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New technologies

Development of Waste Thermal Energy Recovery Heat Pump

Hiroyuki Ohno* Jun Hatakeyama* Mitsuaki Nagata* Tomohiro Maeda*

Abstract　In winter season, using heater for air condition decreases cruising range of electric vehicle (EV). It is one of the issues for EVs’ popularization. So the high efficiency and low cost heating system is desired for EVs. For this demand, a heat pump system has been developed that recovered waste thermal energy in EV and had an HVAC unit without changing from mass product one. This paper reports the detail of this system and experimental results.

Key Words : Electric Vehicle (EV), Air conditioning, Heat pump, heater

1. Introduction In vehicles with internal combustion engine, heat energy during winter is obtained from waste heat of the engines. In electric vehicles (EVs), heat energy is needed to be obtained from another heat sources, therefore electric energy is usually used. Consequently, EV has reduced driving distance due to electric power consumption for heating operation. In recent, in order to improve the reduction of driving distance due to heater operation, air conditioning system with heat pump having high-efficiency are developed and made available in the market for EVs. However, in case of the system is used the outside air as the heat source, other various subjects are occurred that heat exchanger freezing, COP decreasing on low ambient, and big design chang-ing of HVAC in cabin . This paper describes about the new heating system that we developed (designed and tested). This system is utilizes the cabin air, battery- heat storage, and waste heat from each component as the heat source for reducing electrical energy consumption during heating operations instead of outside air.

2. Features of the system The new developed system can be easy installed in the vehicle by carrying across current air conditioning system of internal combustion engine vehicles without changing HVAC design. The new one has a structure that the heat storage with battery, and waste heat from driving components such as the motor and inverter are to be used as the heat source for cabin heating (Fig. 1).

Fig. 1 System structure

2.1. System Design Intents

Waste heat in EVs is indicated as the following thermal energies.(1) Heat radiated during interior air ventilation for

window defrosting (Ventilation heat loss)(2) Heat generated from batteries during charging(3) Electric power loss of inverters and motors during

vehicle drivingTill date, these heat energies cannot be used for heat-ing function due to low temperature, they are ex-hausted outside with the air or water. The new system has been developed such that these thermal energies are used for heating by recovering with a heat pump. The heat recovered, is used in an evaporator and cooler. The refrigerant is compressed at high tempera-ture and pressure. This refrigerant heats the water

* Global Technology Division Green Technology Development Group

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Development of Waste Thermal Energy Recovery Heat Pump

used for Cooling in a water-cooled condenser. This system is not used to pipe a high pressure refrigerant into the cabin, furthermore it is easy to apply new refrigerants and so on.

2.2. Recovery of ventilation heat loss

When the low humidity outside air is lead into the cabin for window defrosting on heating, thermal energy with warm air is released outside. This ventilation loss is accounted for as large as approximately 50 to 60% against vehicle heat loss at 0 ℃ ambient temperature (Fig. 2). This ventilation loss can be reduced by recircu-lating the internal air in cabin. But this typical solution has become worse due to the window defrosting caus-ing increase of humidity in cabin.In order to contain these issues, the new developed de-humidity heat recovering system is applied. This system re-heats the inside air by the heat recovered by a heat pump during cooling and de-humidification operation with evaporator. This system could secured the de-humidity performance with ventilation heat loss reduction. And in order to prevent the deterioration by recirculation, the internal air recirculation ratio was set to 60%.

Fig. 2 Vehicle heat load ratio

2.3. Thermal energy recovery from batteries

Battery for driving is heated by operating the current inputs and outputs while a vehicle running or being charged. In particular on quick charging, cell of battery generates heat and has more increase in temperature. (Fig. 3). As he charging/discharging performance reduces at low temperature, a battery-heating device is installed with the battery to avoid it.

Fig. 3 Cell temperature and SOC at QC

On the other hand, battery modules have large heat capacity, they can be considered as a sort of heat stor-age. Therefore, if batteries are charged or heated at appropriate temperatures before vehicle driving, they can be used for recovering heat lost during driving. Fig. 4 shows the relation of usage heat energy with the battery capacity in case that the heat is being absorbed during the battery is cool down 10 ℃ from warmed 30 ℃ Although the usable energy is varied within the slash zone by the mass energy density of the battery. For example, the vehicle with a 40 kWh battery capac-ity dissipates heat energy of 1200 to 2200 Wh. If this heat can be fully recovered during driving, it can be used the heating for cabin.

Fig. 4 Heat storage capacity

2.4. Thermal energy recovery from high voltage components

Inverters and motors used for driving vehicles have a much higher energy efficiency than internal combustion

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engines. However, they generate a comparable heat, with vehicle driving force. Fig. 5 shows the example where the inverter and motor with an EV equivalent D-segment are used to calculate the power loss on driv-ing in JC08 mode. Although the power loss is occurred around 250 W with large fluctuation at driving condi-tion. Since inverters and motors require high power and have high heat generation density in vehicles, they are water cooled.

Fig. 5 Vehicle energy loss

3. System components The description components in the new system is as follows.

3.1. Water heater

A new high voltage water heater has been installed．this is a sheathed heater with low cost and high safety function (Fig. 6). This water heater is less expensive compared to the ones installed with PTC elements having installed Nichrome wires. We solve the subject of excessive heating to apply our own unique technology that installed bi-metal for failsafe.

Fig. 6 High voltage water heater

3.2. Water-cooled condenser

In general, the condenser for air conditioning systems is air-cooled and is located at the front –end of a

vehicle. In the new system, a water-cooled condenser is applied with the aim of using the whole condensation heat of the refrigerant for heating. The condensation heat that is transferred to the water by water cooled condenser is transferred to the radiator or heater core by switching the circuit of water. The condenser is located near the cabin to shorten the refrigerant pipe, thus the amount of refrigerant charged can be decreased when compared the conventional heat pump system. But water cooled condenser that radiates heat to air from water indirectly has higher condensation tem-perature of the refrigerant, than air-cooled condensers that radiated heat to air directly. Therefore, the power consumption of compressor is increased due to high operation pressure for cooling in summer. (Fig. 7).

Fig. 7 Comparison Of AC power consumption

For decreasing the compressor power consumption, the water cooled condenser is installed, which has a high heat transfer coefficient. And by setting the sub-cooled condenser with a liquid tank, the compressor power consumption can be minimized by increasing the enthalpy difference of condensation (Fig. 8).

Fig. 8 Sub cooled condenser system

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Development of Waste Thermal Energy Recovery Heat Pump

3.3. Cooling plate

Cooling plates for battery cooling and heating has to be small and thin for easy installation of the battery pack. In the new system, battery modules are located on the upper and lower side and thin cooling plate is located in the middle so that battery module can be kept in close contact with both flat surface of cooling plate. (Fig. 9). The designed cooling plate has low heat resistance and equalized cooling water flow on the whole surface for quick operation managing battery heat storage and hence cooling performance fluctuation within battery module can be minimized.

Fig. 9 Structure of battery modules and cooling plate

4. System operation The system has the following heating operation modes on cabin heating(1) Heat absorption by evaporator on internal air recir-

culation in cabin(2) In addition to mode (1), using of heat storage with

battery- absorbed by the chiller.(3) In addition to mode (2), using of waste heat from

motor/inverter absorbed by chiller.

The control method of system operation is mode (1) on basic condition that the evaporator dehumidifica-tion performance is controlled by the compressor speed. In case of battery temperature is reaching the usable range for heating, it is shifted into mode (2). Furthermore in case of the cooling water temperature of the motor and inverter is reaching the usable range for heating, it is sifted to mode (3). Therefore, the heat

absorbed from each component is transferred by the water-cooled condenser to the hot water circuit of the heater. The water temperature target Twh of the hot water circuit is determined by the outlet air tempera-ture target Xm in the cabin. If the water temperature, by radiation heat from cooled condenser does not reach the target temperature, the electric water heater will be operated to heat the water. The heat balance of the above system is shown in the following equations.

Qcond=QEvap+QChiller+PComp (1)

QHC=PHeater+QCond (2)

QHC: Heat release at heater coreQCond: Heat release at condenserQEvap: Heat absorption at evaporatorQChiller: Heat absorption at chillerPComp: Compressor powerPHeate: Electric heater power

The conventional system shown in Fig. 1, has only the electric water heater for heat source. In order to get the radiation heat that is recovered by a chiller and evaporator for heater core as shown in Fig. 10, new system can reduce the electric power consumption of water heater. Further, when heat is recovered in high amounts, water heater can stop operating according to eqn. (2), and heating is done only by compressor power. The waste heat from the motor and inverter is basi-cally radiated from the sub radiator. The system also controls absorption of heat from the chiller by switching the three-way valve, according to the relation between battery cooling circuit water temperature Twc and the motor/inverter cooling circuit water temperature Tws.

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CALSONIC KANSEI TECHNICAL REVIEW vol.11 2014

Fig. 10 p-h diagram

5. Verification results We verified an effect of the system on the prototype vehicle No. 4 built by SIM-Drive Corporation (Fig. 11).

Fig. 11 Test vehicle installed proposal system

5.1 Effect of electric power consumption reduction

Fig. 12 shows a comparison of results of total electric power consumption for heating during 40-miniute driving in JC08 mode at 0 ℃ ambient in case of below 3 mode: Electric heater: only operation by the electric water heater, REC: combined with heat recovery by internal air recirculation, and REC Heat Storage: further com-bined with 1200 Wh of heat storage from the battery. On the result, we confirmed that electric power consump-tion reduction is 17% on REC, 53% on REC Heat Storage with only water heater operation. Therefore, the driving distance can be extended 16% and 49%, respectively with this electric power consumption reduction. In the above evaluating conditions where the driv-ing load is light, the amount of heat generated by the inverter and motor is small. Therefore the components do not reach heat recovery temperature, hence the heat recovery function is not operated.

Fig. 12 Consumed power after 40 min. run

5.2 Heat balance during heating operation

Fig. 13 shows transitions of power consumption and cabin heating performance during 40-minute driving with an initial battery temperature of 30 ℃. In the be-ginning of the driving, larger amounts of electric power (input) is consumed for required thermal energy of cabin heating (output) and for heating the coolant water and heat exchanger. After five minutes, the electric power consumption is reduced and falls much below the heating energy curve. This gap is due to the effect of using the heat storage of battery and recovering of ventilation heat loss. After ten minutes, the electric power consumption is almost half and stable.

Fig. 13 Energy balance

5.3 Effect of heat recovery due to internal air recirculation

Since the heat from battery can’t be always used it, the recovering heat amount is shown as Fig. 14 is by

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Development of Waste Thermal Energy Recovery Heat Pump

operating only internal air recirculation. After a heating operation starts, the evaporation inlet temperature rises along with the cabin temperature, and the heat recov-ery amount is increased. After the cabin temperature becomes stable, an 800 W heat can be recovered. This amount is equal to approximately 30% of the energy required for cabin heating.

Fig. 14 Heat recovery by recirculation

After the 40-minute driving, total energy amount of 400 Wh, heat recovered. We can confirm that the internal air recirculation can be enough for recovering. As EVs and PHVs, vehicle with air conditioning sys-tems if operated for a little while, by using household electric power, prior to boarding them for use, can have more efficient heating.

6. Conclusion As described in this paper, we have confirmed that electric power consumption for heating can be significantly reduced by recovering and re-using waste thermal energy w, together with battery- storage heat. However, the proportion of electric power consumption for the heating is still high for driving in EVs, and therefore further improvements are required. It is necessary not only to reduce the heat capacity and heat loss of an air conditioning system but also all heat load of vehicle.The new developed system can be applied to pure EV’s as well as other electric-driven vehicles. We are continuing our research to enhance this technology.

Hiroyuki Ohno Jun Hatakeyama

Mitsuaki Nagata Tomohiro Maeda