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Potential energy consumption reduction of automotive climate control systems
Accepted Manuscript Potential Energy Consumption Reduction of Automotive Climate Control Systems Filip Nielsen, Åsa Uddheim, Jan-Olof Dalenbäck PII: DOI: Reference:
S1359-4311(16)30815-8 http://dx.doi.org/10.1016/j.applthermaleng.2016.05.137 ATE 8352
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
8 February 2016 20 May 2016 23 May 2016
Please cite this article as: F. Nielsen, a. Uddheim, J-O. Dalenbäck, Potential Energy Consumption Reduction of Automotive Climate Control Systems, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2016.05.137
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Potential Energy Consumption Reduction of Automotive Climate Control Systems Authors Filip Nielsena, b, Åsa Uddheima, Jan-Olof Dalenbäckb a
Volvo Car Corporation, 405 31 Gothenburg, Sweden Chalmers University of Technology, Department of Civil and Environmental Engineering, Division of Building Services Engineering, Gothenburg, Sweden b
Corresponding author: Filip Nielsen, [email protected]
Abstract In recent years fuel consumption of passenger vehicles has received increased attention by customers, the automotive industry, regulatory agencies and academia. One area which affect the fuel consumption is climate control systems. Twenty-one energy saving measures were evaluated regarding the total energy use for vehicle interior climate using simulation. Evaluated properties were heat flow into the passenger compartment, electrical and mechanical work. The simulation model included sub models of the passenger compartment, air-handling unit, AirConditioning (AC) system, engine and engine cooling system. A realworld representative test cycle, which included tests in cold, intermediate and warm conditions, was used for evaluation. In general, few single energy saving measures could reduce the energy use significantly. The measures with most potential were increased blower efficiency with a reduction of 46% of the electrical work and increased AC-system disengage temperature with a reduction of 27% of the mechanical work. These results show that the operation of the climate control system had a large effect on the energy use, especially compared to the required heating and cooling of the passenger compartment. As a result energy saving measures need to address how heating and cooling is generated before reducing the heat flow into the passenger compartment.
Keywords Air conditioning Fuel consumption Automotive HVAC Simulation Climate Control Systems
1. Introduction Fuel consumption of passenger vehicles has received more and more attention in recent years, particularly from regulatory agencies. Many
countries have created, implemented or tightened regulations on fuel consumption and emissions, especially regarding greenhouse gases [1,2]. This emphasis on fuel consumption has led to several improvements, mostly in areas that have large effects on certification cycles. However, other areas not currently included can also affect real-world fuel consumption, for example total energy use of the climate control system. The Air-Conditioning (AC) system is the largest auxiliary load on a vehicle and according to [3] 3.9% of the total greenhouse gas emissions of cars and light trucks are due to the AC-system. The climate control system energy use has received increased attention by research communities, industry and regulatory agencies, mainly due to the relatively large impact on range for electric vehicles, see Farrington et al. [4] and Kambly and Bradley [5]. In general, the solutions to the problem of climate control system energy use can be divided into two areas: decreasing the required heating or cooling and increasing the efficiency of the systems providing heating and cooling. One method to increase the efficiency of the system is to use a heat pump. Ahn et al. [6] compared single source heat pumps with dual source using both air and waste heat as heat source. Hosoz et al. [7] compared a heat pump using different heat sources with a heat pump using waste heat directly from a diesel engine. Fleming et al. [8] investigated the effect of thermal batteries added to the heat pump setup. Diehl et al. [9] and Chiew et al. [10] used a different approach to increase the heating efficiency, waste heat in the exhaust was used to heat the passenger compartment. Another approach was to use the waste heat to cool the passenger compartment using an absorption or adsorption cycle, see for example Talom and Beyene [11] and Verde et al. [12]. A different method to increase the efficiency of the system is to improve the AC-system. According to Lin et al. [13], a desiccant wheel assisted AC-system could decrease the total power input with 20%. Ünal and Yilmaz [14] described how a two phase ejector increased the efficiency of the AC-system. Khayyam et al. [15] reported that a coordinated energy management system could reduce energy consumption of the air conditioning system and still maintain comfort. The impact on fuel consumption when preventing the AC-system to be used at lower ambient temperatures was investigated by Monforte and Mandrile [16]. A lot of research has also focused on decreasing the required cooling and heating. One method that could achieve this is localized cooling and heating, i.e. the passenger compartment is only heated or cooled where it’s needed. Huang et al. [17], Oh et al. [18] and Wang et al. [19] have all presented research in this area. Another method is to reduce the thermal loads on the vehicle, for example by using solar reflective shells, see Levinson et al. [20], or using more advanced glazing, see Rugh et al. [21], Gravelle et al. [22] and Bridge et al. [23]. Türler et al. [24] also investigated glazing technologies as well as passenger compartment insulation. Wirth et al. [25] investigated insulation of the roof structure. Zhang et al. [26] simulated different energy saving ideas such as insulation and reducing the transmitted radiation into the passenger compartment. Obviously a lot of ideas, research and systems exist that could reduce the energy use of climate control systems. However, one major
obstacle exists for the automotive industry; how to decide which solution or solutions that have the largest potential. Consequently what research and development should be prioritized? Often the different solutions found in the literature are not evaluated under the same conditions; regularly conditions that are favorable to that particular solution are used. In addition, the potential of solutions to many real problems have not been investigated, such as the effect of reducing heat pickup. The purpose of the presented research is to compare the potential of different energy saving measures for the same conditions. Some ideas in the cited research will be investigated as well as other less researched areas. More specifically the average electric and mechanical power and the heat flow into the passenger compartment are analyzed. Prior steps in the research includes comprehensive laboratory measurements on a typical vehicle, see Nielsen et al. [27]. These measurements were used to verify a 1 Dimensional (1D) model reported by Nielsen et al. [28]. The simulation model was used to investigate combined energy saving measures with a realistic test cycle, see Nielsen et al. [29]. The simulated vehicle has an Internal Combustion Engine (ICE) although many results are valid for other powertrains, e.g. electric.
2. Method The different measures were investigated with a 1D simulation model of the important systems for the interior climate, using a test cycle based on different real-world conditions. The modelled vehicle was a Volvo S60, D5, model year 2012. It had a 2.4 l diesel engine, Electronic Climate Control (ECC), AC-system with a variable displacement compressor and a Fuel Operated Heater (FOH).
2.1 Model In this section the model is only briefly described, for more details on the model and model verification see Nielsen et al. [28] and Nielsen et al. [29]. The 1D model consisted of sub models of the passenger compartment, the air-handling unit, the AC-system, the ECC, the engine cooling system and the engine. The focus of the model was the energy use of the climate control system, i.e. heating, cooling, electrical work and mechanical work. Consequently, the sub models with greatest impact on these attributes were the most detailed. In general the components were exchangeable to simplify investigation of different measures. The integrated climate control software was the same as in the production vehicle, this ensures a realistic operation of the climate control system. The model was developed in the commercial software GT-SUITE by Gamma Technologies, a modeling software that solves the equations of mass-, energy- and momentum-conservation. It solves the equations in one dimension, which means that the quantities are averaged over a volume and the system is discretized into one or more volumes connected by boundaries. Scalar quantities are assumed to be uniform in the volume, and the vector quantities are uniform for each boundary. Examples of scalar quantities are temperature, pressure and enthalpy, and examples of vector quantities are mass flow and velocity. For more information regarding the software see [30].
The model was verified with testing for a wide range of different conditions, for example, temperatures from -18°C to 43°C and sun load from 0 W/m² to 1000 W/m². See Figure 1 for an overview of the model.
Figure 1. Overview of model
2.2 Test Cycle One important aspect when evaluating energy saving measures is that the comparison is done under similar and relevant conditions. In many studies the energy saving measures are only investigated in a few special cases and it’s difficult to understand the effect on the total energy use of the climate control system. In this research the test cycle developed by Nielsen et al. [29] was used. The test cycle conditions were based on hourly real-world conditions, weighted with sales distribution of Volvo Cars and departure time. It included one test in cold, one in intermediate and one in warm conditions, each weighted for occurrence. See Table 1 for the conditions for each test. The velocity profile of the test cycle was the new Worldwide harmonized Light vehicles Test Procedure (WLTP) class 3 [31]. The climate control system was in automatic mode with a temperature setting of 22°C. There was only one person, the driver, in the passenger compartment. The test time, for each condition, was 30 minutes, i.e. the length of WLTP. Table 1. Conditions for the tests included in the test cycle Weight
Temperature
Dewpoint
Sun load
°C
°C
W/m²
Cold
0.2
0
-4
0
Intermediate
0.6
15
7
200
Warm
0.2
27
17
400
14
7
200
Weighted average
The sun height for the intermediate test was 37° and 50° for the warm test. The azimuth angle changed 5° each second, completing a revolution in 72 seconds. One hour sun soak for both the intermediate and warm case was used. For the cold test all materials had ambient temperature at start. In all tests the engine was initially at ambient temperature.
2.3 Energy Saving Measures In total 21 different cases with energy saving measures were investigated. Additionally, one unmodified base case and one case
without recirculation were also simulated. The energy saving measures were divided into two different categories, source and sink. In the source category, Cases 3-12, measures that affected how the heating and cooling was generated were included, such as recirculation degree and blower efficiency. The sink category, Cases 13-23, included modifications that affected the required heat flow into the passenger compartment, for example, insulation and solar reflecting windows. In this section the modifications of each case are described. Case 2, no recirculation, was included for comparison to the base case, that is, to show the effect of the base level recirculation. See Table 2 for a summary of all cases. Table 2. Investigated cases Case no:
Short description
Category
1
Base case
-
2
No recirculation
-
3
Full recirculation
Source
4
Perfect HVAC blower
Source
5
Perfect condenser cooling fan
Source
6
No heat heat pickup
Source
7
Heat exchanger valve
Source
8
AC off 15°C and below
Source
9
Increased evaporator temperature
Source
10
Increased AC-system heat transfer
Source
11
Perfect AC compressor
Source
12
No AC pressure drop
Source
13
Decreased comfort
Sink
14
No sun soak
Sink
15
Perfect preconditioning
Sink
16
No duct losses
Sink
17
No interior masses
Sink
18
Perfect shell insulation
Sink
19
Perfect glass insulation
Sink
20
Opaque glass, absorption
Sink
21
Opaque glass, reflection
Sink
22
No windows
Sink
23
Perfect reflecting exterior color
Sink
2.3.1 Base Case and Climate Control In the base case, Case 1, the standard vehicle was tested without any energy saving measures. The operation of the climates control systems is quite complex during both transient and steady state. Therefore a, somewhat simplified, summary of the operation for the different conditions is presented. For comparable results between the different cases both the transient and steady state temperature had to be similar in cold, intermediate and warm conditions. The climate control software use a temperature sensor in the passenger compartment for feedback and could to a large degree achieve comparable temperatures in steady state. However, the transient state were much shorter with implemented energy saving measures and therefore the airflow was reduced until comparable transient states were obtained. For the cold test, 0°C, the systems start with a low airflow level due to lack of available heat. The FOH starts immediately, however, it takes a couple of minutes before it provides heat to the coolant. The air distribution focus on defrosting, that is, releasing air mainly on the windows. As more heat becomes available from the engine and FOH the airflow increases and the system start releasing air to the floor.
When the temperature in the passenger compartment approaches the comfort level the airflow decreases and the temperature flaps decreases the heating. When the coolant temperature is sufficiently high the FOH is disengaged. In intermediate temperature, 15°C, the systems functions somewhat differently. As in 0°C the airflow level starts low because of insufficient heat and air is directed to the windows. Furthermore, the AC is engaged, mainly to provide dehumidification. This operation mode is called reheat because the air is first cooled and then heated. The required heat flow is relatively small, therefore the FOH is not engaged. As more waste heat is available the airflow increases and more air is directed to the floor. When the temperature in the passenger compartment approaches the comfort level the airflow decreases, the temperature flaps decrease the heating and air is directed to the panel vents. For the warm test, in 27°C, the airflow rapidly increases to a high level because cooling is available almost immediately. All air is directed through the panel vents. As soon as the passenger compartment temperature has decreased to approximately the outside temperature all air is recirculated. When the passenger compartment temperature approaches the comfort level the airflow is decreased, the level of recirculation is decreased and some air is directed through the defroster outlet for a more homogenous cooling of the passenger compartment. In all tests all actuators were automatically controlled by the climate control system.
2.3.2 Source Energy Saving Measures In Case 3 the recirculation degree was 100% for both the cold and the warm test. Due to reheat in intermediate conditions recirculation was not engaged as it would increase the energy use. The effect of humidity, i.e. risk of mist and ice, in the cold test was ignored. In Case 4 a 100% efficient blower, electric motor and blower control was used. All electric power was converted to flow and pressure increase. Case 5 used the same assumptions as Case 4 for the condenser cooling fan. The climate control system air intake is located between the windshield and engine hood. Surfaces in this area can be heated by the engine and sun load, these surfaces can then heat the incoming air. In Case 6 it was assumed that no heat was added to the air before entering the system. In general coolant is always circulating through the climate control heat exchanger and air leakage can transfer unwanted heat into the passenger compartment. In Case 7 a valve closed the coolant loop when no heat was required. In Case 8 the AC-system was turned off for ambient temperatures below 15°C compared to 5°C of the base system. It was assumed that there were only a few cases when the AC -system was needed for cooling and dehumidification in this temperature range, i.e. these situations does not affect the overall energy consumption. To evaluate this measure two tests were made in intermediate conditions, one with the AC-system engaged and one with the system disengaged. Both tests were weighted equally, 0.3 each, because the sum of the compressor load was roughly equal for the intervals 5°15°C and 16°-22°C. For Case 9 the maximum allowed evaporator air temperature was increased from 8° to 12°C, decreasing the need for reheat in intermediate conditions. More changes were done on the AC-system in Case 10 where the heat transfer for the condenser and the evaporator was greatly increased, simulating much better heat
exchangers. Furthermore, in Case 11 the mechanical efficiency of the compressor was assumed to be 100%. The last energy saving measure on the source side was Case 11 which assumed zero pressure drop in the evaporator and condenser.
2.3.3 Sink Energy Saving Measures
The ducts between the air-handling unit and the passenger compartment can be long and of a different temperature than the conditioned air. In Case 16 the heat transfer to the ducts was assumed to be zero. For Case 17 all the interior masses were removed, i.e. a vehicle without any heat transfer to and from the seats, dashboard and other interior trim. Case 18 assumed that the shell, which includes the doors and roof, was perfectly insulated. The same assumption was used in Case 19 for the windows, however, the radiation heat transfer was unaffected. Case 20 and 21 assumed oblique glass, the former by absorbing all radiation and the latter by reflecting all radiation, in both cases the heat transfer trough conduction was unaffected. In Case 22 the windows were changed to the same structure as the roof, i.e. one plastic layer, one air gap and one outer metallic layer. The reflectivity of the exterior color was increased from 24% to 100% for the last case, 23.
2.4 Compared Properties Table 3 contains a brief explanation of the different compared properties. All properties are weighted averages that include both the cold, the intermediate and the warm test. The heat flows were in general calculated as:
(1) were was the heat flow, the mass flow, the specific heat capacity, the temperature of the flow and was the ambient temperature. The heat flow into the compartment was calculated as the sum of all other sources except latent cooling, see equation (2).
(2) is the heat flow into the passenger compartment, , is the heat pickup,
is recirculated heat flow,
, is sensible cooling over the evaporator,
is the heat
flow from the engine to the heater and is the heat flow from the FOH to the heater. Note that the latent cooling, , is not included in the heat flow into the passenger compartment, however, it
is
Table 3. Compared Properties Variable
In Case 13 the comfort was decreased by lowering the passenger compartment temperature with three degrees in the cold and intermediate case. Furthermore, in the warm case the temperature was increased with three degrees. For Case 14 the intermediate and warm tests did not include a sun soak, i.e. the vehicle was parked in the same temperature but under a roof for instance. In Case 15 the passenger compartment, including interior masses, shell and windows were conditioned to steady state temperatures before the start of the tests. The energy used for this was excluded from the evaluation.
where
is included as a load on the evaporator. Note that reported either as heating flow or a cooling flow.
Description
Blower
Compartment blower power (including motor and control)
FOH electrical load
FOH electrical power
Pump
Climate circuit pump power
Cooling fan
The power for the twin cooling fan (including motor and control).
Comp. clutch & control
Compressor electrical power for clutch and control
Compressor
Compressor mechanical power
Total electrical power
The sum of blower, FOH electrical load, pump, cooling fans and Comp. clutch & control
Engine (
Heat flow from the engine through the climate system heater
FOH (
Heat flow from the FOH through the climate system heater
)
Heat pickup (
Heat flow to the air up to the evaporator
)
Cooling, sensible (
)
Cooling, latent ( Recirculation (
Sensible cooling over the evaporator Latent cooling over the evaporator
)
Heat flow received through recirculation
)
Heating flow (
)
Positive heat flow into the compartment with the airflow
Cooling flow (
)
Negative heat flow into the compartment with the airflow
3. Results The results are divided into two different areas. First the weighted average electrical and mechanical power are presented and thereafter the different heat flows.
3.1 Electrical power In Figures 2 and 3 the weighted average electrical power for the climate control system is presented. The different components such as blower, FOH electrical load, coolant pump, cooling fan and compressor clutch and control are included in the figures. The worst case was the non-energy saving Case 2 without recirculation and the best case was the perfect HVAC blower, Case 4. Other notable measures were Case 8 which decreased the average clutch power and Case 17 which decreased the blower power during the transients. In general the energy saving measures had quite small effect on the average electrical power.
In Figures 4 and 5 the compressor power is presented. As for the electrical power the non-energy saving Case 2 without recirculation was the worst case and the best cases was the 100% efficient AC compressor, Case 11, with a reduction of mechanical power of 36%. Note that the savings for Case 8, AC off 15°C and below, shows savings of 27%. One general results is that it was quite difficult to reduce the average power by changes in the sink category, the main reason for this was the maximum limit on evaporator temperature of 8°C. For an elaboration on this subject see the discussion section. Some cases show an increase in the average compressor power, Case 13 and 15. The main reason for this is increased airflow in the intermediate test which outweighs the potential savings in other conditions.
Figure 4. Average mechanical loads, Cases 1-12
Figure 2. Average electrical loads, Cases 1-12
Figure 5. Average mechanical loads, Cases 13-23
3.3 FOH Figure 3. Average electrical loads, Cases 13-23
3.2 Mechanical power The only part of the climate control system that directly use mechanical power is the AC-system, more precisely the compressor.
The FOH was only activated in the cold test and only in the early parts of the test. When activated the FOH supplied approximately 5 kW to the coolant. See Figures 6 and 7 for the heat flow from the FOH. With more available heat through recirculation the FOH could be disengaged earlier, see Case 3. With no heat pickup, Case 6, the FOH heat flow was increased with 4%. In Case 15, perfect preconditioning, no extra heat was required from the FOH.
Figure 6. Average heat flows from the FOH, Cases 1-12
Figure 8. Average heat flows from sources, Cases 1-12
Figure 7. Average heat flows from the FOH, Cases 13-23
Figure 9. Average heat flows from sources, Cases 13-23
3.4 Sources of Heating and Cooling
3.5 Heat Flow into the Passenger Compartment
There are many different sources for heating and cooling in the climate control system. For example, heat flow can originate from the engine, FOH and due to heat pickup of the incoming air. Sources of cooling can be sensible cooling over the evaporator and recirculated air. See Figures 8 and 9 for the effect on these sources for the different energy saving measures. Note the small effect of recirculation, this was due to the small temperature difference with the ambient temperature in the warm test, the only test with recirculation. However, recirculation reduced heat pickup and latent cooling, compare Case 2 and 3 in Figure 8.
Air is used to condition the passenger compartment, that is, add or remove energy. In Figures 10 and 11 the heating and cooling flow into the passenger compartment is presented. Cases 3 to 12 only have changes affecting the source and the heat flow into the passenger compartment should be roughly the same for all cases. However, due to the difficulty to achieve exactly the same conditions there were some differences. Three cases which affect the sun load, Case 14, 20 and 21, shows reduction of the required cooling but an increase in the required heating. In the intermediate test the sun load was used for heating, reducing the demand from other sources. Large reductions of required heating can be achieved by many sink cases; the cold masses and large temperature difference with the ambient temperature increases the effect of many saving measures.
Most of the energy saving measures could not realistically be implemented into a vehicle, however, this was not the purpose of the research. Instead the results can show which measures that have the most potential for further investigation. The passenger compartment temperature was comparable for all tests. This was mainly achieved by reducing the airflow during the transient state and reduce heating and cooling during steady state. For cases with the solar radiation reduced the climate comfort would be better with higher temperatures, however, the same passenger compartment temperatures were used throughout all tests. As a result, there is more potential for energy savings for Case 20-22 than reported.
4.2 Results Figure 10. Heat flows into the passenger compartment, Cases 1-12
Figure 11. Heat flows into the passenger compartment, Cases 13-23
4. Discussion 4.1 Method The base model was extensively verified and demonstrated good agreement between measurement and simulation except for the compressor power, see [28]. In this investigation the main focus was on relative differences which reduce the sensitivity to absolute errors. Furthermore, the compressor power was never compared to any other load. For an extensive discussion regarding the test cycle, see [29]. In summary the test cycle although simplified accounted for many realworld effects. Some electrical loads related to the climate control system were excluded, for example seat heating. The reason for excluding seat heating was that it was directly controlled by the customer. Furthermore, the mechanical power of the engine coolant pump was not included, despite being important for the heat flow from the engine to the heater. The electrical power of the condenser cooling fan can also be requested by the engine, however, only the ACsystem request was included.
The effect on the average electrical power for the energy saving measures were in general small, especially for the sink category, see Figure 2 and 3. The main reason was that the linear blower controller controls the airflow by reducing the voltage through heat dissipation. In the intermediate range a decrease of voltage will not save much energy, instead more energy will be dissipated. For Case 4 the reduction is large, mainly due to the changed blower control unit but obviously also due to the efficiency increase. The results showed that the blower power is the most important electrical load, using more than 50% of the total electrical power with a large potential for reduction. The compressor clutch and control power can be reduced by not engaging the AC-system, however, other electrical loads have small potential for a reduction of the energy use. The same tendency as for the electrical power could also be seen for the mechanical power, Figure 4 and 5. That is, the sink measures does not have a large effect on the average compressor power. The main reason for this is the maximum evaporator air temperature of 8°C. If the system operates at this temperature a decrease in required cooling for the passenger compartment is achieved by increasing heat flow, i.e. it will no reduced the compressor load. In Case 9 the maximum evaporator temperature is increased with four degrees to 12°C and it reduced the average power with 13%. The main reason for a low maximum evaporator temperature is noise, odor and controllability. One of the best measures for reducing mechanical power is to avoid engaging the AC-system when it is not needed for cooling or dehumidification. In this investigation that is simulated by disengaging the AC-system below 15°C and this reduces the average load with 27%. To be able to implement this for an automatic climate control system the humidity levels need to be monitored and the ACsystem engaged if there is a risk of condensation on the inside of the windows. Reducing the temperature and/or humidity before the evaporator could also save energy, see Case 3 and 6 for examples of this. There could be a lot of energy savings by increasing the mechanical efficiency of the compressor, Case 11, however, what types of measures this could be was not investigated in this research and the final realistic potential was therefore unknown. In general it was difficult to reduce the heat flow from the FOH, primarily due to the deactivation condition. The main condition for deactivation was achieving a specific coolant temperature, therefore even with reduced heat demand the FOH operated quite long. A more demand focused control could reduce the heat flow.
The airflow has a large effect on the energy use and this can clearly be seen for Case 15, perfect preconditioning. Despite significantly reducing the energy for cooling in the warm test the total cooling demand is increase by 6%, which increases the average compressor power with 2%. The reason is higher airflow from the start of the intermediate test thanks to available heat. Note that the energy used for the preconditioning is not included. In general the ventilation airflow of a vehicle is high compared to other environments such as buildings and this have a large effect on the energy use.
Acknowledgements This study was sponsored by FFI Energi & Miljö.
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The results from a comparison of the heat flow into the passenger compartment with the heating and cooling sources are noteworthy. First, the sources are much larger than the flow into the compartment, mainly due to reheat in intermediate conditions. Furthermore, the heat pickup need to be cooled when the AC-system is engaged. This indicate that all energy saving measures in the sink category will have smaller percentage effect on the sources than the actual percentage decrease of the heat flow into the passenger compartment. Clearly energy saving measures need to address the source initially, primarily reducing the reheat by using less AC and increasing maximum evaporator temperature. Without these actions other energy saving measures will have much less effect. How the system operates in intermediate climate defines the energy consumption!
In total 21 energy saving measures divided into two categories, source and sink, were investigated. The source category included measures affecting how the heating and cooling flows were generated and the sink category affected the required heat flow into the passenger compartment. In general the reduction of average electrical power for the 21 tested measures was small, although the blower showed most potential for improvement with a decrease of 46%. For the average mechanical power the measures with most potential were disengaging the ACsystem below 15°C ambient temperature and increasing the evaporator temperature. These measures showed reductions of 27% and 13%. Measures in the sink category could decreased the heat flow into the passenger compartment substantially, however the decrease of the total energy use was much smaller, mainly due to reheat. This means that energy saving measures on the sink will regularly have small effect and energy saving measures in the source category, i.e. how the heating and cooling is generated, needs to be addressed first. How the system operates, i.e. the source category, in intermediate conditions determines the overall energy consumption. Furthermore, few single energy saving measure can reduce the overall energy use substantially.
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Definitions/Abbreviations
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[21]
[22]
[23]
[24]
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1D
One dimensional
AC
Air Conditioning
Blower
Climate system blower
ECC
Electronic Climate Control
Cooling fan
Electric cooling fan for the radiator and ACsystem condenser Fuel Operated Heater
FOH
HVAC
Temperature increase of air due to hot conditions (surfaces) Heating, Ventilation and Air Conditioning
PWM
Pulse Width Modulation
Reheat
The climate control unit cools the air, mainly for dehumidification, and then reheats the air The three tests in cold, intermediate and warm conditions combined Worldwide harmonized Light vehicles Test Procedure Heat flow (W)
Heat pickup
Test cycle WLTP
Ambient temperature (°C) Average temperature for a test condition (°C)
[25]
S. Wirth, F. Niebling, U. Logasanjeevi, V. Premchandran, Improved thermal insulation for contemporary automotive roof structures based on a computational fluid dynamics heat flux approach, Heat Transfer Engineering. (2016) 1–31. doi:10.1080/01457632.2015.1136170.
Flow temperature (°C)
Heat capacity (J/kg K) Mass flow (kg/s)
Graphical abstract
Highlights
Twenty-on energy saving measures for vehicle interior climate were evaluated Few single energy saving measures could reduce the energy use significantly The operation of the system in intermediate conditions determines the energy use Required heating/cooling of passenger compartment had small effect on energy use
S1359-4311(16)30815-8 http://dx.doi.org/10.1016/j.applthermaleng.2016.05.137 ATE 8352
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
8 February 2016 20 May 2016 23 May 2016
Please cite this article as: F. Nielsen, a. Uddheim, J-O. Dalenbäck, Potential Energy Consumption Reduction of Automotive Climate Control Systems, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2016.05.137
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Potential Energy Consumption Reduction of Automotive Climate Control Systems Authors Filip Nielsena, b, Åsa Uddheima, Jan-Olof Dalenbäckb a
Volvo Car Corporation, 405 31 Gothenburg, Sweden Chalmers University of Technology, Department of Civil and Environmental Engineering, Division of Building Services Engineering, Gothenburg, Sweden b
Corresponding author: Filip Nielsen, [email protected]
Abstract In recent years fuel consumption of passenger vehicles has received increased attention by customers, the automotive industry, regulatory agencies and academia. One area which affect the fuel consumption is climate control systems. Twenty-one energy saving measures were evaluated regarding the total energy use for vehicle interior climate using simulation. Evaluated properties were heat flow into the passenger compartment, electrical and mechanical work. The simulation model included sub models of the passenger compartment, air-handling unit, AirConditioning (AC) system, engine and engine cooling system. A realworld representative test cycle, which included tests in cold, intermediate and warm conditions, was used for evaluation. In general, few single energy saving measures could reduce the energy use significantly. The measures with most potential were increased blower efficiency with a reduction of 46% of the electrical work and increased AC-system disengage temperature with a reduction of 27% of the mechanical work. These results show that the operation of the climate control system had a large effect on the energy use, especially compared to the required heating and cooling of the passenger compartment. As a result energy saving measures need to address how heating and cooling is generated before reducing the heat flow into the passenger compartment.
Keywords Air conditioning Fuel consumption Automotive HVAC Simulation Climate Control Systems
1. Introduction Fuel consumption of passenger vehicles has received more and more attention in recent years, particularly from regulatory agencies. Many
countries have created, implemented or tightened regulations on fuel consumption and emissions, especially regarding greenhouse gases [1,2]. This emphasis on fuel consumption has led to several improvements, mostly in areas that have large effects on certification cycles. However, other areas not currently included can also affect real-world fuel consumption, for example total energy use of the climate control system. The Air-Conditioning (AC) system is the largest auxiliary load on a vehicle and according to [3] 3.9% of the total greenhouse gas emissions of cars and light trucks are due to the AC-system. The climate control system energy use has received increased attention by research communities, industry and regulatory agencies, mainly due to the relatively large impact on range for electric vehicles, see Farrington et al. [4] and Kambly and Bradley [5]. In general, the solutions to the problem of climate control system energy use can be divided into two areas: decreasing the required heating or cooling and increasing the efficiency of the systems providing heating and cooling. One method to increase the efficiency of the system is to use a heat pump. Ahn et al. [6] compared single source heat pumps with dual source using both air and waste heat as heat source. Hosoz et al. [7] compared a heat pump using different heat sources with a heat pump using waste heat directly from a diesel engine. Fleming et al. [8] investigated the effect of thermal batteries added to the heat pump setup. Diehl et al. [9] and Chiew et al. [10] used a different approach to increase the heating efficiency, waste heat in the exhaust was used to heat the passenger compartment. Another approach was to use the waste heat to cool the passenger compartment using an absorption or adsorption cycle, see for example Talom and Beyene [11] and Verde et al. [12]. A different method to increase the efficiency of the system is to improve the AC-system. According to Lin et al. [13], a desiccant wheel assisted AC-system could decrease the total power input with 20%. Ünal and Yilmaz [14] described how a two phase ejector increased the efficiency of the AC-system. Khayyam et al. [15] reported that a coordinated energy management system could reduce energy consumption of the air conditioning system and still maintain comfort. The impact on fuel consumption when preventing the AC-system to be used at lower ambient temperatures was investigated by Monforte and Mandrile [16]. A lot of research has also focused on decreasing the required cooling and heating. One method that could achieve this is localized cooling and heating, i.e. the passenger compartment is only heated or cooled where it’s needed. Huang et al. [17], Oh et al. [18] and Wang et al. [19] have all presented research in this area. Another method is to reduce the thermal loads on the vehicle, for example by using solar reflective shells, see Levinson et al. [20], or using more advanced glazing, see Rugh et al. [21], Gravelle et al. [22] and Bridge et al. [23]. Türler et al. [24] also investigated glazing technologies as well as passenger compartment insulation. Wirth et al. [25] investigated insulation of the roof structure. Zhang et al. [26] simulated different energy saving ideas such as insulation and reducing the transmitted radiation into the passenger compartment. Obviously a lot of ideas, research and systems exist that could reduce the energy use of climate control systems. However, one major
obstacle exists for the automotive industry; how to decide which solution or solutions that have the largest potential. Consequently what research and development should be prioritized? Often the different solutions found in the literature are not evaluated under the same conditions; regularly conditions that are favorable to that particular solution are used. In addition, the potential of solutions to many real problems have not been investigated, such as the effect of reducing heat pickup. The purpose of the presented research is to compare the potential of different energy saving measures for the same conditions. Some ideas in the cited research will be investigated as well as other less researched areas. More specifically the average electric and mechanical power and the heat flow into the passenger compartment are analyzed. Prior steps in the research includes comprehensive laboratory measurements on a typical vehicle, see Nielsen et al. [27]. These measurements were used to verify a 1 Dimensional (1D) model reported by Nielsen et al. [28]. The simulation model was used to investigate combined energy saving measures with a realistic test cycle, see Nielsen et al. [29]. The simulated vehicle has an Internal Combustion Engine (ICE) although many results are valid for other powertrains, e.g. electric.
2. Method The different measures were investigated with a 1D simulation model of the important systems for the interior climate, using a test cycle based on different real-world conditions. The modelled vehicle was a Volvo S60, D5, model year 2012. It had a 2.4 l diesel engine, Electronic Climate Control (ECC), AC-system with a variable displacement compressor and a Fuel Operated Heater (FOH).
2.1 Model In this section the model is only briefly described, for more details on the model and model verification see Nielsen et al. [28] and Nielsen et al. [29]. The 1D model consisted of sub models of the passenger compartment, the air-handling unit, the AC-system, the ECC, the engine cooling system and the engine. The focus of the model was the energy use of the climate control system, i.e. heating, cooling, electrical work and mechanical work. Consequently, the sub models with greatest impact on these attributes were the most detailed. In general the components were exchangeable to simplify investigation of different measures. The integrated climate control software was the same as in the production vehicle, this ensures a realistic operation of the climate control system. The model was developed in the commercial software GT-SUITE by Gamma Technologies, a modeling software that solves the equations of mass-, energy- and momentum-conservation. It solves the equations in one dimension, which means that the quantities are averaged over a volume and the system is discretized into one or more volumes connected by boundaries. Scalar quantities are assumed to be uniform in the volume, and the vector quantities are uniform for each boundary. Examples of scalar quantities are temperature, pressure and enthalpy, and examples of vector quantities are mass flow and velocity. For more information regarding the software see [30].
The model was verified with testing for a wide range of different conditions, for example, temperatures from -18°C to 43°C and sun load from 0 W/m² to 1000 W/m². See Figure 1 for an overview of the model.
Figure 1. Overview of model
2.2 Test Cycle One important aspect when evaluating energy saving measures is that the comparison is done under similar and relevant conditions. In many studies the energy saving measures are only investigated in a few special cases and it’s difficult to understand the effect on the total energy use of the climate control system. In this research the test cycle developed by Nielsen et al. [29] was used. The test cycle conditions were based on hourly real-world conditions, weighted with sales distribution of Volvo Cars and departure time. It included one test in cold, one in intermediate and one in warm conditions, each weighted for occurrence. See Table 1 for the conditions for each test. The velocity profile of the test cycle was the new Worldwide harmonized Light vehicles Test Procedure (WLTP) class 3 [31]. The climate control system was in automatic mode with a temperature setting of 22°C. There was only one person, the driver, in the passenger compartment. The test time, for each condition, was 30 minutes, i.e. the length of WLTP. Table 1. Conditions for the tests included in the test cycle Weight
Temperature
Dewpoint
Sun load
°C
°C
W/m²
Cold
0.2
0
-4
0
Intermediate
0.6
15
7
200
Warm
0.2
27
17
400
14
7
200
Weighted average
The sun height for the intermediate test was 37° and 50° for the warm test. The azimuth angle changed 5° each second, completing a revolution in 72 seconds. One hour sun soak for both the intermediate and warm case was used. For the cold test all materials had ambient temperature at start. In all tests the engine was initially at ambient temperature.
2.3 Energy Saving Measures In total 21 different cases with energy saving measures were investigated. Additionally, one unmodified base case and one case
without recirculation were also simulated. The energy saving measures were divided into two different categories, source and sink. In the source category, Cases 3-12, measures that affected how the heating and cooling was generated were included, such as recirculation degree and blower efficiency. The sink category, Cases 13-23, included modifications that affected the required heat flow into the passenger compartment, for example, insulation and solar reflecting windows. In this section the modifications of each case are described. Case 2, no recirculation, was included for comparison to the base case, that is, to show the effect of the base level recirculation. See Table 2 for a summary of all cases. Table 2. Investigated cases Case no:
Short description
Category
1
Base case
-
2
No recirculation
-
3
Full recirculation
Source
4
Perfect HVAC blower
Source
5
Perfect condenser cooling fan
Source
6
No heat heat pickup
Source
7
Heat exchanger valve
Source
8
AC off 15°C and below
Source
9
Increased evaporator temperature
Source
10
Increased AC-system heat transfer
Source
11
Perfect AC compressor
Source
12
No AC pressure drop
Source
13
Decreased comfort
Sink
14
No sun soak
Sink
15
Perfect preconditioning
Sink
16
No duct losses
Sink
17
No interior masses
Sink
18
Perfect shell insulation
Sink
19
Perfect glass insulation
Sink
20
Opaque glass, absorption
Sink
21
Opaque glass, reflection
Sink
22
No windows
Sink
23
Perfect reflecting exterior color
Sink
2.3.1 Base Case and Climate Control In the base case, Case 1, the standard vehicle was tested without any energy saving measures. The operation of the climates control systems is quite complex during both transient and steady state. Therefore a, somewhat simplified, summary of the operation for the different conditions is presented. For comparable results between the different cases both the transient and steady state temperature had to be similar in cold, intermediate and warm conditions. The climate control software use a temperature sensor in the passenger compartment for feedback and could to a large degree achieve comparable temperatures in steady state. However, the transient state were much shorter with implemented energy saving measures and therefore the airflow was reduced until comparable transient states were obtained. For the cold test, 0°C, the systems start with a low airflow level due to lack of available heat. The FOH starts immediately, however, it takes a couple of minutes before it provides heat to the coolant. The air distribution focus on defrosting, that is, releasing air mainly on the windows. As more heat becomes available from the engine and FOH the airflow increases and the system start releasing air to the floor.
When the temperature in the passenger compartment approaches the comfort level the airflow decreases and the temperature flaps decreases the heating. When the coolant temperature is sufficiently high the FOH is disengaged. In intermediate temperature, 15°C, the systems functions somewhat differently. As in 0°C the airflow level starts low because of insufficient heat and air is directed to the windows. Furthermore, the AC is engaged, mainly to provide dehumidification. This operation mode is called reheat because the air is first cooled and then heated. The required heat flow is relatively small, therefore the FOH is not engaged. As more waste heat is available the airflow increases and more air is directed to the floor. When the temperature in the passenger compartment approaches the comfort level the airflow decreases, the temperature flaps decrease the heating and air is directed to the panel vents. For the warm test, in 27°C, the airflow rapidly increases to a high level because cooling is available almost immediately. All air is directed through the panel vents. As soon as the passenger compartment temperature has decreased to approximately the outside temperature all air is recirculated. When the passenger compartment temperature approaches the comfort level the airflow is decreased, the level of recirculation is decreased and some air is directed through the defroster outlet for a more homogenous cooling of the passenger compartment. In all tests all actuators were automatically controlled by the climate control system.
2.3.2 Source Energy Saving Measures In Case 3 the recirculation degree was 100% for both the cold and the warm test. Due to reheat in intermediate conditions recirculation was not engaged as it would increase the energy use. The effect of humidity, i.e. risk of mist and ice, in the cold test was ignored. In Case 4 a 100% efficient blower, electric motor and blower control was used. All electric power was converted to flow and pressure increase. Case 5 used the same assumptions as Case 4 for the condenser cooling fan. The climate control system air intake is located between the windshield and engine hood. Surfaces in this area can be heated by the engine and sun load, these surfaces can then heat the incoming air. In Case 6 it was assumed that no heat was added to the air before entering the system. In general coolant is always circulating through the climate control heat exchanger and air leakage can transfer unwanted heat into the passenger compartment. In Case 7 a valve closed the coolant loop when no heat was required. In Case 8 the AC-system was turned off for ambient temperatures below 15°C compared to 5°C of the base system. It was assumed that there were only a few cases when the AC -system was needed for cooling and dehumidification in this temperature range, i.e. these situations does not affect the overall energy consumption. To evaluate this measure two tests were made in intermediate conditions, one with the AC-system engaged and one with the system disengaged. Both tests were weighted equally, 0.3 each, because the sum of the compressor load was roughly equal for the intervals 5°15°C and 16°-22°C. For Case 9 the maximum allowed evaporator air temperature was increased from 8° to 12°C, decreasing the need for reheat in intermediate conditions. More changes were done on the AC-system in Case 10 where the heat transfer for the condenser and the evaporator was greatly increased, simulating much better heat
exchangers. Furthermore, in Case 11 the mechanical efficiency of the compressor was assumed to be 100%. The last energy saving measure on the source side was Case 11 which assumed zero pressure drop in the evaporator and condenser.
2.3.3 Sink Energy Saving Measures
The ducts between the air-handling unit and the passenger compartment can be long and of a different temperature than the conditioned air. In Case 16 the heat transfer to the ducts was assumed to be zero. For Case 17 all the interior masses were removed, i.e. a vehicle without any heat transfer to and from the seats, dashboard and other interior trim. Case 18 assumed that the shell, which includes the doors and roof, was perfectly insulated. The same assumption was used in Case 19 for the windows, however, the radiation heat transfer was unaffected. Case 20 and 21 assumed oblique glass, the former by absorbing all radiation and the latter by reflecting all radiation, in both cases the heat transfer trough conduction was unaffected. In Case 22 the windows were changed to the same structure as the roof, i.e. one plastic layer, one air gap and one outer metallic layer. The reflectivity of the exterior color was increased from 24% to 100% for the last case, 23.
2.4 Compared Properties Table 3 contains a brief explanation of the different compared properties. All properties are weighted averages that include both the cold, the intermediate and the warm test. The heat flows were in general calculated as:
(1) were was the heat flow, the mass flow, the specific heat capacity, the temperature of the flow and was the ambient temperature. The heat flow into the compartment was calculated as the sum of all other sources except latent cooling, see equation (2).
(2) is the heat flow into the passenger compartment, , is the heat pickup,
is recirculated heat flow,
, is sensible cooling over the evaporator,
is the heat
flow from the engine to the heater and is the heat flow from the FOH to the heater. Note that the latent cooling, , is not included in the heat flow into the passenger compartment, however, it
is
Table 3. Compared Properties Variable
In Case 13 the comfort was decreased by lowering the passenger compartment temperature with three degrees in the cold and intermediate case. Furthermore, in the warm case the temperature was increased with three degrees. For Case 14 the intermediate and warm tests did not include a sun soak, i.e. the vehicle was parked in the same temperature but under a roof for instance. In Case 15 the passenger compartment, including interior masses, shell and windows were conditioned to steady state temperatures before the start of the tests. The energy used for this was excluded from the evaluation.
where
is included as a load on the evaporator. Note that reported either as heating flow or a cooling flow.
Description
Blower
Compartment blower power (including motor and control)
FOH electrical load
FOH electrical power
Pump
Climate circuit pump power
Cooling fan
The power for the twin cooling fan (including motor and control).
Comp. clutch & control
Compressor electrical power for clutch and control
Compressor
Compressor mechanical power
Total electrical power
The sum of blower, FOH electrical load, pump, cooling fans and Comp. clutch & control
Engine (
Heat flow from the engine through the climate system heater
FOH (
Heat flow from the FOH through the climate system heater
)
Heat pickup (
Heat flow to the air up to the evaporator
)
Cooling, sensible (
)
Cooling, latent ( Recirculation (
Sensible cooling over the evaporator Latent cooling over the evaporator
)
Heat flow received through recirculation
)
Heating flow (
)
Positive heat flow into the compartment with the airflow
Cooling flow (
)
Negative heat flow into the compartment with the airflow
3. Results The results are divided into two different areas. First the weighted average electrical and mechanical power are presented and thereafter the different heat flows.
3.1 Electrical power In Figures 2 and 3 the weighted average electrical power for the climate control system is presented. The different components such as blower, FOH electrical load, coolant pump, cooling fan and compressor clutch and control are included in the figures. The worst case was the non-energy saving Case 2 without recirculation and the best case was the perfect HVAC blower, Case 4. Other notable measures were Case 8 which decreased the average clutch power and Case 17 which decreased the blower power during the transients. In general the energy saving measures had quite small effect on the average electrical power.
In Figures 4 and 5 the compressor power is presented. As for the electrical power the non-energy saving Case 2 without recirculation was the worst case and the best cases was the 100% efficient AC compressor, Case 11, with a reduction of mechanical power of 36%. Note that the savings for Case 8, AC off 15°C and below, shows savings of 27%. One general results is that it was quite difficult to reduce the average power by changes in the sink category, the main reason for this was the maximum limit on evaporator temperature of 8°C. For an elaboration on this subject see the discussion section. Some cases show an increase in the average compressor power, Case 13 and 15. The main reason for this is increased airflow in the intermediate test which outweighs the potential savings in other conditions.
Figure 4. Average mechanical loads, Cases 1-12
Figure 2. Average electrical loads, Cases 1-12
Figure 5. Average mechanical loads, Cases 13-23
3.3 FOH Figure 3. Average electrical loads, Cases 13-23
3.2 Mechanical power The only part of the climate control system that directly use mechanical power is the AC-system, more precisely the compressor.
The FOH was only activated in the cold test and only in the early parts of the test. When activated the FOH supplied approximately 5 kW to the coolant. See Figures 6 and 7 for the heat flow from the FOH. With more available heat through recirculation the FOH could be disengaged earlier, see Case 3. With no heat pickup, Case 6, the FOH heat flow was increased with 4%. In Case 15, perfect preconditioning, no extra heat was required from the FOH.
Figure 6. Average heat flows from the FOH, Cases 1-12
Figure 8. Average heat flows from sources, Cases 1-12
Figure 7. Average heat flows from the FOH, Cases 13-23
Figure 9. Average heat flows from sources, Cases 13-23
3.4 Sources of Heating and Cooling
3.5 Heat Flow into the Passenger Compartment
There are many different sources for heating and cooling in the climate control system. For example, heat flow can originate from the engine, FOH and due to heat pickup of the incoming air. Sources of cooling can be sensible cooling over the evaporator and recirculated air. See Figures 8 and 9 for the effect on these sources for the different energy saving measures. Note the small effect of recirculation, this was due to the small temperature difference with the ambient temperature in the warm test, the only test with recirculation. However, recirculation reduced heat pickup and latent cooling, compare Case 2 and 3 in Figure 8.
Air is used to condition the passenger compartment, that is, add or remove energy. In Figures 10 and 11 the heating and cooling flow into the passenger compartment is presented. Cases 3 to 12 only have changes affecting the source and the heat flow into the passenger compartment should be roughly the same for all cases. However, due to the difficulty to achieve exactly the same conditions there were some differences. Three cases which affect the sun load, Case 14, 20 and 21, shows reduction of the required cooling but an increase in the required heating. In the intermediate test the sun load was used for heating, reducing the demand from other sources. Large reductions of required heating can be achieved by many sink cases; the cold masses and large temperature difference with the ambient temperature increases the effect of many saving measures.
Most of the energy saving measures could not realistically be implemented into a vehicle, however, this was not the purpose of the research. Instead the results can show which measures that have the most potential for further investigation. The passenger compartment temperature was comparable for all tests. This was mainly achieved by reducing the airflow during the transient state and reduce heating and cooling during steady state. For cases with the solar radiation reduced the climate comfort would be better with higher temperatures, however, the same passenger compartment temperatures were used throughout all tests. As a result, there is more potential for energy savings for Case 20-22 than reported.
4.2 Results Figure 10. Heat flows into the passenger compartment, Cases 1-12
Figure 11. Heat flows into the passenger compartment, Cases 13-23
4. Discussion 4.1 Method The base model was extensively verified and demonstrated good agreement between measurement and simulation except for the compressor power, see [28]. In this investigation the main focus was on relative differences which reduce the sensitivity to absolute errors. Furthermore, the compressor power was never compared to any other load. For an extensive discussion regarding the test cycle, see [29]. In summary the test cycle although simplified accounted for many realworld effects. Some electrical loads related to the climate control system were excluded, for example seat heating. The reason for excluding seat heating was that it was directly controlled by the customer. Furthermore, the mechanical power of the engine coolant pump was not included, despite being important for the heat flow from the engine to the heater. The electrical power of the condenser cooling fan can also be requested by the engine, however, only the ACsystem request was included.
The effect on the average electrical power for the energy saving measures were in general small, especially for the sink category, see Figure 2 and 3. The main reason was that the linear blower controller controls the airflow by reducing the voltage through heat dissipation. In the intermediate range a decrease of voltage will not save much energy, instead more energy will be dissipated. For Case 4 the reduction is large, mainly due to the changed blower control unit but obviously also due to the efficiency increase. The results showed that the blower power is the most important electrical load, using more than 50% of the total electrical power with a large potential for reduction. The compressor clutch and control power can be reduced by not engaging the AC-system, however, other electrical loads have small potential for a reduction of the energy use. The same tendency as for the electrical power could also be seen for the mechanical power, Figure 4 and 5. That is, the sink measures does not have a large effect on the average compressor power. The main reason for this is the maximum evaporator air temperature of 8°C. If the system operates at this temperature a decrease in required cooling for the passenger compartment is achieved by increasing heat flow, i.e. it will no reduced the compressor load. In Case 9 the maximum evaporator temperature is increased with four degrees to 12°C and it reduced the average power with 13%. The main reason for a low maximum evaporator temperature is noise, odor and controllability. One of the best measures for reducing mechanical power is to avoid engaging the AC-system when it is not needed for cooling or dehumidification. In this investigation that is simulated by disengaging the AC-system below 15°C and this reduces the average load with 27%. To be able to implement this for an automatic climate control system the humidity levels need to be monitored and the ACsystem engaged if there is a risk of condensation on the inside of the windows. Reducing the temperature and/or humidity before the evaporator could also save energy, see Case 3 and 6 for examples of this. There could be a lot of energy savings by increasing the mechanical efficiency of the compressor, Case 11, however, what types of measures this could be was not investigated in this research and the final realistic potential was therefore unknown. In general it was difficult to reduce the heat flow from the FOH, primarily due to the deactivation condition. The main condition for deactivation was achieving a specific coolant temperature, therefore even with reduced heat demand the FOH operated quite long. A more demand focused control could reduce the heat flow.
The airflow has a large effect on the energy use and this can clearly be seen for Case 15, perfect preconditioning. Despite significantly reducing the energy for cooling in the warm test the total cooling demand is increase by 6%, which increases the average compressor power with 2%. The reason is higher airflow from the start of the intermediate test thanks to available heat. Note that the energy used for the preconditioning is not included. In general the ventilation airflow of a vehicle is high compared to other environments such as buildings and this have a large effect on the energy use.
Acknowledgements This study was sponsored by FFI Energi & Miljö.
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The results from a comparison of the heat flow into the passenger compartment with the heating and cooling sources are noteworthy. First, the sources are much larger than the flow into the compartment, mainly due to reheat in intermediate conditions. Furthermore, the heat pickup need to be cooled when the AC-system is engaged. This indicate that all energy saving measures in the sink category will have smaller percentage effect on the sources than the actual percentage decrease of the heat flow into the passenger compartment. Clearly energy saving measures need to address the source initially, primarily reducing the reheat by using less AC and increasing maximum evaporator temperature. Without these actions other energy saving measures will have much less effect. How the system operates in intermediate climate defines the energy consumption!
In total 21 energy saving measures divided into two categories, source and sink, were investigated. The source category included measures affecting how the heating and cooling flows were generated and the sink category affected the required heat flow into the passenger compartment. In general the reduction of average electrical power for the 21 tested measures was small, although the blower showed most potential for improvement with a decrease of 46%. For the average mechanical power the measures with most potential were disengaging the ACsystem below 15°C ambient temperature and increasing the evaporator temperature. These measures showed reductions of 27% and 13%. Measures in the sink category could decreased the heat flow into the passenger compartment substantially, however the decrease of the total energy use was much smaller, mainly due to reheat. This means that energy saving measures on the sink will regularly have small effect and energy saving measures in the source category, i.e. how the heating and cooling is generated, needs to be addressed first. How the system operates, i.e. the source category, in intermediate conditions determines the overall energy consumption. Furthermore, few single energy saving measure can reduce the overall energy use substantially.
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Definitions/Abbreviations
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[21]
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1D
One dimensional
AC
Air Conditioning
Blower
Climate system blower
ECC
Electronic Climate Control
Cooling fan
Electric cooling fan for the radiator and ACsystem condenser Fuel Operated Heater
FOH
HVAC
Temperature increase of air due to hot conditions (surfaces) Heating, Ventilation and Air Conditioning
PWM
Pulse Width Modulation
Reheat
The climate control unit cools the air, mainly for dehumidification, and then reheats the air The three tests in cold, intermediate and warm conditions combined Worldwide harmonized Light vehicles Test Procedure Heat flow (W)
Heat pickup
Test cycle WLTP
Ambient temperature (°C) Average temperature for a test condition (°C)
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S. Wirth, F. Niebling, U. Logasanjeevi, V. Premchandran, Improved thermal insulation for contemporary automotive roof structures based on a computational fluid dynamics heat flux approach, Heat Transfer Engineering. (2016) 1–31. doi:10.1080/01457632.2015.1136170.
Flow temperature (°C)
Heat capacity (J/kg K) Mass flow (kg/s)
Graphical abstract
Highlights
Twenty-on energy saving measures for vehicle interior climate were evaluated Few single energy saving measures could reduce the energy use significantly The operation of the system in intermediate conditions determines the energy use Required heating/cooling of passenger compartment had small effect on energy use