Pressure from the European Union for more environmentally friendly A/C systems has forced manufacturers to look for alternative refrigerants or technologies for HVAC units. A great deal of controversy exists on which technology the EU, US and the automotive industry wants to supersede R134a or in fact just improve the current R134a into a leak-free system.The EU will phase out R134a from 2011 with a complete ban on its use by 2014 to 2017 (dates to be finalised). Possible alternatives are within this section although it seems inevitable that CO2-based A/C systems will replace the R134a system which is currently used.
● CO2-based system (R744);
● absorption refrigeration;
● secondary loop system (HFC152);
● gas refrigeration (R729);
● evaporative cooling;
● thermo electric cooling (Peltier effect).
The production of R744 (CO2)-based HVAC systems will be included on vehicles being mass produced by 2008. The following information has been provided to aid the reader in predicting what refrigerant and accompanying technology will be implemented to supersede R134a.
The CO2 (R744)-based refrigeration cycle (transcritical system)
Refrigeration and air-conditioning systems where the cycle incurs temperatures and pressures both above and below the refrigerant’s critical point are often called transcritical systems. Transcritical systems are somewhat similar to the subcritical systems described above although they do have some different components.
Figure 1.55 illustrates the transcritical vapour compression process. It begins when the superheated refrigerant enters the compressor at point 1. Its pressure, temperature and enthalpy are increased until it leaves the compressor at point 2 located in the supercritical region. Next the refrigerant enters the gas cooler whose function is to transfer heat from the fluid to the environment. Unlike the condensing process in the subcritical system, the refrigerant has not undergone a distinct phase change when it leaves the gas cooler at point 3. Note that this gas cooling
Figure 1.55
process does not occur at constant temperature. The cooled gas then enters an internal heat exchanger (sometimes called a ‘suction line heat exchanger’), which transfers heat to that portion of the refrigerant that is just about to enter the compressor.
This results in additional cooling of the refrigerant to point 4 on the figure, improving performance at high ambient temperatures. From there, the flow undergoes a constant-enthalpy expansion process that decreases its temperature and pressure until it exits at point 5 in the mixed liquid/vapour region, at temperature and pressure well below the critical values. Next, the refrigerant enters an evaporator where it absorbs heat from the cooled space and its enthalpy and vapour fraction gradually increase until it exits at point 6. Finally, the flow enters the internal heat exchanger where it absorbs more heat, until it is ready to enter the compressor again at point 1 to repeat the cycle.
Note – the R744 cycle will also work in the subcritical region (i.e. some condensation in the gas cooler will take place) in case the ambient temperature is considerably lower than the critical temperature of R744, which is about 31°C.
System operation
Figure 1.56 shows an R744 closed loop A/C system capable of acting as a heat pump.The increased
use of highly efficient diesel engines, particularly direct injection models as presently occurring
in Europe, and the anticipated increase in the use of hybrid vehicles, means that engine coolant will no longer have the customary temperatures and capacities for acceptable passenger compartment heating and window defrosting/demisting operations. However, a heat pump can be used to heat the passenger compartment boosted to temperature levels to which vehicle occupants are accustomed.
The diagram (also reproduced in colour in the plate section) shows arrows in blue which represent refrigerant flow when in A/C mode and red arrows when in heat pump mode.
A/C operation (blue arrows)
1. Compressor
Superheated refrigerant enters the compressor (temperature 30°C, pressure 35 bar). The refrigerant is compressed increasing its pressure, temperature and enthalpy (130 bar, 160°C). The given values represent a high load point.
2. The gas cooler (replaces the condenser)
The refrigerant enters the gas cooler (via the active switching valve), upon entering the gas cooler the superheated gas allows heat to be transferred to the walls of the gas cooler and air travelling through it. The refrigerant does not go through a distinct phase change although its temperature (at the gas cooler outlet a few kelvin above inlet air temperature, for example 40°C for a 35°C ambient temperature) and enthalpy reduce. The refrigerant is still operating above its critical point.
3. The accumulator/internal heat exchanger
The refrigerant flows to the high pressure side of the internal heat exchanger which removes heat by transferring it to the refrigerant that is about to enter the compressor. This again reduces the refrigerant’s temperature (30°C).
4. Electronic expansion valve
The refrigerant flows to the electronically controlled expansion device which creates a large pressure drop promoting the constant-enthalpy expansion process. The reduced pressure and temperature of the refrigerant now allows the device to operate below its critical point. The refrigerant is now a mixture of liquid and a flash vapour having the ability to change state with additional heat input.
5. Evaporator
The refrigerant flows into the evaporator absorbing heat through evaporation until it exits the evaporator as a saturated vapour.
6/7. Accumulator/Internal heat exchanger
The refrigerant flows (through the passive switching valve) to the accumulator/internal heat exchanger. This component combines the accumulator and internal heat exchanger functionalities into one part. The internal heat exchanger section enables the refrigerant to become slightly superheated. The accumulator portion separates the liquid and gaseous phase, stores the unused liquid refrigerant and allows the compressor oil together with the gaseous refrigerant to return to the compressor for lubrication.
8. Slightly superheated
refrigerant flows back to the suction side of the compressor and the
refrigerant flows back to the suction side of the compressor and the
process repeats.
Note – temperatures and pressures are approximate and are dependent on system load.
Heating operation (red arrows)
Heating is achieved by directing the flow of heated refrigerant to a secondary gas cooler
positioned inside the vehicle which heats the incoming/recycled air and is distributed through
conventional ducting.The refrigerant then flows to the accumulator/heat exchanger and external gas cooler which is positioned at the front of the vehicle.
1. Compressor
For an assumed ambient temperature of 18°C, the refrigerant enters the compressor at a temperature of about 20°C and a pressure of 18 bar. The refrigerant will be compressed to about 90 bar at 90°C.
2. The secondary gas cooler
The active valve will divert the flow that – in an A/C cycle – would usually go to the gas cooler, to the secondary gas cooler. Upon entering the gas cooler the superheated/high temperature gas allows heat to be transferred to the walls of the gas cooler and the air travelling through it. The refrigerant is still operating above its critical point. Both temperature and enthalpy reduce.
3/4. The electronic expansion device
The refrigerant flows to the electronically controlled expansion device which creates a large pressure drop promoting the constant-enthalpy expansion process. The reduced pressure and temperature of the refrigerant allows it to operate below its critical point. The refrigerant is now a liquid and flash vapour having the ability to change state with additional heat input. Note that for a heat pump cycle the expansion device needs to be a bi-directional type as flow enters from both sides.
5. The accumulator/internal heat exchanger
The internal heat exchanger (IHX) section has no duties in the heat pump cycle (both the former high and low pressure sides of the IHX are now located on the low pressure side of the cycle) and is only a passage for the refrigerant.
6. The gas cooler
The refrigerant flows from the accumulator/internal heat exchanger to the gas cooler, which in fact now acts as the evaporator for the heat pump cycle, where it will absorb more heat to change from a saturated vapour to become slightly superheated.
7. The accumulator/heat exchanger
7. The accumulator/heat exchanger
The passive valve collects the flow of the heat pump branch of the circuit and directs the flow to the accumulator/internal heat exchanger. Again, the IHX is pretty much without function, but the accumulator section acts exactly as in A/C operation.
8. The compressor
The refrigerant flows from the accumulator/heat exchanger back to the compressor and the process repeats itself.
Note – pressure and temperature figures depend on system load.
Heat pump operation will only be used at very cold ambient temperatures allowing for faster
defrost and/interior warm-up. It does not replace the normal heater core and is just a supplement.Thus the mixed A/C/heater mode is still available, but there will be no combined A/C/heat
pump operation (as both functions rely on the same refrigerant circuit). A/C will continue to
be used for dehumidification in warmer ambient temperatures or during comfort drive, assisted
by the normal heater core for reheating. Removing particles is the duty of the air filter that is
located upstream of the evaporator.
R744 properties
R744 has a corrosive effect on polymers so metal pipes are used. R744 is non-flammable and
relatively cheap compared to R134a. R744 is also easier to recycle than R134a.
Table 1.4
(Specifications and descriptions contained in this book were in effect at the time of publication.Visteon reserves the right to discontinue any equipment or change specifications without
notice and without incurring obligation (07/05).)
Component information
Sanden and LuK have been assisting with the research and development of compressor
designs for R744 systems.
● Research in 2000 was based on compressor Luk Variable Technology (LVT) (30–36 cm3), the
parts are interchangeable with compressor Verband der Automobilindustrie (VDA)
(160 cm3) – R134a.
Maximum pressures and temperatures to withstand:
● high side 16.0 MPa (2320 psi);
● low side 12.0 MPa (1740 psi);
● high side 180ºC (356ºF) (discharge temperature).
Possible lubricants for the system
POE or PAG are the most likely options. Surplus oil will be stored in the accumulator and sucked back to the compressor via an oil bleed hole in the accumulator through the suction line. Refrigerant filters will most likely be applied in an R744 system, one location could be within the accumulator, another one in front of the expansion device.
Accumulator/Internal heat exchanger
The accumulator is required to store lubricant for the compressor operation.The internal heat exchanger acts as a heat exchanger to increase cooling capacity which is required mainly at high ambient air temperatures. The accumulator also ensures that no liquid refrigerant enters the compressor during the system operation.
Figure 1.57
Gas cooler
The efficiency of the system’s operation is highly dependent on the air flow through the gas cooler ensuring enough heat is removed or absorbed.
Expansion valve
A solenoid operated valve which can be operated by a high frequency pulse width modulated signal or an analogue DC voltage.
Visteon multi zone modular HVAC system
The modular multi zone HVAC system offers manufacturers the flexibility to personalise the HVAC system depending on model specification and provides up to four temperate controlled zones from one unit (this number of temperature controlled zones is usually provided by two or more units). Modular units allow for mass production of common components gaining efficiencies of scale while still providing flexibility to the customer.
System benefits:
System benefits:
● All metal sealing promotes no leak concept.
● Cross platform usage – modular multi zone design.
● Heat pump facility.
● Full electronic control.
● Low Global Warming Potential (GWP) (1) and zero Ozone Depleting Potential (ODP).
● Improvement in fuel economy.
● No additional fuel or electric heater required.
● Reduction in emissions.
Negative aspects:
- A higher level of technology is required and additional components are necessary.
- Additional cost to suppliers exists due to large investment in research and development (whether this cost will be passed on to the consumer or absorbed by the OEM is speculation).
- There will be an impact on the service and training industry requiring new knowledge, skills, equipment and possibly a certificate of competence to service and repair such systems.
Absorption refrigeration
The absorption refrigeration cycle is attractive when there is a source of inexpensive or waste heat readily available.This cycle uses a refrigerant that is readily soluble in a transport medium. In brief, the condensation, expansion and evaporation processes are identical to those of the vapour/compression cycle. But instead of the latter’s compression process, the absorption cycle’s liquid transport medium absorbs the refrigerant vapour upon leaving the evaporator, creating a liquid solution. This solution is then pumped to a higher pressure, and then heat is used to separate the refrigerant from the solution, whereupon the high pressure refrigerant flows to the condenser to continue the familiar cycle. The equipment used to accomplish the solution/ dissolution processes is complex and heavy, but the advantage lies in the low work input requirement to raise the pressure of a liquid solution as compared to that required for compressing a gas. If the heat utilised is otherwise wasted heat, the low operating costs of absorption systems can be quite attractive.The two most common refrigerants used in absorption systems are ammonia, with water as the transport medium, and lithium bromide in water. However, toxicity issues with ammonia require safeguards, adding to system cost and complexity. Lithium bromide can be corrosive to most common materials, again adding to cost and complexity.Absorption systems are used mostly in large non-vehicular building applications, though occasionally there has been advocacy of their use as mobile air-conditioning systems.
Secondary loop system HFC152a
HFC152a (Fig. 1.58), which is flammable, must be used in a ‘secondary loop’ A/C system that uses a chiller to transfer cooling from the refrigerant in the engine compartment to coolant that is circulating into the passenger compartment.The secondary loop is required as opposed to a primary loop using a hydrocarbon-based refrigerant because if a leak occurs in the evaporator and hydrocarbons are released then an explosion could occur.
The primary loop (refrigerant circuit) operates in the same manner as the vapour compression cycle using a hydrocarbon-based refrigerant instead of R134a. It is positioned under the bonnet. The secondary system (coolant system) positioned inside the vehicle uses brine as a cooling medium which is under pressure by the circulating front and rear pump to transfer the heat from the front and rear coolers to the cooling medium. The reservoir is required to allow for the expansion of the coolant. The system is a dual A/C system with front and rear coolers. This system allows the easy addition of multiple cooling points with no additional expansion device.The refrigerant charge in the primary system is about half when compared to an R134a system with the same HVAC specification.
System benefits:
● Wide choice of refrigerants can be used.
● Enhances city traffic and idle cooling performance.
● Potential for targeted cooling (e.g. seats).
● Reduces refrigerant charge and leakage.
● Potential for elimination of heater core resulting in smaller HVAC case and cost savings (single heat exchanger for heating and cooling).
● Refrigerant NVH reduction – front and rear.
● Eliminates refrigerant maldistribution (coolant exhibits more uniform temperature distribution than refrigerant).
System drawbacks:
● The use of flammable refrigerant.
● Cost of the system.
● Energy penalty.
● Weight of the system.
Gas refrigeration
The gas refrigeration cycle
In the past the gas refrigeration cycle, which is common on aircraft, has been considered for the automobile industry. Research on the use of an air refrigeration system exists and cannot be excluded as a future option due to this fact.
The gas refrigeration cycle is appropriately named due to the refrigerant remaining in a gaseous state throughout the entire cycle. R729, otherwise known as air, is used as the refrigerant medium. The pressure enthalpy diagram for a closed gas refrigeration system operates outside of the phase transition of the refrigerant so that the saturation curves do not appear on the diagram
unlike the vapour compression cycle.
To aid the explanation of the system operation a Jaguar aircraft has been used for illustration (see Figure 1.59).
The refrigerant (air) enters a rotary compressor to raise its pressure and enthalpy. Air charge is taken from both the compressors via an outlet as shown circled in Figure 1.60. Only compressed gas at a temperature of about 190°C enters the A/C system because the feed is situated on the outlet of the compression stage and not ignition.
The air then travels to a primary heat exchanger where it gives up heat at constant pressure. The exchanger has ram air flowing through it to reduce the temperature of the air feed from the gas turbine engines.
In Figure 1.61 the primary heat exchanger is fitted along the spine of the aircraft (large circle) and is fed from the gas turbine engine (small circle).
The gas then flows to a turbine unit (cold air unit) where it expands reducing in enthalpy, temperature and pressure. Some units connect the turbine to a compressor to recover some of the energy given up during isenthalpic expansion. The air is at a temperature of about 100°C upon leaving the turbine (cool air unit).
Upon leaving the turbine the gas flows to a secondary heat exchanger (Figs 1.62 and 1.63), situated behind the cockpit. Cabin air circulates through the fins of the heat exchanger releasing heat and then a water extractor removes moisture in the air.
The system can be operated manually or automatically to control the internal temperature of the cockpit. Heating is achieved by bypassing the heat exchangers thus transferring heat laden gas to the air distribution system. This is electronically sensed using thermistors and directed using control flaps.
Evaporative cooling
The latent heat of vaporisation of water can provide cooling to vehicle occupants. A crude
approach is to spray one’s face with a water mist, then place the head outside the window of a
moving vehicle into the free air stream – the evaporating water carries away heat from the skin.
There have been devices, mostly in the 1950s – ‘The Weather Eye’ – that worked on evaporative
Figure 1.63
cooling.They consisted of a box or cylinder fitted to the window of the vehicle.The intake of the unit would allow air to enter from outside and travel through a water soaked wire mesh grille and excelsior cone inside the unit. The water would evaporate due to absorbing the heat in the air and travel through the outlet of the unit which acted as a feed to the inside of the vehicle.At one time, these devices had a certain attractiveness, particularly in hot, low humidity regions like the US southwest. However, their performance compared to modern vehicular air-conditioning systems is generally inadequate and they currently do not have significant popularity.
Thermo electric cooling (Peltier effect)
The basic concept behind thermoelectric (TE) technology is the Peltier effect – a phenomenon first discovered in the early 19th century. The Peltier effect occurs whenever electrical current flows through two dissimilar conductors; depending on the direction of current flow, the junction of the two conductors will either absorb or release heat.
Semiconductor material, usually bismuth and telluride, are generally used within the thermoelectric industry. This is due to the type of charge carrier employed within the conductor (see Chapter 3). Using this type of material, a Peltier device can be constructed. In its simplest form it consists of a single semiconductor ‘pellet’ which is soldered to electrically conductive material on each end (usually plated copper). In this ‘stripped-down’ configuration (Figure 1.65), the second dissimilar material required for the Peltier effect is the copper connection which also acts as a conductor for the power supply.
N type
Once impurities are added to a base material their conductive properties are radically affected. For example, if we have a crystal formed primarily of silicon (which has four valence electrons), but with arsenic impurities (having five valence electrons) added, we end up with ‘free electrons’ which do not fit into the crystalline structure. These electrons are loosely bound. When a voltage is applied, they can be easily set in motion to allow electrical current to pass. The loosely bound electrons are considered the charge carriers in this ‘negatively doped’ material, which is referred to as ‘N’ type material. The electron flow in an N type material is from negative to positive. This is due to the electrons being repelled by the negative pole and attracted by the
positive pole of the power supply.
P type
It is also possible to form a more conductive crystal by adding impurities which have one less valence electron. For example, if indium impurities (which have three valence electrons) are used in combination with silicon, this creates a crystalline structure which has ‘holes’ in it, that is, places within the crystal where an electron would normally be found if the material was pure. These so called ‘holes’ make it easier to allow electrons to flow through the material with the application of a voltage. In this case,‘holes’ are considered to be the charge carriers in this ‘positively doped’ conductor, which is referred to as ‘P’ material. Positive charge carriers are repelled by the positive pole of the DC supply and attracted to the negative pole; thus ‘hole’ current flows in a direction opposite to that of electron flow.
Figure 1.64 shows two dissimilar materials, one is N type and the other P type. It is important to note that the heat will be moved (or ‘pumped’) in the direction of charge carrier movement throughout the circuit (it is the charge carriers that transfer the heat).Thus the electrons
flow continuously from the negative pole of the voltage supply, through the N pellet, through
the copper tab junction, through the P pellet, and back to the positive terminal of the supply.
The positive charge carriers (i.e.‘holes’) in the P material are repelled by the positive voltage potential and attracted by the negative pole and flow through the positive pole through the P pellet and copper tab to the N pellet and the negative terminal.When a DC current is applied heat is moved from one side of the device to the other – where it must be removed with a heat-sink.
The ‘cold’ side is commonly used to cool. If the current is reversed the device makes an excellent heater.
Arranging N and P type pellets in a ‘couple’ and forming a junction between them with a plated copper tab, it is possible to configure a series circuit which can keep all of the heat moving in the same direction. Using these special properties of the TE ‘couple’, it is possible to team many pellets together in rectangular arrays to create practical thermoelectric modules. These devices can pump appreciable amounts of heat, and with their series electrical connection, are suitable for commonly available DC power supplies.The most common TE modules in use connect 254 alternating P and N type pellets and use 12 to 16 VDC supply and draw 4 to 5 amps.
Thermoelectric modules look like small solid-state devices that can function as heat pumps. A ‘typical’ unit is a few millimetres square to a few centimetres square. It is a sandwich formed by two ceramic plates with an array of small bismuth telluride cubes (‘couples’) in between.
Heatsinks are used to either collect heat (in heating mode) or dissipate collected heat into another medium (air, water). Heat must be transferred from the object being cooled (or heated) to the Peltier module and heat must be transferred from the Peltier module to a heatsink.Systems are often designed for pumping heat from both liquids and solids. In the case of solids, they are usually mounted right on the TE device; liquids typically circulate through a heat exchanger (usually fabricated from an aluminium or copper block) which is attached to the Peltier unit. Occasionally, circulating liquids are also used on the hot side of TE cooling systems to effectively dissipate all of the heat.
Peltier device cooling and heating speeds can change temperatures extremely quickly, but to avoid damage from thermal expansion the rate of change is controlled to about 1°C per second. It is theoretically possible to get a temperature difference across a Peltier module of 75°C although it has been stated that in practice this is not achieved. In practice the results are about half this figure.
If a TE module is to be used to cool anywhere near freezing then water condensation must be considered. Ever-present water vapour begins to drop out of the air at the ‘dew point’.This will result in the TE module, and what it is being used to cool, to get wet. Moisture inside of the TE module will cause corrosion and can result in a short-circuit. A solution to this problem is to operate the TE module in a vacuum (best) or a dry nitrogen atmosphere.
System control
Varying the power supply is often used. Pulse width modulation can be used, but a frequency above 2 kHz is recommended and the voltage applied must not exceed the recommended maximum voltage (Tmax).
Peltier devices are best suited to smaller cooling applications.They can be stacked to achieve lower temperatures, but are not very ‘efficient’ as coolers due to the heating effect of the current flowing as well as drawing a great deal of current but act as very good heaters. This disadvantage can be offset by the advantages of no moving parts, no Freon refrigerant, no noise, no vibration, very small size, long life, and the capability of precision temperature control.
Automotive application – Amerigon’s proprietary CCS system
Vehicle cabin air is drawn into the cushion and back TE modules and, based on inputs from individual seat controls and from temperature sensors, the unit will either add heat or remove heat to the air flow.
The basis of the system is the Peltier circuit. The Peltier circuit, heatsink (heat exchanger) and fan assembly are mounted as one module.Air is used as a medium to move heat around the seat through the perforated seat layers. Conditioned air is ported to the top surface of the foam
through channels which evenly distribute the conditioned air over the surface. Breathable trim covers allow the conditioned air to pass through to the occupant.When the Peltier device is cooling, heat is generated on the opposite side of the device which must be removed to allow the temperature differential to exist.
This heat is pumped into the cabin space and is labelled on Figure 1.66 as ‘waste heat’. This will create an extra load on the A/C system if being operated to cool the interior space. When voltage is applied to the Peltier module in one direction one side of the Peltier device will be hot and the other cool due to the direction of the charge carriers creating a T across the Peltier device. Switching polarity of the circuit creates the opposite effect.
Figure 1.66 1.67
Amerigon’s Peltier module proprietary CCS system allows occupants to select seat temperatures to promote comfort and reduce driver fatigue through the use of a solid-state heat pump combined with an active, microprocessor controlled temperature management system to vary heating and cooling capacity.
Amerigon states that it is the first to have successfully packaged this technology for use in
automotive seating applications.
0 comments :
Post a Comment