gkh decreased with increasing outside air temperature (fig. 4). The hourly CO2 emissions per floor area in the Ghp were in the range of 9.5 to 24 g m-2, and in the Gkh were in the range of 31 to 55 g m-2. Thus, CO2 emissions were reduced by 56% to 79% in the Ghp compared with the Gkh. This reduction increased with increasing outside air temperature. Tin : 16.8±0.4 (ºC)

1.8

EUE

R 2 = 0.66

0

6

Figure 2. Hourly energy consumption affected by outside air temperature (Tout) with different COPs (coefficient of performance) of the heat pumps in the greenhouse with heat pumps and the greenhouse with a kerosene heater. The solid line shows the actual data of this experiment and the dotted line shows the simulated data.

EUEhp 1.2

0.6

y = 0.16 x 2 − 2.5 x + 46

60

Tout (ºC)

2.4

Tin: 16.8±0.4 (ºC)

Tin: 16.8±0.4 (ºC)

heater

60

40

COP :3 COP : 4 COP :5 COP : 6

20

EUEkh 0

0

-8

-6

-4

-2

0

2

4

6

Tout (ºC) Figure 3. The energy utilization efficiency of the heat pumps and the kerosene heater (EUEhp and EUEkh, respectively) affected by the outside air temperature (Tout). Each data point represents the mean of the energy utilization efficiency with its standard deviation.

404

-6

-4

-2

0

2

4

6

Tout (ºC) Figure 5. Hourly CO2 emissions affected by outside air temperature (Tout) with different COPs (coefficient of performance) of the heat pumps in the greenhouse with heat pumps and the greenhouse with a kerosene heater. The solid line shows the actual data of this experiment and the dotted line shows the simulated data.


Based on the above discussions, one effective way to mitigate CO2 emissions in the greenhouse industry is to use heat pumps with high COP and switch the energy sources from fossil fuel to renewable energy sources. Heat pumps will be a more attractive option to reduce CO2 emissions as technologies improve and as the power generation system is switched to renewable energy sources without CO2 emissions. ENERGY COSTS AND CO2 EMISSIONS BY USING THE ELECTRICITY-DRIVEN HEAT PUMPS AND COMBUSTIONBASED HEATING SYSTEMS WITH DIFFERENT COPS To estimate if one heating system is economically feasible and environmentally sustainable or not, a real-world analysis is necessary. Therefore, the costs and CO2 emissions per unit heat energy were calculated by using the combustion-based heating systems (taking kerosene/natural gas heaters as examples) and electricity-driven heat pumps with different COPs. Taking the costs per unit heat energy in Japan as an example (fig. 6 and table 1), for achieving the same amount of heat energy in a greenhouse: (1) The electricity-driven heat pumps would be cost-effective compared with the kerosene heater, if their COP was higher than around 2.0. This result indicates that the heat pumps used in the present experiment are cost saving since the COP of the heat pumps was always higher than 2.9. (2) The electricity-driven heat pumps would be cost effective compared with the natural gas heater, if their COP was higher than around 3.9. (3) The cost is much lower when using the natural gas heater compared to the kerosene heater. The energy cost is relatively steady when using the electricitydriven heat pumps since the price of the electricity remains more constant, while the prices of fossil fuels fluctuate over time. Figure 6 and table 2 show that the CO2 emissions were lower using the electricity-driven heat pumps compared to the kerosene/natural gas heaters in the present experiment since the COP of the heat pumps was always higher than 2.3. The CO2 emissions of the natural gas

0.06

0.3

0.04

0.2

0.02

0.1

0

0 1

2

3

4

5

6

COP Figure 6. The costs and CO2 emissions per unit heat energy by using the combustion-based heating systems (taking the kerosene/natural gas heaters as examples) and electricity-driven heat pumps with different COPs. Based on the data in table 1 and 2, three lines were showed as examples.

heater are lower than that of the kerosene heater when delivering the same amount of heat energy.

CONCLUSIONS When the air temperature inside the greenhouse was maintained at around 16°C and the air temperature outside the greenhouse ranged between -5°C and 6°C, the experimental results can be summarized as follows: 1. The hourly energy consumption in the greenhouse with heat pumps was in the range of 0.22 to 0.56 MJ m-2, while that in the greenhouse with a kerosene heater was in the range of 0.42 to 0.76 MJ m-2. 2. The energy utilization efficiency of the heat pumps was 1.3 to 2.6 times higher than that of the kerosene heater. 3. The hourly CO2 emissions in the greenhouse with heat pumps were in the range of 9.5 to 24 g m-2, while that in the greenhouse with the kerosene heater was in the range of 31 to 55 g m-2. 4. During the experiment, the heat pumps used for greenhouse heating were cost effective. Based on these results, it was concluded that heat pumps are energy- and carbon-saving technology, and offer considerable economic benefits. Moreover, heat pumps will become more attractive for efficient multi-purpose environmental control systems as their technology improves. Since the greenhouses used in this experiment are relatively small, thermal property in a commercial greenhouse with a larger floor area (e.g., >1000 m2) is different from that in this experiment. To adopt our evaluation methods with respect to energy consumption and CO2 emissions in larger greenhouses, further experiments are needed.

Table 2. CO2 emissions per unit heat energy from different energy sources. Electricity[a] France Japan EU-25 United States China Poland Energy Source CO2 emissions per unit heat energy (kg MJ-1) 0.014 0.119 0.108 0.156 0.197 0.286 [a] Jenkins et al. (2008). [b] Kozai (2009).

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0.4

combustion-based heaters heat pumps

CO2 emissions (kg MJ-1)

0.08 Energy costs ($ MJ-1)

Table 1. Costs per unit heat energy of three different energy sources in different countries. Cost per unit heat energy ($ MJ-1) Electricity[a] Natural gas[b] Kerosene[c] Country Japan 0.0438 0.0110 0.0216 Taiwan 0.0161 --[d] --[d] Korea 0.0207 0.0124 -USA 0.0190 0.0086 -UK 0.0375 0.0081 0.0220 France 0.0296 0.0145 0.0255 Italy 0.0767 0.0110 -[a] IEA: International Energy Agency (2010), Key World Energy Statistics. [b] OECD/IEA (2009), Energy Prices and Taxes 3rd Quarter. [c] IEA: International Energy Agency (2009), End-Use Petroleum Product Prices and Average Crude Oil Import Costs. [d] No available data.

Natural gas[b] Kerosene[b] 0.051 0.068

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NOMENCLATURE Abbreviations COP = coefficient of performance for heating Ghp = greenhouse with heat pumps Gkh = greenhouse with kerosene heater EUE = energy utilization efficiency Variables M = amount of CO2 emissions generated (kg) P = cost per unit heat energy generated ($ MJ-1) p = energy cost ($ unit-1) Q = hourly energy consumed or generated (MJ) T = air temperature (°C) W = electric-energy consumption (kWh) V = amount of kerosene consumption (L) m = CO2 generation rate (kg unit-1) λ = energy generation coefficient per unit energy source (MJ unit-1) Subscripts e = electricity h = heat energy hp = heat pumps in = inside greenhouses k = kerosene kh = kerosene heater out = outside greenhouses s = energy source
