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3D Thermal simulation of a vehicle cabin with a view to reducing air conditioning loads
3D Thermal simulation of a vehicle cabin with a view to reducing air conditioning loads D BRIDGE MIRA Ltd., UK
ABSTRACT The use of air conditioning for vehicles in hot climates is a significant contributor to both overall fuel consumption and tailpipe emissions. It can be seen, however, that much of the energy required by the air conditioning goes not into cooling the occupants but cooling the vehicle structure. Most air conditioning loads are highest after the vehicle has soaked in the sun for a period of time. Consequently, the focus must be on limiting heat penetration into the vehicle in order to reduce the peak and steady state cooling loads. For this paper a modelling approach is used to assess the impact of various palliative countermeasures. A Radtherm heat transfer model was constructed for a saloon type vehicle to mimic a solar soak and subsequent pull-down for a simulated climatic chamber test. The base vehicle air conditioning load was identified before various material changes to the vehicle structure were implemented. The impact of each, and the results for an optimum combination, is presented in terms of average interior air and selected component surface temperatures at the end of the soak and pull-down periods. Additionally, the impact of forced ventilation pre-conditioning for the soak period, as may be driven by an integrated solar panel, was also investigated. 1
INTRODUCTION
Although clearly dependant upon the prevailing environmental conditions, AirConditioning (AC) provision can represent one of the largest vehicle ancillary loads. Even for a small sized vehicle cabin, cooling requirements can be in the region 3.55kW. Clearly this has a significant bearing on the overall energy management of the vehicle and even modest enhancements in AC efficiency can have an influence on engine load (for a traditional belt driven mechanical compressor) with subsequent improvements to fuel consumption and emissions. The environmental impact of air conditioning use, both in terms of tailpipe emissions and additional fuel consumption, depends on a number of factors particularly local ambient conditions (temperature, humidity and solar load), driving pattern and powertrain type. Typically AC related fuel consumption can increase by 7% in the US/Europe but by 20% in India, for example, where high levels of temperature and humidity are common (1). This, along with the rising global demand for AC equipped
______________________________________________ Copyright of the author(s) and/or their employer(s), 2011
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vehicles, has lead the SAE to adopt a new standard in 2009 whereby the full life cycle environmental impact of a given air conditioning system can be evaluated for a wide range of world wide locations using test bench data (2). The detrimental effect of air conditioning use has been studied by a number of authors. For example, Welstand et al (3) compared a range of vehicles for different drive cycles with AC off versus AC on and found NOx increased by 0.1 to 0.6g/mile, CO by 0.2 to 12g/mile and fuel consumption by 2.5 to 4.5 mpg. The economic impact of AC use and methods of reducing thermal loads has been studied extensively by the National Renewable Energy Laboratory (4) over many years. They have shown that hybrid powertrains are even more sensitive to AC use, showing as much as 35% reduction in fuel economy. Whilst AC load is typically accepted and adsorbed for traditional powertrains, in Electrical Vehicles (EV’s), where the compressor is electrically driven and powered by a battery, then AC load provision has a significant detrimental impact on range, one of the major sales attributes for an EV. According to the US Department of Energy, the impact of AC use on (EV) range can lead to a reduction of up to 30% (5). Consequently, with the move to new powertrain technologies, there is now a strong driver to reduce AC load requirements as historically the emphasis has largely been on capacity targets rather than efficiency. Cooling loads are generally highest for vehicles parked for a period in the sun (solar loading). It follows, therefore, that reducing the thermal loading of the cabin in the first instance reduces the amount of subsequent cooling required. The wisdom of this approach is confirmed when one considers that the metabolic rate by a typical clothed person driving a vehicle is of the order 60 to 115W/m2 (6). Assuming a standard human surface area of 1.6m2 then this equates to ~96-184W per person. Compare this to the 3.5-5kW required cooling capacity. This suggests that cooling the vehicle structure is the dominant factor, not the vehicle occupants. Reducing the thermal load into the cabin structure suggests that energy provision for AC can also be reduced and more efficiently focused and managed onto the vehicle occupants. It was the aim of this work to use 3D modelling techniques to investigate the thermal influence of solar soak conditions on a vehicle structure and then to use the model to explore possible methods of decreasing the AC load. 1.1 Simulation objectives There were three main objectives to the simulation:1. Baseline thermal load: The first objective was to determine the cabin thermal load of the reference vehicle under harsh but realistic climatic conditions. 2. Thermal Load minimisation: Once the base load had been established, the second objective was to determine what changes could be made to the vehicle materials in order to reduce/minimise the cabin thermal load, thereby directly reducing both the peak and steady state AC energy demand. 3. Forced ventilation cooling: The use of the photovoltaic (PV) panels has been looked at by a number of OEMs, as a means of additional ‘free’ electrical energy when the vehicle is exposed to sunny conditions. Furthermore, with the vehicle static, this energy can be used to recharge the battery and/or drive the HVAC blower, whereby cooler exterior air is blown into the cabin and the hot internal air is expelled out
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through the vehicle extracts (’fresh’ air mode). The advantage is that when cabin heating is worst (sunny conditions) the PV panel is also producing the most power and hence airflow rate is highest. 2
SIMULATION APPROACH
The simulation reference vehicle was based on an Alfa Romeo 159 Station Wagon. The complete vehicle CAD data was received from a number of sources and as normal the assembled CAD needed tidying up and refining into a suitable form for thermal analysis. This included removal of unnecessary minor features and stripping away unwanted surfaces, neither of which would affect the resulting simulation. As a final activity a quad dominant surface mesh (consisting of around 0.3 million elements) was created to then import into the heat transfer software package. A sectional view of the final model is shown in Figure 1. Note to save using unnecessary surface elements, the engine was represented as a simple block structure.
Figure 1. Sectional view of model Thermal analysis was undertaken with Radtherm heat transfer analysis software. This is a 3D code, but differs from CFD in that only the surfaces and not the air volumes are meshed. As such, convection is modelled as 0D or 1D rather than 3D. Conduction and radiation are modelled as 3D. This allows for a much coarser mesh than would otherwise be used for traditional CFD models and consequently for a much more rapid solution of complex systems. This is important for transient runs where solutions are computed for small time steps over a relatively long period of time – 3 hours in this case. The lower surface mesh count also greatly reduces the numerically expensive view factor calculation which has to be performed for determining radiation heat exchange between elements. Material properties were allocated to each part, with most parts being multilayers. For instance the roof comprised painted outer steel skin, air gap and interior roof liner. Besides material type, an estimate of the material thickness had to be made. Unless already known, the thickness used was based on the part surface area and the estimated expected weight for that part. Additionally, the surface properties (emissivities) were assigned. For the base vehicle, the exterior was black paint with dark leather interior and standard automotive glazing all round. The ground was uncoloured concrete.
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2.1 Cabin convection simulation Whilst all three heat transfer processes are considered within Radtherm, as there is no air volume mesh, convection is handled in a number of ways, either imported as CFD convection coefficients/film temperatures on a per element basis or defined using inbuilt library 0D convection coefficients or predefined convection coefficients/film temperatures on a part by part basis. The software also allows for the use of 1D fluid nodes, that can be connected to adjacent surfaces and/or other nodes and this was the approach used for the vehicle interior spaces. For the purposes of this analysis the cabin was assumed perfectly sealed although in practise some exchange with the outside air will occur.
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Figure 2. Cabin air-nodes and their interdependance The placement of the interior cabin nodes is shown in Figure 2. Each node was ‘connected’ to its adjacent surfaces ie convective heat transfer occurred between that surface and the node. For instance, the parcel shelf node was connected to the upper surface of the parcel shelf, rear quarter lights and D pillars, rear roof liner section and rear screen. Figure 2 also shows how the interior cabin nodes were connected with each other. As the boot space is independent of other areas no connection to this node was made, the same being true of the node within the IP. Additionally, two HVAC nodes were also connected to the inner face of bespoke plastic ducting under the IP (not shown), the outer faces of which were connected to the independent IP node. This allowed for a degree of heat pick-up as air left the HVAC and transited into the cabin. Figure 2 also illustrates how the HVAC outlet flow was simplified as being equally split on left and the right hand sides. During the soak phase the air nodes were allowed to ‘mix’ at a fairly low rate in order to account for natural convection buoyancy. With the vehicle running, the HVAC outlet flows were set at 60l/s per side and, as the system was assumed to be in recirculation mode, flowed to the row two seats before retuning
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via the lower air nodes to the front. Some intermixing with the parcel shelf node was also allowed for. Other independent air nodes were also used within the model, such as inside the tyres, the spare wheel well, the centre console, seats, doors and under the bonnet. On the exterior surfaces, air flow velocity was set according to the vehicle speed. 2.2 Test profile In the model all parts and air nodes were initially set to 43°C and a horizontal solar load then applied, at a constant 1000W/m2. After 90 minutes, and with the solar still present, the vehicle was ‘driven’ at a constant 50kph. After a further 30 minutes the speed was increased to 100kph for 30 minutes before the vehicle was stopped and idled for the last 30 minutes. In the simulation, the impact of driving the vehicle was simulated in two ways:• •
Airflow (at 43°C) over the vehicle exterior at 50, 100 and 0kph Appropriate increase in engine and exhaust temperatures
2.3 Evaporator loading One role of the evaporator is to remove sensible heat from the air passing through it and transfer it to the refrigerant. A typical (airside) cooling capacity curve for sensible heat for a similar vehicle is shown in Figure 3 for the pull-down profile discussed in 2.2. It can be seen that the peak load is ~4.4 kW and gradually drops off to a steady state value of ~2.9kW during the idle phase. The gradual drop is due to the fact that the recirculated air incident on the evaporator is obviously being progressively cooled until the heat extracted from the cabin via the HVAC matches the input heat from the ambient (conduction, radiation etc). In the simulation, the evaporator capacity curve was replicated as shown in Figure 3 and used to extract heat from air returning from the cabin once the AC pull-down had begun. 5.0 4.5 4.0
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Figure 3. Initial evaporator capacity curve used in the simulation
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3
RESULTS
The resultant model consisted of some 0.3 million elements and 0.8 million thermal nodes when accounting for multiplayer parts. The simulation was run over the 180 minutes with a time step of 2.5 minutes, using an convergence tolerance of 0.2°C. Results were saved every 10 minutes. Typical run times were ~12.5 hours on four processors. Although it was expected to have only a minor influence on internal temperatures, during the pull-down phase the simulated engine surface was allowed to eventually heat up to 90°C and the exhaust surface up to 150°C, dropping to 100°C at the end of the idle phase. 3.1 Evaporator loading Figure 4 shows the average internal air temperature for the vehicle during the soak and pull-down phases. Also shown is a breakdown of this average into front row (Row 1) and second row (Row 2) cabin air temperatures. Several observations can be made:• • •
As required, the start temperature is 43°C. After 90 minutes the internal temperature (average) has reached ~63°C. During the heating phase Row 1 is hotter than Row 2 primarily due to the effects the IP surface heat pick-up – Row 2 is almost directly shielded from the overhead solar load as no sunroof or panoramic roof was present. During the pull-down, Row 1 and 2 temperatures are similar as they are dominated by the high interior airflow (120 l/s). The internal temperatures cool rapidly at the start of the cool-down and by the end of the 90 minutes drive profile the internal temperature has reached 12°C
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Figure 4. Interior cabin air temperatures during soak and subsequent pull-down
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Figure 5 shows the temperature variations over the internal and external surfaces at the end of the 90 minute soak (left) and at the end of the pull-down (right). At the end of the soak the high temperatures on the upper exterior surfaces can be clearly seen. As the solar load is directly and continually overhead, the internal trim directly below the glazing reaches a higher temperatures than the rest of the trim. This can be seen particularly in the fronts seats (mainly the right hand side as the left side is partially obscured by the steering wheel) and the IP topper, which reaches nearly 85°C.
Figure 5. Vehicle interior/exterior at the end of soak (left) and pull-down (right) As would be expected, at the end of the pull-down the interior trim surfaces are substantially cooler although the upper exterior panels are relatively unchanged. It is this heat, the constant solar load (either directly through the glazing or via conduction and convection predominantly through the roof) and the high ambient temperature that the AC has to remove. As the interior target temperature for comfort is in the region 20-22°C, Figure 4 suggests that the 4.4 kW (peak) curve is too high and that the capacity of the evaporator can be reduced. Several more iterations were run, where the applied heat rate curve of Figure 4 was progressively reduced each time, but retaining the same overall shape. It was found that a curve with a peak value of 4kW was sufficient to give an end point of 20°C. This curve was then used in the subsequent analysis runs reported in section 3.2. For this work the actual vehicle performance curves were not available so the reported temperatures cannot be correlated against physical test data. However the main focus here was to look at changes relative to the baseline rather than the absolute values themselves. 3.2 Reduction in thermal load The aim of this section of work was to see how variations in the vehicles construction could be used to reduce the transmission of external heat (solar and ambient air temperature) via conduction, radiation and convection, into the vehicle interior during hot soak conditions. The objective was to reduce the final soak internal temperature and
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consequently to reduce the amount of AC load capacity required to still provide a comfortable interior. The following alterations were made to the base vehicle model:•
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Black exterior paint swapped to white – this was investigated as lighter coloured surfaces reflect the visible portion of the electromagnetic spectrum more than darker ones. This visible portion accounts for ~50% of the thermal load (the other portion being in the IR band, with ~5% in the UV band) Standard glazing replaced with IR Reflective Glazing (IRG) all round. About 5075% of the heat entering the cabin can do so via the glazing (7). Consequently any chance of eliminating heat transfer via this route will be beneficial. IRG on front and front side screens, privacy glazing to remainder – privacy glazing differs from IRG in that the coloured glass absorbs a certain amount of both visible and IR bands, which is then re-radiated and convected on both inside and outside faces, as the glass itself heats up. This is not as efficient as IRG as 50% of the heat is still transmitted inside the vehicle but as the glass is just coloured, rather than laminated, it is a significantly cheaper option. Base car but with light coloured interior trim surfaces – The base car has simulated with dark coloured leather trim, so a lighter coloured material (seats, IP and door panels) would expect to be slightly cooler as some of the heat is reflected away, although some of this will impinge on other adjacent surfaces. Base car but with insulation in roof and door panels – Insulation should be helpful in stopping heat getting into the vehicle and in the model 10mm of glass-fibre insulation was used in the roof and door panels. Base car with sun blinds at all surfaces. As most heat entering a vehicle does so through the glazed surfaces. The use of sun blinds is a possible way in reducing this heat transfer path. However, as they are positioned inside the car, the heat (visible and IR) has already entered the vehicle. In the model, blinds were placed 15mm behind every glazed surface and additional air nodes introduced linking the back face of the window with the front face of the adjacent blind. The rear faces of each blind were then connected to the relevant adjacent interior air nodes (Figure 2). Two blind types were investigated, dark (emissivity & absorptivity on both faces = 0.9) and silvered (emissivity = 0.22, absorptivity = 0.49).
The results for relevant interior temperatures at the end of the 90 minute soak period are shown in Figure 6a and at the end of the idle phase in Figure 6b. The reported results break down into two sections (front row and second row air temperatures along with the overall interior average) and three surface temperatures, steering wheel and (front) seat cushion and seat back. The latter are relevant as it is these surfaces that are in contact with the driver and thus directly impact on occupant comfort. Figure 6a shows that as expected the base vehicle is the worst case and that at the end of the soak period the interior surface temperatures are substantial, nearly 90°C for the front portion of the front seat cushion. Changes to vehicle materials and finish largely have a bigger effect on the surface temperatures than they do for interior air temperatures. The same can be said for temperatures the end of the idle phase (Figure
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6b). Note in this figure, for reference, the two blind results are also reported but clearly the vehicle would not normally be run with the all the blinds deployed. From these results an ‘optimised’ vehicle solution was chosen. This consisted:• Light coloured exterior (white) • IRG for all glazed surfaces • Light coloured interior trim 90 80 70
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Figure 6a – Peak temperatures at the end of a 90 minute solar soak 90 80 70
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When this new optimised vehicle was run at the existing 4kW peak evaporator capacity, then the bottom curve depicted in Figure 7 was the result. As the heat load into the vehicle is reduced for both the soak and running phases, not surprisingly the resultant interior temperatures are much reduced, by around 10°C at the end of the idle phase. Therefore there is scope to reduce the AC capacity to yield the same level of comfort achieved for the base vehicle. This has the obvious benefit that the energy consumption of the vehicle is also reduced. 70
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Figure 7 – Interior temperatures at differing levels of evaporator capacity Consequently, the evaporator capacity curve depicted in Figure 3 was further factored down to give peak loads of both 3.5 and then 3.0 kW. The resultant interior air temperatures can be seen in Figure 7. By reducing the AC (peak) load to 3 kW in the optimised vehicle the same level of comfort can be achieved as using 4 kW of cooling in the base vehicle. 3.3 Impact of active cooling The 3D cabin model was also used to determine the likely effect pre-ventilation, or active cooling, would have on interior temperatures and hence any further opportunities in AC load reduction. Here an 80W (peak) solar panel was assumed to drive the HVAC blower whilst the vehicle was parked in the sun prior to the standard pull-down. To calculate the expected level of air flow, the following assumptions were made:• • • • •
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~15% of maximum power is lost in conversion efficiencies and other losses. As the model simulates a chamber test with 1000W/m2 solar load, this power is available constantly. 68W at 12.8V equates to 5.3A available at the blower motor. From the blower characteristic curve, this would yield an airflow of ~38 l/s from the face vent outlets. In the model this was split into 19 l/s per side.
To have the maximum benefit, the cabin air inlet operation needs to be in ‘fresh air’ mode. To accommodate this change, the interior air node paths (Figure 2) were rearranged to reflect the active cooling approach. Here the inlet ambient air is allowed to flow out the back of the vehicle via the parcel shelf. 70
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Figure 8 – Interior temperatures for base car with and without active air cooling The resultant interior air temperatures during the soak, followed by the normal pulldown at 4kW, are shown for the base vehicle in Figure 8. It can be seen that the effect of pre-ventilation is to reduce interior air temperatures by ~10°C. Additionally, the pulldown is initially slightly quicker as the starting point is lower. However, as the final temperature at the end of the idle was unchanged, no further reduction in AC load was advised although in practise a small reduction may be realsied. 4
DISCUSSION
The interior average air temperatures depicted in Figure 4 are typical for this type of test. In the soak phase the vehicle is effectively sealed so air trapped inside the vehicle cabin rapidly warms up, driven by the high level of solar loading on the external and some internal surfaces. As soon as the pull-down phase starts the air temperatures drop rapidly due to the low thermal inertia of the air. By the end of the 100kph phase (60 minutes elapsed driving time) steady state conditions are being achieved as the heat inputs start to match the heat being removed:Input Mechanisms ΔT outside to inside (43-~20°C) Solar load at 1000W/m2
Removal Mechanisms Ambient airflow over vehicle structure Heat pick-up in refrigerant (rejected at condenser)
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Additionally, as the evaporator cools towards zero, compressor control will also start to regulate the amount of heat removed from the cabin in order to prevent evaporator icing. For the idle phase the temperatures rise again due to several factors:• • •
Airflow over the vehicle exterior has ceased, thus the exterior starts to warm back up again, under the constant solar load. At idle the engine runs at low speed and consequently so will a mechanically driven compressor with a net reduction in refrigerant mass flow (not implicitly modelled here). Air re-circulation effects at the front of the vehicle degrades condenser performance; air being drawn onto the condenser is hotter than ambient air (not implicitly modelled here).
For the base vehicle, the peak AC load requirement was determined as being ~4kW based on sensible heat transfer, settling at ~3kW steady state for the idle (Figure 3). Sensible heat only was considered as Radtherm does not take into account air humidity changes. In reality the evaporator capacity is a combination of both sensible and latent heats, as moisture is removed from the air passing over the cold evaporator core. As the air is in recirculation mode, eventually the dew point temperature will stabilise with little additional latent heat removal, so the assumption made above would be valid. However, at the start of the pull-down a large amount of moisture has to be removed, and this can easily constitute 50% of the initial load, and so a more realistic load requirement for the base car would be nearer ~8kW peak, falling to ~3.5kW steady state. 4.1 Impact of preconditioning technologies As expected, the base vehicle with its dark exterior and interior gave the worst performance in terms of soak and pull down temperatures for both air and internal trim temperatures. The aim of this section of work was to identify possible ways of reducing the AC load, without necessarily compromising interior comfort, through changes to the vehicles final finish i.e. the same target interior temperature at the end of the pull-down phase. The list of changes made is outlined in section 3.2 and is comprised mostly of changes that could be easily implemented by an OEM although not necessarily embraced by the customer! For this vehicle, swapping to a white exterior finish had a noticeable impact on air temperatures, resulting in a 6°C reduction in average internal air temperature at the end of the vehicle soak. Trim surfaces temperatures were also reduced. Whilst choice of colour still needs to reside with the customer, modest reductions can also be achieved through the use of IR reflective paints that have been fashioned to give the same visible appearance but reflect a higher proportion of incident energy (6). A similar argument over customer choice holds for the colour of the internal trim. As expected, seat and steering wheel temperatures were reduced by moving to a lighter colour, especially for the front of the seat cushion that was exposed to direct solar energy transmitted through the wind screen. Air temperatures are only reduced a little however, as the increase in reflected energy is still absorbed within the vehicle. As glazing represents such a significant transmission path for thermal energy, it has been studied by a number of authors (e.g. 8, 9, 10, 11). IRG is a laminated based
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product that allows the transmission of visible light, making it acceptable for use in front of vehicle glazing, but the silver doped interlayer acts to preferentially reflect Infra Red light. Its replacement for all the glazing in the model was also beneficial compared to the base vehicle, again particularly for the steering wheel and front seat area that are subjected to direct radiation through the front screen. Besides cost increase, one disadvantage of IRG is that it stops electromagnetic signals required for phones, GPS etc. Consequently, a compromise solution was also examined, whereby the IRG from the B pillar rearwards was replaced with privacy glazing. As expected, the performance of this arrangement was slightly worse than 100% IRG due to 50% of the absorbed heat in the privacy glass being partly retransmitted inside the vehicle. Insulation potentially offers a low cost means of AC (and heater) load reduction. In the model the base car had insulation added to the roof and door panels and proved largely beneficial. However, these panels, particularly on the roof, can also act as a means of loosing heat from the vehicle as the exterior ambient cools. As a result, insulated vehicles left at the end of the day may end up hotter than their uninsulated equivalents. The use of sun blinds is an alternative method of disrupting the solar radiation path into the vehicle and temporary reflective blinds can often be seen in vehicles parked up in hot countries. From a heat transfer viewpoint, blinds would ideally be placed outside the glazing; by having blinds on the inside, the solar radiation has already entered the vehicle by the time it meets the blind surface. Black blinds will effectively absorb this radiation yielding a hotter interior compared with more reflective material and the results in Figure 6 confirm this, especially for the trim surfaces. Reflective blinds appear to significantly reduce front row air temperatures (~5°C reduction) and steering wheel, front seat (cushion) temperatures (27°C reduction). Whilst taken on their own, the reductions brought about by these preconditioning technologies are relatively modest compared with the base vehicle. However, when used in combination, the benefit was shown to be significant. The example in Figure 6 combines just three; white exterior paint, light coloured interior trim and IRG for all glazing surfaces. This combination results in an 11°C reduction in final soak temperature and, when using the same evaporator capacity curve, a similar reduction in final cabin temperature at the end of the pull-down. This suggests that the capacity can be reduced by ~25% in order to achieve a similar level of comfort experienced in the base car if these changes were to be adopted. 4.2 Impact of active cooling Inclusion of a photovoltaic panel on the roof of the vehicle opens up many options for energy management, provided a reasonable level of sunlight is available. The opportunity of using this available power to drive the HVAC blower whilst the vehicle is in its soak state is well known in the automotive industry and solar panel equipped roof panels are offered as optional fit on some higher end vehicles. Figure 8 shows the benefits of drawing in exterior ambient air which, when in fresh mode, displaces the hotter air inside the vehicle cabin. At the end of the soak the interior air temperature is cooler so the pull-down is, initially, slightly more rapid. However, this benefit is quickly lost as the pull-down becomes dominated by the vehicle interior structure which still retains a significant amount of heat and it is this heat that dictates
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the rest of the pull-down. This result appears consistent with the findings of other authors (12,13). Furthermore, the benefits described here would be diminished in reality as air entering the vehicle in the early mornings and late afternoons, where the reduced solar levels would yield lower flow rates, would give higher levels of heat pick-up in the inlet system than resulted here. 5
CONCLUSION
A 3D thermal cabin model has been used to determine the heat transfer characteristics of a typical saloon vehicle. Specifically, it has been used to investigate ways in which air conditioning loads can be both quantified and then reduced in order to reduce the overall energy demand on the vehicle. This approach is important in lowering fuel consumption in conventional vehicles but is particularly important in those vehicles that rely on battery power as the principal energy source as AC provision has a significant detrimental impact on vehicle range. The approach adopted here suggests that AC load reductions of 25% are possible when vehicle materials are selectively used to minimise heat penetration into the cabin, although the choices made may not be acceptable to the customer for other reasons. Drawing in ambient air during the vehicle soak though the use of the HVAC blower driven by the energy from a photovoltaic panel is also beneficial. However, although interior air temperatures are reduced during the soak, possibly leading to improved comfort on entry, the AC load requirement is not significantly impacted due to retained heat within the vehicle structure. The software code appears well suited in determining the thermal environment in the cabin. It is critical that any code can robustly handle the radiation effects of heat transfer, and the related view factor calculations that are also required, as thermal radiation dominates when AC loads are highest, both directly, from the sun, and via reflections, from the IP and ground for example. Ideally the results would have been correlated against actual test data but this is not within the scope of the project. However, just focusing on the sensible heat capacity of the evaporator underestimates the total AC load required, especially at the start of the pull-down where humidity levels are highest. ACKNOWLEDGEMENTS This work was supported by funding from the EU Commission under the Seventh Framework programme as part of the EE-VERT project. Contributing partners in the project are Beespeed Automatizari, Bosch, CRF, Lear, Engineering Center Steyr, MIRA, Volvo Technology and the Universities of FH Joanneum Gesellschaft and Politehnica din Timisoara
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Papasavva, S and Monforte, R., “An update on the LCCP approach for MAC systems”, 3rd European Workshop on Mobile Air Conditioning, Vehicle Thermal Systems and Auxiliaries”, Turin, 2009. “Life cycle analysis to estimate the CO2-equivalent emissions from MAC operation”, SAE Standard J2766, 2009. Welstand, S.J. et al, “Evaluation of the effects of air conditioning operation and associated environmental conditions on vehicle emissions and fuel economy”, SAE Paper 2003-01-2247, 2003. Bharathan, D., “An overview of vehicle test and analysis results from NREL’s AC fuel reduction research”, VTMS-8, Nottingham 2007. http://www1.eere.energy.gov/vehiclesandfuels/avta/light_duty/fsev/fsev_auxiliary.html Accessed February, 2011. Ashrae handbook, Fundamentals, ASHRAE, Atlanta, 2009, ISBN 1-933742-55-7. Shimizu, S., Hara, H. and Asakawa, F., “Analysis of air-conditioning heat load of a passnger vehicle”, Int. J. Vehicle design (4:3); p292-311, 1983. Rugh, J. et al, “Reduction in vehicle temperatures and fuel use from cabin ventilation, solar-reflective paint and a new solar-reflective glazing”, SAE Paper 2007-01-1194, 2007 Rugh, J., Hendricks, T. and Koram K., “Effect of solar reflective glazing on Ford Explorer climate control, fuel economy and emissions”, SAE Paper 2001-01-3077, 2001 Turler, D., Hopkins, D. and Goudey, H., “Reducing vehicle auxiliary loads using advanced thermal insulation and window technologies”, SAE Paper 2003-01-1076, 2003 Han, T et al, “Assessment of various environmental thermal loads on passenger thermal comfort”, SAE Paper 2010-01-1205, 2010. Parrino, M. et al, “Investigation of pre-cooling effectiveness in vehicle cabin cooldown”, SAE Paper 2004-01-1380, 2004. Roy, D. et al, “Experimental investigation of a thermal preconditioning of a car cabin”, SAE Paper 2005-01-2057, 2005.
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ABSTRACT The use of air conditioning for vehicles in hot climates is a significant contributor to both overall fuel consumption and tailpipe emissions. It can be seen, however, that much of the energy required by the air conditioning goes not into cooling the occupants but cooling the vehicle structure. Most air conditioning loads are highest after the vehicle has soaked in the sun for a period of time. Consequently, the focus must be on limiting heat penetration into the vehicle in order to reduce the peak and steady state cooling loads. For this paper a modelling approach is used to assess the impact of various palliative countermeasures. A Radtherm heat transfer model was constructed for a saloon type vehicle to mimic a solar soak and subsequent pull-down for a simulated climatic chamber test. The base vehicle air conditioning load was identified before various material changes to the vehicle structure were implemented. The impact of each, and the results for an optimum combination, is presented in terms of average interior air and selected component surface temperatures at the end of the soak and pull-down periods. Additionally, the impact of forced ventilation pre-conditioning for the soak period, as may be driven by an integrated solar panel, was also investigated. 1
INTRODUCTION
Although clearly dependant upon the prevailing environmental conditions, AirConditioning (AC) provision can represent one of the largest vehicle ancillary loads. Even for a small sized vehicle cabin, cooling requirements can be in the region 3.55kW. Clearly this has a significant bearing on the overall energy management of the vehicle and even modest enhancements in AC efficiency can have an influence on engine load (for a traditional belt driven mechanical compressor) with subsequent improvements to fuel consumption and emissions. The environmental impact of air conditioning use, both in terms of tailpipe emissions and additional fuel consumption, depends on a number of factors particularly local ambient conditions (temperature, humidity and solar load), driving pattern and powertrain type. Typically AC related fuel consumption can increase by 7% in the US/Europe but by 20% in India, for example, where high levels of temperature and humidity are common (1). This, along with the rising global demand for AC equipped
______________________________________________ Copyright of the author(s) and/or their employer(s), 2011
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vehicles, has lead the SAE to adopt a new standard in 2009 whereby the full life cycle environmental impact of a given air conditioning system can be evaluated for a wide range of world wide locations using test bench data (2). The detrimental effect of air conditioning use has been studied by a number of authors. For example, Welstand et al (3) compared a range of vehicles for different drive cycles with AC off versus AC on and found NOx increased by 0.1 to 0.6g/mile, CO by 0.2 to 12g/mile and fuel consumption by 2.5 to 4.5 mpg. The economic impact of AC use and methods of reducing thermal loads has been studied extensively by the National Renewable Energy Laboratory (4) over many years. They have shown that hybrid powertrains are even more sensitive to AC use, showing as much as 35% reduction in fuel economy. Whilst AC load is typically accepted and adsorbed for traditional powertrains, in Electrical Vehicles (EV’s), where the compressor is electrically driven and powered by a battery, then AC load provision has a significant detrimental impact on range, one of the major sales attributes for an EV. According to the US Department of Energy, the impact of AC use on (EV) range can lead to a reduction of up to 30% (5). Consequently, with the move to new powertrain technologies, there is now a strong driver to reduce AC load requirements as historically the emphasis has largely been on capacity targets rather than efficiency. Cooling loads are generally highest for vehicles parked for a period in the sun (solar loading). It follows, therefore, that reducing the thermal loading of the cabin in the first instance reduces the amount of subsequent cooling required. The wisdom of this approach is confirmed when one considers that the metabolic rate by a typical clothed person driving a vehicle is of the order 60 to 115W/m2 (6). Assuming a standard human surface area of 1.6m2 then this equates to ~96-184W per person. Compare this to the 3.5-5kW required cooling capacity. This suggests that cooling the vehicle structure is the dominant factor, not the vehicle occupants. Reducing the thermal load into the cabin structure suggests that energy provision for AC can also be reduced and more efficiently focused and managed onto the vehicle occupants. It was the aim of this work to use 3D modelling techniques to investigate the thermal influence of solar soak conditions on a vehicle structure and then to use the model to explore possible methods of decreasing the AC load. 1.1 Simulation objectives There were three main objectives to the simulation:1. Baseline thermal load: The first objective was to determine the cabin thermal load of the reference vehicle under harsh but realistic climatic conditions. 2. Thermal Load minimisation: Once the base load had been established, the second objective was to determine what changes could be made to the vehicle materials in order to reduce/minimise the cabin thermal load, thereby directly reducing both the peak and steady state AC energy demand. 3. Forced ventilation cooling: The use of the photovoltaic (PV) panels has been looked at by a number of OEMs, as a means of additional ‘free’ electrical energy when the vehicle is exposed to sunny conditions. Furthermore, with the vehicle static, this energy can be used to recharge the battery and/or drive the HVAC blower, whereby cooler exterior air is blown into the cabin and the hot internal air is expelled out
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through the vehicle extracts (’fresh’ air mode). The advantage is that when cabin heating is worst (sunny conditions) the PV panel is also producing the most power and hence airflow rate is highest. 2
SIMULATION APPROACH
The simulation reference vehicle was based on an Alfa Romeo 159 Station Wagon. The complete vehicle CAD data was received from a number of sources and as normal the assembled CAD needed tidying up and refining into a suitable form for thermal analysis. This included removal of unnecessary minor features and stripping away unwanted surfaces, neither of which would affect the resulting simulation. As a final activity a quad dominant surface mesh (consisting of around 0.3 million elements) was created to then import into the heat transfer software package. A sectional view of the final model is shown in Figure 1. Note to save using unnecessary surface elements, the engine was represented as a simple block structure.
Figure 1. Sectional view of model Thermal analysis was undertaken with Radtherm heat transfer analysis software. This is a 3D code, but differs from CFD in that only the surfaces and not the air volumes are meshed. As such, convection is modelled as 0D or 1D rather than 3D. Conduction and radiation are modelled as 3D. This allows for a much coarser mesh than would otherwise be used for traditional CFD models and consequently for a much more rapid solution of complex systems. This is important for transient runs where solutions are computed for small time steps over a relatively long period of time – 3 hours in this case. The lower surface mesh count also greatly reduces the numerically expensive view factor calculation which has to be performed for determining radiation heat exchange between elements. Material properties were allocated to each part, with most parts being multilayers. For instance the roof comprised painted outer steel skin, air gap and interior roof liner. Besides material type, an estimate of the material thickness had to be made. Unless already known, the thickness used was based on the part surface area and the estimated expected weight for that part. Additionally, the surface properties (emissivities) were assigned. For the base vehicle, the exterior was black paint with dark leather interior and standard automotive glazing all round. The ground was uncoloured concrete.
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2.1 Cabin convection simulation Whilst all three heat transfer processes are considered within Radtherm, as there is no air volume mesh, convection is handled in a number of ways, either imported as CFD convection coefficients/film temperatures on a per element basis or defined using inbuilt library 0D convection coefficients or predefined convection coefficients/film temperatures on a part by part basis. The software also allows for the use of 1D fluid nodes, that can be connected to adjacent surfaces and/or other nodes and this was the approach used for the vehicle interior spaces. For the purposes of this analysis the cabin was assumed perfectly sealed although in practise some exchange with the outside air will occur.
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Figure 2. Cabin air-nodes and their interdependance The placement of the interior cabin nodes is shown in Figure 2. Each node was ‘connected’ to its adjacent surfaces ie convective heat transfer occurred between that surface and the node. For instance, the parcel shelf node was connected to the upper surface of the parcel shelf, rear quarter lights and D pillars, rear roof liner section and rear screen. Figure 2 also shows how the interior cabin nodes were connected with each other. As the boot space is independent of other areas no connection to this node was made, the same being true of the node within the IP. Additionally, two HVAC nodes were also connected to the inner face of bespoke plastic ducting under the IP (not shown), the outer faces of which were connected to the independent IP node. This allowed for a degree of heat pick-up as air left the HVAC and transited into the cabin. Figure 2 also illustrates how the HVAC outlet flow was simplified as being equally split on left and the right hand sides. During the soak phase the air nodes were allowed to ‘mix’ at a fairly low rate in order to account for natural convection buoyancy. With the vehicle running, the HVAC outlet flows were set at 60l/s per side and, as the system was assumed to be in recirculation mode, flowed to the row two seats before retuning
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via the lower air nodes to the front. Some intermixing with the parcel shelf node was also allowed for. Other independent air nodes were also used within the model, such as inside the tyres, the spare wheel well, the centre console, seats, doors and under the bonnet. On the exterior surfaces, air flow velocity was set according to the vehicle speed. 2.2 Test profile In the model all parts and air nodes were initially set to 43°C and a horizontal solar load then applied, at a constant 1000W/m2. After 90 minutes, and with the solar still present, the vehicle was ‘driven’ at a constant 50kph. After a further 30 minutes the speed was increased to 100kph for 30 minutes before the vehicle was stopped and idled for the last 30 minutes. In the simulation, the impact of driving the vehicle was simulated in two ways:• •
Airflow (at 43°C) over the vehicle exterior at 50, 100 and 0kph Appropriate increase in engine and exhaust temperatures
2.3 Evaporator loading One role of the evaporator is to remove sensible heat from the air passing through it and transfer it to the refrigerant. A typical (airside) cooling capacity curve for sensible heat for a similar vehicle is shown in Figure 3 for the pull-down profile discussed in 2.2. It can be seen that the peak load is ~4.4 kW and gradually drops off to a steady state value of ~2.9kW during the idle phase. The gradual drop is due to the fact that the recirculated air incident on the evaporator is obviously being progressively cooled until the heat extracted from the cabin via the HVAC matches the input heat from the ambient (conduction, radiation etc). In the simulation, the evaporator capacity curve was replicated as shown in Figure 3 and used to extract heat from air returning from the cabin once the AC pull-down had begun. 5.0 4.5 4.0
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3
RESULTS
The resultant model consisted of some 0.3 million elements and 0.8 million thermal nodes when accounting for multiplayer parts. The simulation was run over the 180 minutes with a time step of 2.5 minutes, using an convergence tolerance of 0.2°C. Results were saved every 10 minutes. Typical run times were ~12.5 hours on four processors. Although it was expected to have only a minor influence on internal temperatures, during the pull-down phase the simulated engine surface was allowed to eventually heat up to 90°C and the exhaust surface up to 150°C, dropping to 100°C at the end of the idle phase. 3.1 Evaporator loading Figure 4 shows the average internal air temperature for the vehicle during the soak and pull-down phases. Also shown is a breakdown of this average into front row (Row 1) and second row (Row 2) cabin air temperatures. Several observations can be made:• • •
As required, the start temperature is 43°C. After 90 minutes the internal temperature (average) has reached ~63°C. During the heating phase Row 1 is hotter than Row 2 primarily due to the effects the IP surface heat pick-up – Row 2 is almost directly shielded from the overhead solar load as no sunroof or panoramic roof was present. During the pull-down, Row 1 and 2 temperatures are similar as they are dominated by the high interior airflow (120 l/s). The internal temperatures cool rapidly at the start of the cool-down and by the end of the 90 minutes drive profile the internal temperature has reached 12°C
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Figure 5 shows the temperature variations over the internal and external surfaces at the end of the 90 minute soak (left) and at the end of the pull-down (right). At the end of the soak the high temperatures on the upper exterior surfaces can be clearly seen. As the solar load is directly and continually overhead, the internal trim directly below the glazing reaches a higher temperatures than the rest of the trim. This can be seen particularly in the fronts seats (mainly the right hand side as the left side is partially obscured by the steering wheel) and the IP topper, which reaches nearly 85°C.
Figure 5. Vehicle interior/exterior at the end of soak (left) and pull-down (right) As would be expected, at the end of the pull-down the interior trim surfaces are substantially cooler although the upper exterior panels are relatively unchanged. It is this heat, the constant solar load (either directly through the glazing or via conduction and convection predominantly through the roof) and the high ambient temperature that the AC has to remove. As the interior target temperature for comfort is in the region 20-22°C, Figure 4 suggests that the 4.4 kW (peak) curve is too high and that the capacity of the evaporator can be reduced. Several more iterations were run, where the applied heat rate curve of Figure 4 was progressively reduced each time, but retaining the same overall shape. It was found that a curve with a peak value of 4kW was sufficient to give an end point of 20°C. This curve was then used in the subsequent analysis runs reported in section 3.2. For this work the actual vehicle performance curves were not available so the reported temperatures cannot be correlated against physical test data. However the main focus here was to look at changes relative to the baseline rather than the absolute values themselves. 3.2 Reduction in thermal load The aim of this section of work was to see how variations in the vehicles construction could be used to reduce the transmission of external heat (solar and ambient air temperature) via conduction, radiation and convection, into the vehicle interior during hot soak conditions. The objective was to reduce the final soak internal temperature and
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consequently to reduce the amount of AC load capacity required to still provide a comfortable interior. The following alterations were made to the base vehicle model:•
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Black exterior paint swapped to white – this was investigated as lighter coloured surfaces reflect the visible portion of the electromagnetic spectrum more than darker ones. This visible portion accounts for ~50% of the thermal load (the other portion being in the IR band, with ~5% in the UV band) Standard glazing replaced with IR Reflective Glazing (IRG) all round. About 5075% of the heat entering the cabin can do so via the glazing (7). Consequently any chance of eliminating heat transfer via this route will be beneficial. IRG on front and front side screens, privacy glazing to remainder – privacy glazing differs from IRG in that the coloured glass absorbs a certain amount of both visible and IR bands, which is then re-radiated and convected on both inside and outside faces, as the glass itself heats up. This is not as efficient as IRG as 50% of the heat is still transmitted inside the vehicle but as the glass is just coloured, rather than laminated, it is a significantly cheaper option. Base car but with light coloured interior trim surfaces – The base car has simulated with dark coloured leather trim, so a lighter coloured material (seats, IP and door panels) would expect to be slightly cooler as some of the heat is reflected away, although some of this will impinge on other adjacent surfaces. Base car but with insulation in roof and door panels – Insulation should be helpful in stopping heat getting into the vehicle and in the model 10mm of glass-fibre insulation was used in the roof and door panels. Base car with sun blinds at all surfaces. As most heat entering a vehicle does so through the glazed surfaces. The use of sun blinds is a possible way in reducing this heat transfer path. However, as they are positioned inside the car, the heat (visible and IR) has already entered the vehicle. In the model, blinds were placed 15mm behind every glazed surface and additional air nodes introduced linking the back face of the window with the front face of the adjacent blind. The rear faces of each blind were then connected to the relevant adjacent interior air nodes (Figure 2). Two blind types were investigated, dark (emissivity & absorptivity on both faces = 0.9) and silvered (emissivity = 0.22, absorptivity = 0.49).
The results for relevant interior temperatures at the end of the 90 minute soak period are shown in Figure 6a and at the end of the idle phase in Figure 6b. The reported results break down into two sections (front row and second row air temperatures along with the overall interior average) and three surface temperatures, steering wheel and (front) seat cushion and seat back. The latter are relevant as it is these surfaces that are in contact with the driver and thus directly impact on occupant comfort. Figure 6a shows that as expected the base vehicle is the worst case and that at the end of the soak period the interior surface temperatures are substantial, nearly 90°C for the front portion of the front seat cushion. Changes to vehicle materials and finish largely have a bigger effect on the surface temperatures than they do for interior air temperatures. The same can be said for temperatures the end of the idle phase (Figure
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6b). Note in this figure, for reference, the two blind results are also reported but clearly the vehicle would not normally be run with the all the blinds deployed. From these results an ‘optimised’ vehicle solution was chosen. This consisted:• Light coloured exterior (white) • IRG for all glazed surfaces • Light coloured interior trim 90 80 70
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When this new optimised vehicle was run at the existing 4kW peak evaporator capacity, then the bottom curve depicted in Figure 7 was the result. As the heat load into the vehicle is reduced for both the soak and running phases, not surprisingly the resultant interior temperatures are much reduced, by around 10°C at the end of the idle phase. Therefore there is scope to reduce the AC capacity to yield the same level of comfort achieved for the base vehicle. This has the obvious benefit that the energy consumption of the vehicle is also reduced. 70
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Figure 7 – Interior temperatures at differing levels of evaporator capacity Consequently, the evaporator capacity curve depicted in Figure 3 was further factored down to give peak loads of both 3.5 and then 3.0 kW. The resultant interior air temperatures can be seen in Figure 7. By reducing the AC (peak) load to 3 kW in the optimised vehicle the same level of comfort can be achieved as using 4 kW of cooling in the base vehicle. 3.3 Impact of active cooling The 3D cabin model was also used to determine the likely effect pre-ventilation, or active cooling, would have on interior temperatures and hence any further opportunities in AC load reduction. Here an 80W (peak) solar panel was assumed to drive the HVAC blower whilst the vehicle was parked in the sun prior to the standard pull-down. To calculate the expected level of air flow, the following assumptions were made:• • • • •
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~15% of maximum power is lost in conversion efficiencies and other losses. As the model simulates a chamber test with 1000W/m2 solar load, this power is available constantly. 68W at 12.8V equates to 5.3A available at the blower motor. From the blower characteristic curve, this would yield an airflow of ~38 l/s from the face vent outlets. In the model this was split into 19 l/s per side.
To have the maximum benefit, the cabin air inlet operation needs to be in ‘fresh air’ mode. To accommodate this change, the interior air node paths (Figure 2) were rearranged to reflect the active cooling approach. Here the inlet ambient air is allowed to flow out the back of the vehicle via the parcel shelf. 70
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Figure 8 – Interior temperatures for base car with and without active air cooling The resultant interior air temperatures during the soak, followed by the normal pulldown at 4kW, are shown for the base vehicle in Figure 8. It can be seen that the effect of pre-ventilation is to reduce interior air temperatures by ~10°C. Additionally, the pulldown is initially slightly quicker as the starting point is lower. However, as the final temperature at the end of the idle was unchanged, no further reduction in AC load was advised although in practise a small reduction may be realsied. 4
DISCUSSION
The interior average air temperatures depicted in Figure 4 are typical for this type of test. In the soak phase the vehicle is effectively sealed so air trapped inside the vehicle cabin rapidly warms up, driven by the high level of solar loading on the external and some internal surfaces. As soon as the pull-down phase starts the air temperatures drop rapidly due to the low thermal inertia of the air. By the end of the 100kph phase (60 minutes elapsed driving time) steady state conditions are being achieved as the heat inputs start to match the heat being removed:Input Mechanisms ΔT outside to inside (43-~20°C) Solar load at 1000W/m2
Removal Mechanisms Ambient airflow over vehicle structure Heat pick-up in refrigerant (rejected at condenser)
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Additionally, as the evaporator cools towards zero, compressor control will also start to regulate the amount of heat removed from the cabin in order to prevent evaporator icing. For the idle phase the temperatures rise again due to several factors:• • •
Airflow over the vehicle exterior has ceased, thus the exterior starts to warm back up again, under the constant solar load. At idle the engine runs at low speed and consequently so will a mechanically driven compressor with a net reduction in refrigerant mass flow (not implicitly modelled here). Air re-circulation effects at the front of the vehicle degrades condenser performance; air being drawn onto the condenser is hotter than ambient air (not implicitly modelled here).
For the base vehicle, the peak AC load requirement was determined as being ~4kW based on sensible heat transfer, settling at ~3kW steady state for the idle (Figure 3). Sensible heat only was considered as Radtherm does not take into account air humidity changes. In reality the evaporator capacity is a combination of both sensible and latent heats, as moisture is removed from the air passing over the cold evaporator core. As the air is in recirculation mode, eventually the dew point temperature will stabilise with little additional latent heat removal, so the assumption made above would be valid. However, at the start of the pull-down a large amount of moisture has to be removed, and this can easily constitute 50% of the initial load, and so a more realistic load requirement for the base car would be nearer ~8kW peak, falling to ~3.5kW steady state. 4.1 Impact of preconditioning technologies As expected, the base vehicle with its dark exterior and interior gave the worst performance in terms of soak and pull down temperatures for both air and internal trim temperatures. The aim of this section of work was to identify possible ways of reducing the AC load, without necessarily compromising interior comfort, through changes to the vehicles final finish i.e. the same target interior temperature at the end of the pull-down phase. The list of changes made is outlined in section 3.2 and is comprised mostly of changes that could be easily implemented by an OEM although not necessarily embraced by the customer! For this vehicle, swapping to a white exterior finish had a noticeable impact on air temperatures, resulting in a 6°C reduction in average internal air temperature at the end of the vehicle soak. Trim surfaces temperatures were also reduced. Whilst choice of colour still needs to reside with the customer, modest reductions can also be achieved through the use of IR reflective paints that have been fashioned to give the same visible appearance but reflect a higher proportion of incident energy (6). A similar argument over customer choice holds for the colour of the internal trim. As expected, seat and steering wheel temperatures were reduced by moving to a lighter colour, especially for the front of the seat cushion that was exposed to direct solar energy transmitted through the wind screen. Air temperatures are only reduced a little however, as the increase in reflected energy is still absorbed within the vehicle. As glazing represents such a significant transmission path for thermal energy, it has been studied by a number of authors (e.g. 8, 9, 10, 11). IRG is a laminated based
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product that allows the transmission of visible light, making it acceptable for use in front of vehicle glazing, but the silver doped interlayer acts to preferentially reflect Infra Red light. Its replacement for all the glazing in the model was also beneficial compared to the base vehicle, again particularly for the steering wheel and front seat area that are subjected to direct radiation through the front screen. Besides cost increase, one disadvantage of IRG is that it stops electromagnetic signals required for phones, GPS etc. Consequently, a compromise solution was also examined, whereby the IRG from the B pillar rearwards was replaced with privacy glazing. As expected, the performance of this arrangement was slightly worse than 100% IRG due to 50% of the absorbed heat in the privacy glass being partly retransmitted inside the vehicle. Insulation potentially offers a low cost means of AC (and heater) load reduction. In the model the base car had insulation added to the roof and door panels and proved largely beneficial. However, these panels, particularly on the roof, can also act as a means of loosing heat from the vehicle as the exterior ambient cools. As a result, insulated vehicles left at the end of the day may end up hotter than their uninsulated equivalents. The use of sun blinds is an alternative method of disrupting the solar radiation path into the vehicle and temporary reflective blinds can often be seen in vehicles parked up in hot countries. From a heat transfer viewpoint, blinds would ideally be placed outside the glazing; by having blinds on the inside, the solar radiation has already entered the vehicle by the time it meets the blind surface. Black blinds will effectively absorb this radiation yielding a hotter interior compared with more reflective material and the results in Figure 6 confirm this, especially for the trim surfaces. Reflective blinds appear to significantly reduce front row air temperatures (~5°C reduction) and steering wheel, front seat (cushion) temperatures (27°C reduction). Whilst taken on their own, the reductions brought about by these preconditioning technologies are relatively modest compared with the base vehicle. However, when used in combination, the benefit was shown to be significant. The example in Figure 6 combines just three; white exterior paint, light coloured interior trim and IRG for all glazing surfaces. This combination results in an 11°C reduction in final soak temperature and, when using the same evaporator capacity curve, a similar reduction in final cabin temperature at the end of the pull-down. This suggests that the capacity can be reduced by ~25% in order to achieve a similar level of comfort experienced in the base car if these changes were to be adopted. 4.2 Impact of active cooling Inclusion of a photovoltaic panel on the roof of the vehicle opens up many options for energy management, provided a reasonable level of sunlight is available. The opportunity of using this available power to drive the HVAC blower whilst the vehicle is in its soak state is well known in the automotive industry and solar panel equipped roof panels are offered as optional fit on some higher end vehicles. Figure 8 shows the benefits of drawing in exterior ambient air which, when in fresh mode, displaces the hotter air inside the vehicle cabin. At the end of the soak the interior air temperature is cooler so the pull-down is, initially, slightly more rapid. However, this benefit is quickly lost as the pull-down becomes dominated by the vehicle interior structure which still retains a significant amount of heat and it is this heat that dictates
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the rest of the pull-down. This result appears consistent with the findings of other authors (12,13). Furthermore, the benefits described here would be diminished in reality as air entering the vehicle in the early mornings and late afternoons, where the reduced solar levels would yield lower flow rates, would give higher levels of heat pick-up in the inlet system than resulted here. 5
CONCLUSION
A 3D thermal cabin model has been used to determine the heat transfer characteristics of a typical saloon vehicle. Specifically, it has been used to investigate ways in which air conditioning loads can be both quantified and then reduced in order to reduce the overall energy demand on the vehicle. This approach is important in lowering fuel consumption in conventional vehicles but is particularly important in those vehicles that rely on battery power as the principal energy source as AC provision has a significant detrimental impact on vehicle range. The approach adopted here suggests that AC load reductions of 25% are possible when vehicle materials are selectively used to minimise heat penetration into the cabin, although the choices made may not be acceptable to the customer for other reasons. Drawing in ambient air during the vehicle soak though the use of the HVAC blower driven by the energy from a photovoltaic panel is also beneficial. However, although interior air temperatures are reduced during the soak, possibly leading to improved comfort on entry, the AC load requirement is not significantly impacted due to retained heat within the vehicle structure. The software code appears well suited in determining the thermal environment in the cabin. It is critical that any code can robustly handle the radiation effects of heat transfer, and the related view factor calculations that are also required, as thermal radiation dominates when AC loads are highest, both directly, from the sun, and via reflections, from the IP and ground for example. Ideally the results would have been correlated against actual test data but this is not within the scope of the project. However, just focusing on the sensible heat capacity of the evaporator underestimates the total AC load required, especially at the start of the pull-down where humidity levels are highest. ACKNOWLEDGEMENTS This work was supported by funding from the EU Commission under the Seventh Framework programme as part of the EE-VERT project. Contributing partners in the project are Beespeed Automatizari, Bosch, CRF, Lear, Engineering Center Steyr, MIRA, Volvo Technology and the Universities of FH Joanneum Gesellschaft and Politehnica din Timisoara
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