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The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015 DOI Number: 10.6567/IFToMM.14TH.WC.OS17.008
Design of a Novel Solar Air Conditioning System for Electric Vehicles
Bin-Juine Huang1, J.K. Guan
2, D.F. Hou
3, Y.H. Chuang
4, Y.Y. Hsieh
5, K. Li
6, K.Y. Lee
7
New Energy Center, Department of Mechanical Engineering
National Taiwan University, Taipei, Taiwan
Abstract: In the present study, a direct solar PV-driven air
conditioning system is designed for medium electric bus
(21 passengers). Passive design for energy conservation is
carried out first by improving insulation and reduces
cooling load by 54%. Active design by modification of air
conditioner with inverter-type with high COP (4.0) further
reduces the energy consumption from 5.2 kW to 1.5 kW, by
71% in total. If spot cooling is employed, the energy
consumption is further reduced to 1.1 kW, about 78%.
The present study also designs a novel stand-alone
solar PV system with small battery which assure operation
probability of air conditioner OPB>0.9 at solar radiation
IT>400 W/m2. That is, solar PV will supply most of power
to drive the air conditioning system.
Keywords: solar air conditioner, solar air conditioner for electric bus
I. Introduction
Electric vehicle (EV) is very promising in solving
problems of air pollution, global warming, and oil shortage.
Distance or mileage of EV travel for each battery recharge
is the key factor of acceptance for users. The EU
technology has been developed and commercialized very
fast since advanced motor technology, power controller,
energy management system, and high energy density
battery has been extensively studied. However, it is found
that the energy consumption of air conditioning system in
EV is so large that it may reduce 30-40% mileage. For
example, the existing air conditioner of a medium electric
bus with 21 passengers (Fig.1) consumes about 50 kWh in
a sunny day (8 hour) which is about 40% of total battery
storage.
[email protected] [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]
Fig. 1. Medium electric bus.
In the present study, we intend to develop a solar air
conditioning system for medium electric bus. Energy
conservation with passive and active designs will be
carried out first to reduce energy demand. Then, solar PV
system will be installed to supply power for air conditioner.
II. Passive Design for Energy Conservation
The passive design for energy conservation in
medium electric bus includes three items: (1)insulation of
window using low-E film (Fig.2); (2)coating of sun
reflecting pain on rooftop surface; (3)improving body
foam insulation during manufacture (Fig.3). Tab. 1 shows
that the cooling load is reduced 64%.
Fig.2 Low-E film on window.
Fig.3 Insulation of body.
Tab.1 Cooling load reduction by passive design.
If body heat of 21 passengers is considered, the total
cooling load is reduced from 13.0 kW to 6.0 kW with
passive design, about 54%, as shown in Tab.2.
Tab.2 Total cooling load reduction
Type of heat load No passive
design
With passive
design
Body heat
(21 passengers)
2,100W
(100W/p)
2,100W
(100W/p)
Heat input 10,942W 3,984W
Total cooling load 13,042W 6,084W
Load reduction - 54%
III. Active Design for Energy Conservation
The conventional air conditioner installed on the
medium bus uses scroll compressor with fixed speed. COP
is about 2.5. The power consumption of air conditioning
system is 5.2 kW if no passive design is employed. If the
air conditioner uses inverter-type with COP=4, the power
consumption will be reduced to 1.5 kW with active design,
71% reduction. See Tab.3.
Tab.3 Power consumption of A/C
IV. Installation of Solar PV System
Solar radiation intensity is in phase with the cooling
load as well as power consumption of air conditioner. Solar
cooling is the best solution to electric vehicles. The roof
area of medium electric bus is about 16 m2 which is enough
to install 3 kWp PV modules as shown in Fig.4. The
maximum PV power generation is about 2.1 kW which is
enough to drive the air conditioner (1.5 kW) and has 0.6
kW excess PV power for charging EV power battery to
increase the mileage (Tab.4). Curved light solar PV
modules with high efficiency (> 20%) will be paved on the
top of bus to avoid the increases of drag and weight.
Fig.4 Solar PV modules installed on electric bus.
Tab. 4 Solar PV power supply.
Power consumption of A/C 1.5 kW
Solar PV system installed 3 kWp
Maximum solar power generation in
sunny day 2.1 kW
Net PV power for EV battery charge 0.6 kW
V. Application of Spot Cooling
The spot cooling has been developed in 1950’s for
military application using thermoelectric cooler. It is quite
suitable for electric vehicles to keep the passenger seat cool.
This will make passenger feel comfortable even if the air
temperature in the room is set higher. An experiment was
performed in New Energy Center, National Taiwan
University. It shows that people still feel comfortable at air
temperature 32oC if the seat is kept at 28
oC. In this case, the
power consumption of air conditioner can be reduced about
35% if the air temperature is set 32oC from 25
oC (5%
power reduction for 1oC air temperature increase,
according to the test of ITRI). The total power
consumption for air conditioning is 1.1 kW, 78.8%
reduction from original, see Tab.5.
The cooler for spot cooling is made from mini-
compressor (Freon 134a) with rated 24VDC power
input 70W (COP=2), Fig.5. It is enough to supply spot
cooling load (200W) for 21 passengers. The solar PV
module required is about 250Wp.
Tab. 5 Solar PV power supply.
A water circulation system is designed to circulate
cold water to the cooling pads on passenger seats, as
shown in Fgi.6. The cooling pad installation on electric
bus is shown in Fig.7.
Fig.5 Mini cooler for spot cooling in electric bus.
Fig.6 Mini cold water circulation system for spot cooling.
Fig.7 Cooling pad installation for spot cooling.
VI. Direct Solar PV Driven A/CThe solar air conditioning system is stand-alone solar
system. It requires a steady power input to compressor for
smooth operation under variable solar radiation. Huang et
al [1] developed a stand-alone solar air conditioner driven
directly by solar PV. A small battery is thus used, called
buffer battery, since it acts as a buffer only for supplying
steady energy to air conditioner. A capacitor is connected
to battery in order to suppress the surge power at
compressor startup. A microprocessor-based charge/discharge controller
with long-term measurements of charge/discharge current,
battery voltage, solar irradiation etc. [2] is used to control
the battery charge and discharge and data recording. The
schematic diagram is shown in Fig. 8.
The solar PV system did not use MPPT (maximum-
power-point tracking control) for maximum power
tracking of PV module. Instead, the PV system design is
based on nMPPO (near maximum-power-point operation)
[3] which match the performance of solar PV modules with
the battery voltage. This avoids the energy loss of MPPT
and reduces the cost as well as keeping higher reliability.
Fig.8 Direct PV driven air conditioning system.
The maximum PV power generation of the electric
bus is about 2.1 kW in sunny weather which is enough to
drive the air conditioner with spot cooling (1.1 kW) and
has 1.0 kW excess PV power for charging EV power
battery to increase the mileage (Tab.5).
Tab. 5 Solar PV power supply for EV with spot cooling.
Power consumption of A/C 1.1 kW
Solar PV system installed 3 kWp
Maximum solar power generation in sunny day
2.1 kW
Net PV power for EV battery charge 1.0 kW
In order to stabilize compressor operation and reduce
battery cost, a small buffer battery will be used. An inverter
is used to convert PV power into ac power to drive the air
conditioner. The small battery can supply power for less
than one hour during low solar radiation periods. Hence,
the cooling system may suffer from loss of power.
Huang et al [1] defined the operation probability (OPB)
of solar air conditioner, eqn.(1), as the ratio of total running
time of the air conditioner to total occurrence time of solar
irradiation at specific intensity IT ±△IT where △IT is the
radiation increment chosen as 50 Wm-2
. OPB is used to
characterize the running probability of air conditioner at
given solar irradiation IT.
(1)
It was found that the operation probability (OPB) of
the solar air conditioner built by Huang et al [1] is 1.0 at
solar irradiation > 550 Wm-2
and around 80% at solar
irradiation 400Wm-2
, both at cloudy condition.
Huang et al [1] defined another performance index
called “runtime fraction” (RF) as the ratio of the total
running time tON of air conditioner to the total service time
ttotal (taken 8 h), eqn.(2).
(2)
RF is used to characterize the daily overall performance of
solar air conditioner at daily-total solar irradiation HT.
Actually, 1 - RF is the time fraction of load power loss. The
runtime fraction RF (actual running time/demand time) of
the solar air conditioner built by Huang et al [1] is 0.6-0.8
in clear days.
Huang et al [1] also defined two system design
parameters, rpL and tbp , to study the relationship of PV
power generation, load power, and battery storage with
OPB and RF.
(3)
and
(4)
where Ebat is the usable energy storage capacity of battery
(Wh) (= DOD x Ebat0) ; DOD is the depth of discharge of
battery; Ebat0 is the rated capacity of battery (Wh); Wpv is
the rated PV maximum power generation; WL is the load
power (W).
L
pv
pLW
Wr
TI
jjon
t
t
OPB
,
total
ONF
t
tR
pv
batbp
W
Et
tbp can be interpreted as the time to fully charge the
battery at maximum PV power generation. The PV system
with a higher tbp needs a longer time to charge the battery,
due to a smaller PV panel installed or a larger battery used.
rpL is the ratio of maximum PV power generation to
load power. The PV system with rpL >1.0 means that the
maximum PV power generation is higher than the load
power. For the solar air conditioner built by Huang et al [1],
rpL=2.15 and tbp= 0.33 h. For daily-total performance, RF is
approximately 1.0 at daily-total solar radiation HT > 13 MJ
m-2
day-1
(partly cloudy), if rpL>3. That is, rpL =3 is a
suitable design for high OPB and RF. This applies to the
design of EV solar cooling.
For the present medium electric bus, rpL =2.72 which
assure OPB>0.9 at IT>400 W/m2, as shown in Fig. 9.
Fig. 9 Operation probability of direct PV driven air conditioner [1].
VII. Conclusions
In the present study, a direct solar PV-driven air
conditioning system is designed for medium electric bus
(21 passengers). Passive design for energy conservation is
carried out first and reduces cooling load by 54%. Active
design by modification of air conditioner with
inverter-type with high COP (4.0) further reduces the
energy consumption from 5.2 kW to 1.5 kW, by 71% in
total. If spot cooling is employed, the energy consumption
is further reduced to 1.1 kW, about 78%.
The present study also designs a novel stand-alone
solar PV system with small battery which assure operation
probability of air conditioner OPB>0.9 at solar radiation
IT>400 W/m2. That is, solar PV will supply most of power
to drive the air conditioning system.
References
[1]Huang, B.J., Lin, T.H., Chen, Y.T., Hsu, P.C., Lim K. Solar PV-driven
air conditioner. EuroSun 2014, International Conference on Solar Energy and Buildings. September 16-19, 2014 . Aix-les-Bains,
France.
[2] Huang, B.J., Hsu, P.C., Wu, M.S., Ho, P.Y.. System dynamic model and charging control of lead-acid battery for stand-alone solar PV
system. Solar Energy 84, 822–830, 2010. [3] Huang, B.J., Sun, F.S. and Ho, R.W., Near-Maximum-Power-Point-
Operation Design of Photovoltaic Power Generation System. Solar
Energy 80(8),1003-1020, 2006.
Acknowledgement
This study was supported by RAC Electric Vehicles
Inc, Taiwan, and National Energy Program II, MOST
103-3113-E-002-006 made by Ministry of Science and
Technology, Taiwan.
