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Heating Performance Analysis of an Air-to-Water Heat Pump Using Underground Air for Greenhouse Farming

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Taesub Lim
Taesub Lim
Taesub Lim
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Yong-Kyu Baik
Yong-Kyu Baik
Yong-Kyu Baik
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Daeung Danny Kim at King Fahd University of Petroleum and Minerals
Daeung Danny Kim
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Abstract and Figures

As one of the main businesses in Jeju-do in South Korea, specialized local products are grown in greenhouses. For greenhouse farming, it is preferable to use geothermal heat pump systems for energy conservation because of the stable temperature of the ground. In the same manner, heat pumps using underground air is recommended for greenhouse farming since underground air can easily be obtained from porous volcanic rocks in Jeju-do. However, direct usage of the underground air is not feasible for planting in the greenhouse or livestock care because the underground air is relatively humid and its temperature is low. For the present study, the heating performance of an air-to-water heat pump which used underground air as a heat source for greenhouse farming during the winter was assessed through measurements. In addition, the economic impact of the air-to-water heat pump (AWHP) was compared with a conventional air heater. According to the results, an AWHP can save more than 70% of the total heating costs compared with a conventional air heater. In sum, the utilization of the air-to-water heat pump using underground air can have a positive impact on reducing energy consumption as well as provide direct economic benefits.
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energies
Article
Heating Performance Analysis of an Air-to-Water Heat
Pump Using Underground Air for Greenhouse Farming
Taesub Lim 1, Yong-Kyu Baik 1,* and Daeung Danny Kim 2, *
1Department of Architectural Engineering, Seoil University, Seoul 02192, Korea; francis9@seoil.ac.kr
2Architectural Engineering Department, King Fahd University of Petroleum and Minerals (KFUPM),
Dhahran 31261, Saudi Arabia
*Correspondence: ykbaik29@seoil.ac.kr (Y.-K.B.); dkim@kfupm.edu.sa (D.D.K.)
Received: 29 June 2020; Accepted: 27 July 2020; Published: 28 July 2020


Abstract:
As one of the main businesses in Jeju-do in South Korea, specialized local products are grown
in greenhouses. For greenhouse farming, it is preferable to use geothermal heat pump systems for energy
conservation because of the stable temperature of the ground. In the same manner, heat pumps using
underground air is recommended for greenhouse farming since underground air can easily be obtained
from porous volcanic rocks in Jeju-do. However, direct usage of the underground air is not feasible for
planting in the greenhouse or livestock care because the underground air is relatively humid and its
temperature is low. For the present study, the heating performance of an air-to-water heat pump which
used underground air as a heat source for greenhouse farming during the winter was assessed through
measurements. In addition, the economic impact of the air-to-water heat pump (AWHP) was compared
with a conventional air heater. According to the results, an AWHP can save more than 70% of the total
heating costs compared with a conventional air heater. In sum, the utilization of the air-to-water heat
pump using underground air can have a positive impact on reducing energy consumption as well as
provide direct economic benefits.
Keywords: air-to-water heat pump; greenhouse; heating load; underground air
1. Introduction
While facing significant global warming, a lot of concerns have focused on energy use by buildings,
which accounts for more than half of the total energy consumption [
1
,
2
]. Thus, a reduction in building energy
consumption can lead to a reduction in CO
2
emissions. According to previous studies, energy savings
can be achieved by applying appropriate thermal insulation to building envelopes and the use of
advanced mechanical systems [
3
–
7
]. As building occupants spend more time in an indoor environment,
the importance of the selection of proper mechanical systems has become a main concern for building
stakeholders regarding thermal comfort and energy conservation [8].
Among current technologies for providing both heating and cooling, heat pumps have generally
applied for residential and commercial buildings by implementing demand response measures which
enable one to shift building energy loads [
9
–
11
]. These systems can maintain indoor thermal comfort
as well as provide a potential for energy savings in buildings by extracting energy from the air, water,
and soil [
12
–
15
]. Even though heat pumps often have excessive system complexity, they have increasingly
been used to building applications [
14
,
16
]. Among the broad range of heat pump applications, heat pump
technology can also be utilized in the area of agriculture.
Energies 2020,13, 3863; doi:10.3390/en13153863 www.mdpi.com/journal/energies
Energies 2020,13, 3863 2 of 9
According to a review article of Daghigh et al., heat pumps can be used to dry agricultural and
forest products even under low-temperature conditions [
17
]. In addition, Kosan et al. have analyzed the
efficiency of photovoltaic-thermal assisted heat pump drying systems [
18
]. Kumar et al. have used a
closed-loop heat pump system for drying herbal leaves [
19
]. Besides, several studies have proposed drying
methods based on heat pump technology for preserving agricultural products [
20
–
24
]. Among other
applications, heat pump technology has been utilized to control the thermal performance in greenhouses.
According to a study by Yang et al., the temperature in a greenhouse could be controlled by using an
earth-air heat exchanger system [
25
]. Moreover, Ozgener and Hepbasli have studied the performance
of a solar-assisted geothermal heat pump system for greenhouse heating under the climatic conditions
in Turkey, where the night temperature drops to 14
◦
C while the day temperature is about 31
◦
C [
26
–
28
].
Ozgener has also proposed the use of a hybrid solar-assisted geothermal heat pump technology with a
small wind turbine system for greenhouse heating [29].
For agricultural applications in South Korea, heat pumps have also applied to greenhouse space
heating. In the case of Jeju-do in South Korea, where about 20% of the people work in agriculture cultivating
local specialties like tangerines and mandarins. In the winter, these local specialties and others such
as livestock and flowers are generally raised or planted in greenhouses where heating is provided by
air-source heat pumps. Since the ground temperature is stable at a depth of 3 m–50 m, it is preferable
to use the ground as a heat source for both heating and cooling (i.e., geothermal heat pumps) [
25
,
30
].
Like geothermal heat pumps, air source heat pump systems using underground air are recommended
for greenhouse farming. Since Jeju-do is a dormant volcano in which the geographical features consist of
volcanic and sedimentary rocks, basanites, and so on, it can form underground air and water tables between
rocks and sediment [
31
]. Thus, it is easy to obtain underground air from porous volcanic rocks, in which
the underground air temperature ranges around 15
◦
C–18
◦
C. However, the use of this underground
air for planting or livestock care is not feasible because the underground air is relatively humid and its
temperature is low. Most studies have investigated the performance of the air-source heat pumps and there
have been few studies about air-to-water heat pumps using heat transferred from underground air. For the
present study, the heating performance of an air-to-water heat pump (AWHP) system intended for use in
greenhouse farming was assessed. To analyze the performance of the AWHP, the heating capacity and
COP were analyzed through measurements. In addition, the economic impact of the heating performance
of the AWHP for greenhouse farming was compared with a conventional air heater.
2. Heat Transfer from Underground Air to Heat Pumps
Figure 1shows the schematic diagram of the AWHP. Temperature sensors were located at the
underground air intake of the air/water direct contact heat exchanger, the compressor and evaporator,
the fan-coil unit, and the heat storage tank. In addition, water flow meters were located at each unit of
the AWHP. For the specification, the air/water direct contact heat exchanger is a cylinder of 1 m diameter
with a height of 2.5 m. From a borehole, underground air is drawn by the turbo blower to the air/water
direct contact heat exchanger through a cylinder of 0.3 m diameter. Similar to the process of cooling
towers, the airflow at the lower part of the heat exchanger is mixed with water sprayed from the top.
In addition, this heat exchanger utilizes plastic splash type fill to enhance the contact between air and water.
During the process, the heat is transferred from the humid underground air at 16
◦
C to water. Therefore,
as a heat source, the water flow is sent to the AWHP to provide further greenhouse heating. In the
AWHP, heat transferred from the air/water direct contact heat exchanger is delivered to the evaporator.
Passing the compressor, water is heated at the condenser. The control system uses a Programmable Logic
Controller (PLC).
Energies 2020,13, 3863 3 of 9
Energies 2020, 13, x FOR PEER REVIEW 3 of 9
Figure 1. A schematic diagram of the AWHP.
3. Method
3.1. The AWHP at the Greenhouse
To analyze the heating performance of the AWHP, the AWHP was installed at the greenhouse
for flower production located in Jeju-do in South Korea. The size of the greenhouse was 330 m2. The
specifications of the AWHP are presented in Table 1.
Table 1. Specifications of the AWHP.
Component Specification
Air/water direct contact heat exchanger
Material Stainless steel
Diameter (mm) 1000
Height (mm) 2500
Heat transfer fluid Normal water
Heat pump
Compressor
Type High-temperature scroll type
Capacity 35 kW (10 RT)
Voltage 380 V (3 Phase)
Condenser/Evaporator Flat type heat exchanger
Refrigerant R22
Heat storage tank
Diameter (mm) 2000
Height (mm) 2000
Heat storage fluid Normal water
Turbo blower Capacity 7.5 kW
Airflow rate 102 m3/min
Air/Water
Direct
Contact
Heat
Exchanger
Underground Air
Heat Pump
Heat
Storage
Tan k
Fan
Coil
Unit
Greenhouse
Evaporator
Condenser
Expansion
Valve
Compressor
Underground Air in
Wate r in
Wate r ou t
Underground air out
Wate r ba sin
Figure 1. A schematic diagram of the AWHP.
3. Method
3.1. The AWHP at the Greenhouse
To analyze the heating performance of the AWHP, the AWHP was installed at the greenhouse for flower
production located in Jeju-do in South Korea. The size of the greenhouse was 330 m
2
. The specifications of
the AWHP are presented in Table 1.
3.2. COP of the AWHP
For the control of the AWHP, a blower, an air/water direct contact heat exchanger, water pumps were
operated to heat or cool the water in the heat storage tank to reach the setpoint temperature. In the case
that the setpoint temperature of the AWHP does not reach the heating or cooling set temperatures in the
greenhouse, water pumps of the heat storage tank and the fan of the fan-coil unit were operated. To measure
the Coefficient of Performance (COP) for the AWHP in a heating application, T-type thermocouples and
ultrasonic flowmeters (PT868, Panametrics, Boston, MA, USA) were located at the condenser, the evaporator,
and the heat storage tank. In addition, the power usage of the AWHP was measured by using a power
meter (CW240, Yokogawa, Tokyo, Japan). The specifications of the equipment are presented in Table 2.
Energies 2020,13, 3863 4 of 9
Table 1. Specifications of the AWHP.
Component Specification
Air/water direct contact heat exchanger
Material Stainless steel
Diameter (mm) 1000
Height (mm) 2500
Heat transfer fluid Normal water
Heat pump
Compressor
Type High-temperature scroll type
Capacity 35 kW (10 RT)
Voltage 380 V (3 Phase)
Condenser/Evaporator Flat type heat exchanger
Refrigerant R22
Heat storage tank
Diameter (mm) 2000
Height (mm) 2000
Heat storage fluid Normal water
Turbo blower Capacity 7.5 kW
Airflow rate 102 m3/min
Table 2. Specifications of the equipment.
PT868
- Non-Intrusive Liquid Flowmeter
- Velocity range: −12.2 m/s to 12.2 m/s
- Accuracy: 2%
-
Fluid temperature:
−
40
◦
C to 150
◦
C
CW240
- Voltage: 150 V to 1000 V
- Frequency range: 45 Hz to 65 Hz
- Accuracy: ±0.2% reading
- Display interval: Approx. 0.5 s
Before calculating the COP, the heating capacity was calculated using Equation (1) below:
Qh=ρw·Vw·cw·(Tw,o−Tw,i)(1)
where Q
h
is the heating capacity of the AWHP (kcal/h, 1 kcal/h=0.0012 kW). In addition,
ρw
,V
w
, and c
w
are water density (kg/m
3
), water flow (m
3
/s), and the specific heat capacity of water (kJ/kg
◦
C), respectively.
T
w,o
and T
w,i
are the water temperatures at the outlet and inlet of the condenser (
◦
C). Using Equation (2) [
32
],
the COP was calculated for open-loop and closed-loop systems. In an open-loop system, heated water
from the condenser is dumped directly to the outside, while water at the heat storage tank circulates and is
reheated at the condenser and returns to the heat storage tank for a closed-loop system:
COPh=Qh
PHP
(2)
where COP
h
is the COP of the AWHP for the heating application. Moreover, P
HP
is the power usage
of the AWHP (kW). To compare the heating energy consumption and temperature distributions in the
greenhouse, a conventional heat pump system was also installed in the same size of the greenhouse.
Energies 2020,13, 3863 5 of 9
4. Results
4.1. The Temperature Profiles of the System During the Condenser Operation
It is important to figure out the relationship between the temperature changes of each component
of the AWHP and the outdoor temperature conditions. The measurement results are shown in Figure 2.
For the open-loop system, the heating performance was observed for 7 min until the water was drawn off,
while it took 3 h to heat 6 tons of water from 18 ◦C to 50 ◦C for the closed-loop system on 24 March 2015.
Energies 2020, 13, x FOR PEER REVIEW 5 of 9
runoff. For the closed-loop system, the outdoor air temperature was slightly decreased from 10 °C to
8 °C, while the underground air temperature was also maintained at 15 °C during the measurement.
In the case of the evaporator, both water temperatures at the inlet and outlet were dropped from
about 15 °C to 5 °C after 10 min from the beginning of the measurement. It was caused by the low
temperature in the condenser at the early operation, which required a large amount of heat transfer.
After the temperature in the condenser was increased, the amount of heat transfer was decreased.
Therefore, the temperatures at the inlet and outlet of the evaporator were slightly increased to 8 °C
after the temperature was slightly increased in the condenser. Simultaneously, the underground air
temperature at the outlet of the air/water direct contact heat exchanger was also increased.
Figure 2. Temperature profiles of the system for open-loop and closed-loop application.
4.2. Heating Capacity for the Heating Application
Figure 3 shows the heating capacity for the open-loop and closed-loop applications. The
observed heating capacity for the open-loop and closed-loop systems was ranged from 35,000 to
41,000 kcal/h and 27,000 to 40,000 kcal/h, respectively. In addition, the heat extracted by the air/water
direct contact heat exchanger for the open-loop system was about 28,000 kcal/h. For the closed-loop
system, the extracted heat was decreased from 30,000 kcal/h to 18,000 kcal/h.
Figure 3. The heating capacity for the open-loop and closed-loop systems.
Based on the values of the heating capacity and electricity consumption, the COP values were
calculated using Equation (2). As can be seen in Figure 4, the COP values were ranged from 4.3 to 5.5
for the open-loop system, while the COP value was decreased from 5.0 to 2.5 for the closed-loop
system as the inlet water temperature of the condenser was increased to 50 °C. For energy
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
123456789101112131415
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
1 3 5 7 9 11131517192123252729313335
Figure 2. Temperature profiles of the system for open-loop and closed-loop application.
When measuring the water temperature of the open-loop system, the outdoor air temperature was
about 13.8
◦
C, while the underground air temperature at the inlet of the air/water direct contact heat
exchanger was maintained at 15
◦
C. Because of the short observation, there were a little temperature
changes at both inlet and outlet of the condenser. In the case of the evaporator, about a 5
◦
C decrease in
water temperature was observed at both inlet and outlet due to the rapid water runoff. For the closed-loop
system, the outdoor air temperature was slightly decreased from 10
◦
C to 8
◦
C, while the underground
air temperature was also maintained at 15
◦
C during the measurement. In the case of the evaporator,
both water temperatures at the inlet and outlet were dropped from about 15
◦
C to 5
◦
C after 10 min
from the beginning of the measurement. It was caused by the low temperature in the condenser at the
early operation, which required a large amount of heat transfer. After the temperature in the condenser
was increased, the amount of heat transfer was decreased. Therefore, the temperatures at the inlet and
outlet of the evaporator were slightly increased to 8
◦
C after the temperature was slightly increased in the
condenser. Simultaneously, the underground air temperature at the outlet of the air/water direct contact
heat exchanger was also increased.
4.2. Heating Capacity for the Heating Application
Figure 3shows the heating capacity for the open-loop and closed-loop applications. The observed
heating capacity for the open-loop and closed-loop systems was ranged from 35,000 to 41,000 kcal/h and
27,000 to 40,000 kcal/h, respectively. In addition, the heat extracted by the air/water direct contact heat
exchanger for the open-loop system was about 28,000 kcal/h. For the closed-loop system, the extracted
heat was decreased from 30,000 kcal/h to 18,000 kcal/h.
Energies 2020,13, 3863 6 of 9
Energies 2020, 13, x FOR PEER REVIEW 5 of 9
runoff. For the closed-loop system, the outdoor air temperature was slightly decreased from 10 °C to
8 °C, while the underground air temperature was also maintained at 15 °C during the measurement.
In the case of the evaporator, both water temperatures at the inlet and outlet were dropped from
about 15 °C to 5 °C after 10 min from the beginning of the measurement. It was caused by the low
temperature in the condenser at the early operation, which required a large amount of heat transfer.
After the temperature in the condenser was increased, the amount of heat transfer was decreased.
Therefore, the temperatures at the inlet and outlet of the evaporator were slightly increased to 8 °C
after the temperature was slightly increased in the condenser. Simultaneously, the underground air
temperature at the outlet of the air/water direct contact heat exchanger was also increased.
Figure 2. Temperature profiles of the system for open-loop and closed-loop application.
4.2. Heating Capacity for the Heating Application
Figure 3 shows the heating capacity for the open-loop and closed-loop applications. The
observed heating capacity for the open-loop and closed-loop systems was ranged from 35,000 to
41,000 kcal/h and 27,000 to 40,000 kcal/h, respectively. In addition, the heat extracted by the air/water
direct contact heat exchanger for the open-loop system was about 28,000 kcal/h. For the closed-loop
system, the extracted heat was decreased from 30,000 kcal/h to 18,000 kcal/h.
Figure 3. The heating capacity for the open-loop and closed-loop systems.
Based on the values of the heating capacity and electricity consumption, the COP values were
calculated using Equation (2). As can be seen in Figure 4, the COP values were ranged from 4.3 to 5.5
for the open-loop system, while the COP value was decreased from 5.0 to 2.5 for the closed-loop
system as the inlet water temperature of the condenser was increased to 50 °C. For energy
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
123456789101112131415
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
1 3 5 7 9 11131517192123252729313335
Figure 3. The heating capacity for the open-loop and closed-loop systems.
Based on the values of the heating capacity and electricity consumption, the COP values were
calculated using Equation (2). As can be seen in Figure 4, the COP values were ranged from 4.3 to 5.5 for
the open-loop system, while the COP value was decreased from 5.0 to 2.5 for the closed-loop system as the
inlet water temperature of the condenser was increased to 50
◦
C. For energy consumption, the electricity
usage for the open-loop system was decreased, while it was increased for the closed-loop system. It can be
seen that the continuous operation of the AWHP for increasing the water temperature in the heat storage
tank caused an increase in electricity consumption.
Figure 4. The COP and energy consumption for the open-loop and closed-loop systems.
Figure 5shows the comparison of air temperature in the greenhouse operated by the AWHP and a
conventional air heater. When the outdoor air temperature varied from 7
◦
C to 12
◦
C on 19 April 2015,
the air temperature in the greenhouse heated by the conventional air heater was ranged from 19
◦
C to
26
◦
C, while the greenhouse heated by the AWHP showed a small air temperature fluctuation which
ranged from 20
◦
C to 22
◦
C. Based on the results, the AWHP can provide more stable heating than the
conventional air heater.
Energies 2020,13, 3863 7 of 9
Energies 2020, 13, x FOR PEER REVIEW 6 of 9
consumption, the electricity usage for the open-loop system was decreased, while it was increased
for the closed-loop system. It can be seen that the continuous operation of the AWHP for increasing
the water temperature in the heat storage tank caused an increase in electricity consumption.
Figure 4. The COP and energy consumption for the open-loop and closed-loop systems.
Figure 5 shows the comparison of air temperature in the greenhouse operated by the AWHP and
a conventional air heater. When the outdoor air temperature varied from 7 °C to 12 °C on 19 April 2015,
the air temperature in the greenhouse heated by the conventional air heater was ranged from 19 °C
to 26 °C, while the greenhouse heated by the AWHP showed a small air temperature fluctuation
which ranged from 20 °C to 22 °C. Based on the results, the AWHP can provide more stable heating
than the conventional air heater.
Figure 5. Comparison of heating performance between the AWHP and a conventional air heater.
4.3. Economic Assessment of the AWHP
To analyze the economic impact of the AWHP, the heating cost was compared with that of a
conventional air heater (Table 3). Since each system used different energy sources, the total heating
cost for these two systems was compared. From February to June in 2016, the AWHP consumed 4064
kWh/m2 of electricity at a cost of ₩45.5/kWh (South Korea Won, i.e., $0.04 (US Dollar)). In the case of
the conventional air heater, about 889.8 liters of diesel/m2 were consumed at ₩840 per liter (South
Korea Won, i.e., $0.70 (US Dollar)). As a result, the AWHP only consumed about 25% of the total
heating cost of the conventional air heater.
7
7.5
8
8.5
9
9.5
10
10.5
0.0
1.0
2.0
3.0
4.0
5.0
6.0
123456789101112131415
0
2
4
6
8
10
12
14
0.0
1.0
2.0
3.0
4.0
5.0
6.0
1357911131517192123252729313335
0.0
10.0
20.0
30.0
Figure 5. Comparison of heating performance between the AWHP and a conventional air heater.
4.3. Economic Assessment of the AWHP
To analyze the economic impact of the AWHP, the heating cost was compared with that of a
conventional air heater (Table 3). Since each system used different energy sources, the total heating cost for
these two systems was compared. From February to June in 2016, the AWHP consumed 4064 kWh/m
2
of
electricity at a cost of
₩
45.5/kWh (South Korea Won, i.e., $0.04 (US Dollar)). In the case of the conventional
air heater, about 889.8 liters of diesel/m
2
were consumed at
₩
840 per liter (South Korea Won, i.e., $0.70
(US Dollar)). As a result, the AWHP only consumed about 25% of the total heating cost of the conventional
air heater.
Table 3. Heating cost comparison for the AWHP and a conventional air heater.
System Energy Consumption Total Heating Cost Comparison (%)
South Korea Won US Dollar
AWHP 4064 kWh/m2(Electricity) ₩184,912 $154 24.7
Conventional air heater 889.8 liters of diesel/m2₩747,432 $621 100
5. Discussion and Conclusions
As one of the main businesses in Jeje-do in South Korea, agriculture is supported by the South Korean
Government. Heat pump systems using underground air are recommended for greenhouse farming,
however, the direct use of underground air may harm greenhouse products because of the high humidity
and low temperature of the underground air. The present study assessed the heating performance of an
AWHP for greenhouses in Jeju-do through measurements. Specifically, the AWHP used the heat extracted
from water in which the heat was transferred from underground air through an air/water direct contact
heat exchanger.
During the measurement period, the outdoor air temperature was in the range of 7
◦
C to 12
◦
C,
while the underground air temperature was maintained at 15
◦
C. When the water temperature of the heat
storage tank ranged 40
◦
C–45
◦
C, the COP was 2.1 to 2.7 and the heating capacity was 34.9 kW–44.2 kW.
Comparing the heating cost with the conventional air heater from February to June, the AWHP consumes
about 25% of the total heating cost.
Energies 2020,13, 3863 8 of 9
When delivering underground air at a rate of 102 m
3
/min, the underground air temperature dropped
from 15
◦
C to 10
◦
C and 32.6 kW of thermal energy was obtained. This was slightly lower than that
generated by conventional heat pumps since the heat energy was transferred twice through the air/water
direct contact heat exchanger and the evaporator. In addition, the COP for the closed-loop system decreased
from 5.0 to 2.5. Thus, it can be seen that an increase in water temperature in the heat storage tank during the
early AWHP operation reduced the amount of heat absorbed by the condenser and evaporator. The heat
reduction ultimately caused the AWHP to operate continuously leading to an increase in the electricity
consumption of the AWHP.
Considering the outcome of the present study, the AWHP system is more energy and cost-efficient
than a conventional mechanical system. Moreover, the utilization of the AWHP for greenhouse farming
can reduce greenhouse gas emissions. For further study, it is necessary to find a feasible solution to increase
the COP for the closed-loop application. In addition, energy-advanced or more sophisticated controlled
conventional air heaters could be used for more accurate comparison with the AWHP. Furthermore,
life cycle cost analysis will be conducted considering factors such as capital investments.
Author Contributions:
T.L. and Y.-K.B. designed and performed the experiments; D.D.K. wrote the manuscript and
analyzed the data. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution (CC BY)
license (http://creativecommons.org/licenses/by/4.0/).
... In addition, greenhouse farming has been spreading rapidly in the study area. Some grasslands and shrublands have been used to grow crops [43,44]. ...
Evaluation and Optimization of Landscape Spatial Patterns and Ecosystem Services in the Northern Agro-Pastoral Ecotone, China
Article
Full-text available
  • Sep 2024
The alteration of landscape spatial patterns (LSPs) and ecosystem services (ESs) in watersheds can have detrimental effects on the local environment and community. However, a comprehensive understanding of the current state of LSPs and ESs in watersheds around Winter Olympic venues in China is limited. Here, we assessed current LSPs and ESs and developed optimization strategies for the Xigou watershed around Winter Olympic venues in the northern agro-pastoral ecotone of China. The results indicated that the main land use type was grassland in the Xigou watershed, and landscape types were relatively homogenous. All three ESs (water yield, sediment retention, and carbon storage) generally improved from 2004 to 2020. For ESs, there was the lowest total volume of water yield in 2004 (637.44 × 104 m3). But sediment retention (10.54 × 106 t, 18.13 × 106 t, 13.28 × 106 t, and 16.85 × 106 t) had an upward, then downward, then upward trend before and after ERP. Carbon storage grew steadily. Correlation analysis suggested that the three ESs were closely related to the landscape spatial indices of average patch area (AREA_MN), contagion index (CONTAG), and Shannon’s evenness index (SHEI). AREA_MN, CONTAG, and SHEI in the eastern part of the study area promoted sediment retention and carbon storage, while in the southwestern part of the study area, they inhibited water yield and sediment retention. The results suggest that improving sediment retention by optimizing land use and cover change (LUCC) and LSPs is the main approach to further enhance ESs in the study area. Our study suggests that the inclusion of multiple landscape pattern indices can provide a more comprehensive representation of regional ecosystem service.
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... R. O'Hegarty et al. [14] reviewed and analyzed the performance of air/water heat pumps, and they found that the heating seasonal performance of the monitored case study heat pump was 2.49, mainly attributed to pipeline losses and inaccurate weather data. Lim et al. [15] exploited underground air as a heat source for winter greenhouse agriculture and compared the economics of air/water heat pumps (AWHPs) with those of traditional air heaters, and it was revealed that AWHPs with air preheating can save more than 70% of the total heating costs. Kowalski et al. [16] applied TRNSYS to the waste heat recovery of air/water electric compressor heat pumps in multi-residential buildings at five locations, and they found that the most energy-efficient system was the one with the lowest heating system temperature. ...
Experimental Study on the Combined Heat Storage and Supply of Air/Water-Source Heat Pumps
Article
Full-text available
  • Sep 2024
As the application of renewable energy becomes increasingly extensive, heat pump technology with renewable energy as the heat source is achieving good results. Air-source heat pumps and water-source heat pumps can be widely used in cold areas. In this work, an integrated combined storage and supply system of an air-source heat pump and a water-source heat pump was studied, and the heating characteristics of the system at the beginning, middle, and end of the heating period were examined. It was found that, when the outdoor temperature of the system was very low, the efficiency of the combined storage and supply system reached the highest value of 2.57 when the source-side water tank was kept at 30 °C, and the performance of the combined storage and supply system was better than that of the air-source heat pump and the water-source heat pump in cold regions. Meanwhile, the independent storage of the air-source heat pump and the combined storage and supply system can be used for heating at the beginning and end of the heating period.
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... They have been shown to perform well even at low temperatures, making them suitable for regions with harsh winters. ASHPs, on the other hand, have been found to be effective in greenhouse farming and are recommended for such applications [30]. ...
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... In some of the studies there is indication to the heating performance of the AWHP system with a storage tank for the greenhouse heating by calculating COP of the system [13]. Other researchers have carried out economic analysis of the AWHP system in the heating mode, where the results forecast an average COP of 4.5 and 70% reduction in the energy cost, when compared with a conventional air heating system [14]. The convenient installation and comparatively low cost of the air source heat pumps makes them a viable option for energy to greenhouses [15]. ...
Development of technological process solutions in modular system of solar electricity and heat supply for greenhouses
Conference Paper
  • May 2023
Considering the recent rapid increase of the energy costs for heating commercial greenhouses, solutions for more efficient use of the solar energy are needed. In this study there is investigated application of solar energy for greenhouse heating with a joint equipment, which consists of 8.0 kW solar photovoltaic panels (PV), coupled with a 5.0 kW air-to-water heat pump, 15.0 kW solar collectors (SC), a 10 kWh solar energy accumulator (SEA), and 1600 litre water heat accumulators (WHA). The tests of the joint equipment were made in a 50 m2 experimental greenhouse (EG) in several stages: testing of heat accumulation in the water-based heat tanks, testing of the amount of the solar energy, produced by the photovoltaic panels, used for direct water heating and for the operation of the air-to-water heat pump with and without the solar accumulator; and in the periods with insufficient solar energy directly from the grid. Surplus heat from the system was used to heat the irrigation water or to drive the biomass dryer. The temperatures in various points of the heating system were measured and analysed together with average temperatures and the data about the heat and solar energy. The results of the 2022 study showed that in EG, at the average annual solar intensity of 1227.4 kWh·m-2, 6762.7 kWh of electricity was produced by PV panels, and 12385.2 kWh of heat energy by SC. In the industrial greenhouse (IG), where the total energy consumption during the period, when natural gas was used, was 17.95 m3·m-2 or 163.73 kWh·m-2, causing climate change-promoting gas emissions (GHG) of 30.29 kg CO2·m-2; the experimental greenhouse (EG) emission factor was not detected. We concluded that, without a long–term energy storage system, covering several weeks of heat consumption, the solar energy system should be supplemented with external energy sources to cover the extended periods with insufficient energy supply at the beginning and the end of the vegetation season.
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... They suggested using this technique in a greenhouse with lower heating setpoints to maximize the economy generated by this system. Other authors used underground air from a volcanic site as a source for an air to water heat pump used for greenhouse heating [16]. ...
Runaround coils as a way to improve the efficiency of heat pumps for the dehumidification of nordic greenhouses
Conference Paper
Full-text available
  • May 2023
Greenhouses generate greenhouse gas from the combustion of fossil fuel for heating and dehumidification. Heat pumps can use electricity generated from renewable to do both, but the heat pump performance might be improved by optimizing the cycle for the specific requirements of a greenhouse. In this work, runaround coils are added to a single-speed R-134a heat pump. A Dymola heat pump example model is used with the addition of an air cycle and additional water coils. Results demonstrated that runaround coils are an efficient way to increase simultaneously and significantly the heating Coefficient of Performance, the condensate flowrate and the exergy efficiency of the system, by 8.5%, 16.3% and 13.1%, respectively, for interior conditions typical of Nordic greenhouses.
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... ASHPs are the most widely used heat pumps and are present in numerous countries, being often preferred in moderate climates and in those existing houses located in highdensity urban areas with the limited surrounding land. Air-to-air heating alternatives are widely distributed across southern Europe and Asia, where the climate conditions allow their use for both heating and air conditioning [22][23][24][25]. ...
Comparative Analysis of Ground Source and Air Source Heat Pump Systems under Different Conditions and Scenarios
Article
Full-text available
  • Jan 2023
The current energy context demands the use of environmentally friendly solutions that contribute to the displacement of traditional fossil fuels. In this regard, heat pumps have become an important tool in the decarbonization of the heating and cooling energy system. With the aim of providing new information in the field, this research is conducted to analyze the suitability of a Ground Source Heat Pump (GSHP) and an Air Source Heat Pump (ASHP) in two different scenarios. Systems are designed to cover the heating needs of a building placed in a cold climate area, characterized by being in a thermally and geologically favorable formation (Case 1), and in a mild climate location where the geology is not so appropriate for the thermal exchange with the ground (Case 2). Results highlight the need to perform an exhaustive study of the subsoil and the external conditions of the area for a reliable selection. In Case 1, the ASHP option is discarded due to the demanding outdoor air requirements that rocket the operating costs of the system. In Case 2, both solutions are viable, with the geothermal alternative preferred if the initial investment can be assumed, providing economic advantages from the 17th year of the system operation.
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... The use of industrial waste heat could be a potential solution to lower the heating costs of greenhouses as in the case of low-temperature industrial wastewater [94], lowtemperature waste heat [95] and the waste heat from biogas driven power plants [96]. The technical, economic and environmental performance of hybrid systems for heating and cooling was investigated with the aim of enhancing the utilization of renewable energy resources in the greenhouse sector; this is the case of: solar photovoltaic system assisted earth-to-air heat exchangers consisting of underground air tunnels [97]; solar assisted water-to-water heat pumps [98]; air-to-water heat pumps using underground air [99]; PV assisted ground source heat pumps [100]; and heating systems based on solar-hydrogen energy and biomass or geothermal energy [79,101]. ...
Research Trends on Greenhouse Engineering Using a Science Mapping Approach
Article
Full-text available
  • Sep 2022
Horticultural protected cultivation has spread throughout the world as it has proven to be extremely effective. In recent years, the greenhouse engineering research field has become one of the main research topics within greenhouse farming. The main objectives of the current study were to identify the major research topics and their trends during the last four decades by analyzing the co-occurrence network of keywords associated with greenhouse engineering publications. A total of 3804 pertinent documents published, in 1981–2021, were analyzed and discussed. China, the United States, Spain, Italy and the Netherlands have been the most active countries with more than 36% of the relevant literature. The keyword cluster analysis suggested the presence of five principal research topics: energy management and storage; monitoring and control of greenhouse climate parameters; automation of greenhouse operations through the internet of things (IoT) and wireless sensor network (WSN) applications; greenhouse covering materials and microclimate optimization in relation to plant growth; structural and functional design for improving greenhouse stability, ventilation and microclimate. Recent research trends are focused on real-time monitoring and automatic control systems based on the IoT and WSN technologies, multi-objective optimization approaches for greenhouse climate control, efficient artificial lighting and sustainable greenhouse crop cultivation using renewable energy
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Advances in coupled use of renewable energy sources for performance enhancement of vapour compression heat pump: A systematic review of applications to buildings
Article
  • Feb 2023
  • APPL ENERG
Vapour compression heat pump provides efficient cooling and heating through the use of various renewable energy sources and plays a significant role in the energy conservation of modern buildings. Its performance enhancement gains increasing attention under the background of conventional energy and environmental crisis. Since the efficiency of a heat pump cycle is closely related to its thermal source condition, a stable high-temperature heat source or low-temperature cold source could benefit its heating or cooling efficiency a lot. Following this improvement strategy, significant progress has been made over the past years on the use of different renewable energy sources as coupled thermal sources of heat pumps. However, a specialised review that summarises relevant studies remains to be published. This paper aims at presenting operating mechanisms, characteristics, benefits, and limitations of different energy source coupling methods for heat pump performance enhancement towards its building application. The most concerning research on solar-assisted air source heat pumps, solar-assisted ground source heat pumps, and air-ground dual source heat pumps is emphasised. The existing thermal source coupling attempts under other renewable energy fields, such as waste heat, biomass, and water sources, are also summarised. Recommendations for further research and marketisation of such technologies are made based on detailed performance characterisation and comparison. The review is expected to benefit researchers in heat pumps and relevant applied energy fields to understand the challenges and opportunities in thermal source coupling technology and to promote its research and application development.
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Design and energy flexibility analysis for building integrated photovoltaics-heat pump combinations in a house
Article
  • Aug 2022
  • RENEW ENERG
This paper considers three configurations of air source heat pumps and building-integrated photovoltaic (BIPV) systems in a solar house. This study evaluates energy efficiency and flexibility when interacting with a smart grid. The reference case is a 5 kW BIPV system on the roof with a separate air-source heat pump water heater (HPWH). In the two more novel options we have: first, a 5 kW BIPV/thermal (BIPV/T) roof system where the heated air from the BIPV/T system is ducted to the air source of the HPWH, which also contains integrated thermal storage (hot water); in the second case we have an attached solarium with a 5 kW semi-transparent photovoltaic façade, with the solar heated air in the solarium connected to the air source of the HPWH with integrated water thermal storage. The three cases are modelled with an explicit finite difference thermal network model, and energy performance is determined and compared over a typical heating season in Montreal, Canada. The energy flexibility potential of the options is compared for different scenarios, such as heating the thermal water storage during the daytime (e.g. using the solar heat in the novel options) and using it for space heating during the time that the grid is under stress (and may have price incentives). Results show that the second novel case utilizing the solarium air as the source of the heat pump resulted in an over 80% energy use reduction relative to the reference. In comparison, the BIPV/T configuration had around 20% reduction compared to the reference case. The tank volume and solarium size had the highest impact on the flexibility of the system. Optimal sizes for the tank volume were typically between 300 and 600 L for the house with a floor area of 116 m².
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Model complexity of heat pump systems to investigate the building energy flexibility and guidelines for model implementation
Article
Full-text available
  • Dec 2019
  • APPL ENERG
Building performance simulation (BPS) is a powerful tool for engineers working in building design and heating, ventilation and air-conditioning. Many case studies using BPS investigate the potential of demand response (DR) measures with heat pumps. However, the models are often simplified for the components of the heat pump system (i.e. heat pump, electric auxiliary heater and storage tank) and for their interactions. These simplifications may lead to significant differences in terms of DR performance so that more comprehensive models for a heat pump system may be necessary. The contribution of this work is twofold. Firstly, this work investigates the influence of the modeling complexity of the heat pump control on different key performance indicators for the energy efficiency, the DR potential and the heat pump operation. To this end, the performance of six different heat pump controls is compared. Secondly, it describes the implementation of a comprehensive control for a heat pump system in BPS tools. This control is not often documented in the BPS literature and is error-prone. Generic pseudo-codes are provided, whereas IDA ICE is taken as an example in the case study. A predictive rule-based control is implemented to study price-based DR of residential heating. It is shown that a realistic operation of the heat pump system can be achieved using the proposed modeling approach. The results prove that the modeling complexity of the system control has a significant impact on the performance indicators, meaning that this aspect should not be overlooked. For some performance indicators, e.g. the annual energy use for heating and average water tank temperature, it is shown that a proportional (P-) and proportional-integral (PI-) control can lead to similar results. If the heat pump operation is investigated in detail and a short-time resolution is required, the difference between P- and PI-controls and their tuning is important. As long as the heat pump operation and electrical power at short timescales are not of importance, the choice of controller (P or PI) is not crucial. However, the use of P-control significantly simplifies the modeling work compared to PI-control. If DR is performed for domestic hot water, it is also demonstrated that the prioritization of domestic hot water heating can indirectly influence the operation of auxiliary heaters for space-heating, significantly increasing the use of electricity. However, the electricity use is only slightly increased if DR control is only used for space heating.
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Experimental study of moisture removal rate in Moringa leaves under vacuum pressure in closed-loop heat pump dryer
Article
  • May 2020
Herbal industries are facing challenges while processing the herbal leaves. Herbal leaves are processed using various methods such as convection drying, chemical drying, vacuum drying, microwave drying, etc. The common issues encountered during the time of drying the herbal leaves are lost in phytochemicals, nutrients and colour. Heat pump drying is an important method by which the drying can be made without influencing of environmental condition and by controlling the drying properties without loss in phytochemicals, nutrients and colour. In this proposed work the experimental device, namely closed-loop heat pump dryer is designed and developed for conducting experiments. For this study, Moringa leaves were considered as herbal leaves. The vacuum is introduced in the heat pump dryer and the Moisture Removal Rate (MRR) is calculated under this condition. The experiments were conducted with vacuum pressures of 0.9 & 0.8 bar and the air velocities of 1.1 m/s & 1.4 m/s respectively. In this study, the experiments were conducted under the above-said conditions for several periods with temperature transients. Based on the experimental study it is observed that the moisture removal rate is increased significantly under low pressure with high temperature and velocity.
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Experimental energy, exergy, economic and exergoeconomic analyses of batch-type solar-assisted heat pump dryer
Article
  • Apr 2020
  • RENEW ENERG
In the present study, a batch-type solar-assisted heat pump dryer (SAHPD) has been developed and experimented in both simple heat pump dryer (HPD) and SAHPD modes for closed system drying of banana chips. The effect of drying time on various energy, exergy, economic and exergoeconomic performance parameters has been investigated. The banana chips have been dried to remove moisture from 83.5% to 11.5% at an air velocity of 01.0 m/s in the drying chamber. Both energy efficiency and exergy efficiency are found higher for SAHPD. The specific moisture extraction rate is higher for SAHPD as compared to simple HPD. On the basis of economic analysis, the payback period for the SAHPD over the simple HPD is 46 months. The lowest exergoeconomic factor is for the expansion device in simple HPD and SAHPD with determined values as 0.1395 and 0.2053, respectively. Total exergy destruction cost for simple HPD and SAHPD is 0.1185 $/h and 0.1386 $/h, respectively. It is observed that the most important components that needed to improve based on exergoeconomic factor are expansion device and evaporator. The present study reveals that the SAHPD system is much better than the simple HPD system.
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Investigation of a solar assisted heat pump wheat drying system with underground thermal energy storage tank
Article
  • Feb 2020
  • SOL ENERGY
Storage of solar energy in underground Thermal Energy Storage (TES) tank during sunny days and extraction of the energy in the TES tank and its surrounding ground by a heat pump through the year for drying systems is an attractive subject for effective use of solar energy and ground as heat sources. It is possible to store solar energy in the TES tank and ground to use it for heating systems. In this study, modeling of a wheat drying system with a heat pump and underground TES tank charged by solar energy is developed using energy analysis of the drying system to investigate its long-term performance parameters. The drying system has four components, which are wheat dryer, underground TES tank, solar collectors, and a heat pump. The analytical model consists of a solution of unsteady heat transfer problem around the TES tank and energy expressions for the other components. A MATLAB code is developed to examine the performance parameters hour by hour along the years. These parameters are water temperature in the tank, Coefficient of Performances for the heat pump (COP) and system (COPs), Specific Moisture Evaporation Rate (SMER), and energy fractions for the system. When wheat mass flow rate and Carnot efficiency are selected as 50 kg h−1 and 40% respectively, collector area, TES tank volume, COP, COPs and SMER are 70 m2 and 200 m3, 4.43, 4.3 and 6.05 respectively when the system attains periodic operation time from the 5th year onwards for 10 years of operation.
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Towards the electrification of buildings heating - Real heat pumps electricity mixes based on high resolution operational profiles
Article
  • Mar 2020
  • ENERGY
The energy transition driven by the need of a deep decarbonization to limit the world temperature rise requires coordinated actions across the whole energy sector. Among other measures, the strong development of renewable energy sources in the power sector is gaining momentum in different parts of the world. The possibility of producing low-carbon electricity leads to a renovated interest in increasing electricity penetration in final sectors, especially in transport and buildings heating. However, a large share of renewable electricity comes from non-dispatchable sources, notably wind and solar PV, and their daily and seasonal variability needs to be matched with the demand profiles of those sectors. In particular, the charge of electric vehicles shows a relatively constant demand on a seasonal basis while attention must be paid on the daily operational logic to fully exploit available power from solar and/or wind. On the other hand, the operation of heat pumps for building heating shows a strong seasonality that may be an issue in countries that have a larger renewable production during summer. This research work is focusing on this specific issue, with the aim of improving the common practice of evaluating energy consumption and emissions on an annual basis, thanks to a detailed analysis based on hourly time-step, both for the electricity generation mix and for the heat pumps demand. To increase the significance of the results, different countries across Europe have been analyzed and compared.
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Comparative study of heat pump system and biomass boiler system to a tertiary building using the Life Cycle Assessment (LCA)
Article
  • Jun 2020
  • RENEW ENERG
The high emissions of substances harmful to the environment associated with the activity of people, has become a point of extreme importance, since it depends on the subsistence of life on the planet [1]. Manufacturing processes and the application of new technologies improve substantially the life, but some processes contribute more to the damage to the environment. These manufacturing processes require a high consumption of energy and resources, which entail environmental impacts, some of them not quantified. For this reason, the reduction of emissions has become the battlefield in the fight for the preservation of planet. To determine and quantify the impacts that occur in a product, process or system, it is necessary to perform an analysis of the flows of energy and resources that occur throughout its life cycle. That is why the LCA has become a very important tool in the process of transition to a low-emission production economy [2]. There are systems that, although considered renewable, also produce impacts on the environment. That is why, the present work, and through the LCA, determines the impacts produced by two heat generation systems, to later be able to compare them with each other.
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Performance analysis and mathematical modelling of banana slices in a heat pump drying system
Article
  • May 2020
  • RENEW ENERG
In this study, banana slices were dried at different temperatures in a closed-loop heat pump drying system. As the temperature and the slice thickness increased, the drying time also increased. Midilli & Kucuk model was found to be the best model to explain the experimental data. The effective moisture diffusivity ranged from 1.12 × 10⁻¹⁰ to 1.64 × 10⁻¹⁰ m²/s. The dependence of moisture diffusivity on temperature was described by an Arrhenius-type equation and the activation energy was found to be -51.45 kJ/mol. The highest mean specific moisture extraction ratio (SMER) and coefficient of performance (COP) of HPD system were obtained as 0.212 kg/kWh and 3.059, respectively, at the highest drying air temperature.
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Dynamic coupling method between air-source heat pumps and buildings in China’s hot-summer/cold-winter zone
Article
  • Nov 2019
  • APPL ENERG
The existing selection methods for air-source heat pumps often result in insufficient heating effects and high electricity consumption in winter, in China's hot-summer/cold-winter zone. Because the existing basis for selection are diverse and difficult to convert for comparison, selection methods are not connected with actual dynamic load demand. This paper presents a dynamic coupling selection method for air-source heat pumps based on load balance. Dynamic output capacity models and energy efficiency models for air source heat pumps, dynamic load demand models for buildings, and dynamic coupling models are established to interpret the relationships between heat pumps and buildings. This paper puts forward the stable operation conditions, the output capacity, and the energy efficiency of heat pumps under the most unfavorable operation conditions, in order to select units. Our study finds that the lowest coefficient of performance is 1.12, which is only 43% of the rated value in the region. Heat pumps with higher performance generated excessive cooling capacity relative to load demands of buildings. Hence, the dynamic coupling method is a simple and engineering tool for optimizing entire air-source heat pump energy systems in buildings by matching the performance surfaces of heat pumps and buildings. It is a general method which can be applied by users to directly compare different units in buildings with higher energy efficiency and operation reliability, and also to guide manufacturers with a performance surface to complement existing indexes. This method has great significance for selection optimization and promotion of air-source heat pumps.
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Assessing the reliability of current simulation of thermoelectric heat pumps for nearly zero energy buildings: Expected deviations and general guidelines
Article
  • Oct 2019
  • ENERG CONVERS MANAGE
This paper makes evident that a rigorous review of simulation methods for thermoelectric heat pumps in nearly-zero energy buildings is needed, as incoherent results during verification and validation of simulation models are reported in the literature. Statistical methods based on uncertainty analysis are deployed to calculate the minimum deviations between experimental and simulated values of the main variables that define the performance of a thermoelectric heat pump, within working scenarios expected in nearly-zero energy buildings. Results indicate that the narrower confidence intervals of these deviations are set by the uncertainties in the calculation of the thermoelecric properties of the thermoelectric modules. The minimum deviation in the prediction of the electric power consumed by the thermoelectric heat pump is ±6% in all scenarios. Likewise, confidence intervals for the heat flow emitted to the hot reservoir range from ±8% for high operating voltages of the thermoelectric heat pump to ±23% for low ones. In similar terms, those of the coefficient of performance range from ±4% to ±21%. These lower limits cannot be reduced unless the uncertainties in the measurement of the thermoelectric properties are reduced. In fact, these confidence intervals are due to increase as more uncertainties are added in the analysis, so wider intervals are expected when heat exchangers and complex heat reservoir are introduced in the system. To avoid so, several guidelines for uncertainty reduction are included in the paper, intended to increase the reliability of the simulation of thermoelectric heat pumps. Among them, relevant is the precise account of the aspect ratio in a thermoelectric module, as well as the deployment of temperature and voltage sensors with systematic standard uncertainties lower than 0.3 °C and 0.01 V respectively. The paper demonstrates the relevance of uncertainty propagation analysis in the verification and validation of the simulation models in this field, and underlines how misleading could be just to compare average values of experimental and simulated results.
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