ENVIRONMENTAL IMPACT OF HEAT PUMPING SYSTEMS

Transcription

1 ID: 343 ENVIRONMENTAL IMPACT OF HEAT PUMPING SYSTEMS HAFNER A SINTEF Energy Research, 7465 Trondheim, Norway Armin.Hafner@sintef.no ABSTRACT A calculation tool was developed within the project Thermal Systems Integration for Fuel Economy TIFFE, which is a project within the SEVENTH FRAMEWORK PROGRAMME of EU, under ACTIVITY: The TIFFE-EIT (TIFFE-Environmental Impact Tool) calculates the annual emissions generated due to the additional consumption of fuel used for HVAC (Heating, Ventilation and Air Condition). A flexible daily/weekly driving routine is applied and weather/climate dates from different climate zones are used on an hour by hour base. The operation sequence starts with analyzing the energy demand of the different vehicle rides during a day, week and year. The energy demand for each ride depends on the climate conditions in which the vehicle was parked and the performance of the air conditioning/heat pump system to achieve comfort in the passenger cabin. Thus it is possible to compare the fuel demand related to HVAC for different systems. Specific car parameters (size of cabin, window ratio, etc.) can be varied, i.e. a validation of different vehicle types is possible. A huge difference in the fuel consumption of the HVAC can be observed when comparing short and long distance rides. If the car is just driven a few minutes every time, the HVAC system operates in pull-down or heat-up performance more often than on long distance rides, where the comfort level is just maintained after the initial phase. Parking in the shadow or open the window before you use the air condition has also an influence of the total energy demand. The output of the TIFFE-EIT is the additional fuel consumption of the HVAC during an entire driving season. It only requires MS Excel. The transparent configuration helps to understand the influence of the different parameters. 1. INTRODUCTION Various methods of calculating environmental impacts of refrigeration and air conditioning systems have been introduced during the last decades after the sensitivity arouse due to the harmful behaviour of non natural refrigerants to the ozone layer and the global warming. TEWI (Total Equivalent Warming Impact) was introduced in the early 9 of the last century. The analysis mainly take into account the direct impact of the required refrigerant, i.e. its GWP 1 value and the corresponding equivalent CO 2 emissions due to the operation of the system at a certain design/operating conditions. This method can be applied to compare various systems; however, if the operating conditions are far off the design point, the part load behaviour of systems is not taken into account. Therefore LCCP (Live Cycle Climate Performance) has been developed (HAFNER 26 and 27, PAPASAWA 28), beside the emissions related to the production of the entire system LCCP consists of the following parts: Indirect impact, i.e. GHG emissions mainly dependent on the system performance. Most important parameter since the usage period mainly dominates the environmental impact of HVAC systems. Real system performance data like fuel over consumption measurements, i.e. vehicle tests at certain 1

2 ambient conditions during dedicated driving cycles are the most reliable input values for this analysis. Direct impact, i.e. GHG emissions due to the refrigerant leakage, production of refrigerant and emissions due to special waste crematory processes. GHG emissions due to transportation of the system (mass) which can be a part of the indirect LCCP part. So far these tools do not take into account the entire energy system of vehicles, i.e. both the auxiliary heating devices required in the cold season and the AC loop. As proposed by MONFORTE (28), TIFFE-EIT applies the air enthalpy difference method; between passenger cabin environment and ambient. The air enthalpy is a direct measure for comfort because of the comprehension from air temperature and relative humidity. The definition of comfort (set-point temperature of the HVAC module) inside the passenger compartment, i.e. the required cooling/heating capacity can be adapted to regional preferences and seasonal climate changes. 2. OUTLINE OF THE CALCULATION TOOL A flexible weekly driving routine is applied in TIFFE-EIT, based on particulate set parameter from the user and measured/simulated fuel consumption parameters obtained from experimental results with specific driving cycles and cool down/heat up tests. The user distributes the average annual driving distance for a region/country into a flexible weekly driving pattern. The weekly drives are subdivided and it is distinguished between driving to and from work and weekend rides which are at different times of the day and the distances during the weekend are assumed to be longer. Example: driving to work at 8: and return home at 16:. In some regions people return home for lunch at 12: optional back home or somewhere else. According to this routine, the program counts the energy demand for each ride with respect to heating and cooling. Furthermore, it considers the air enthalpy in the car before the car is used. For the passenger compartment the starting energy demand (start of pull-down / heat up) is estimated, it is defined as the energy difference between the comfort level and the actual air enthalpy. It is dependent on the sunshine radiation, on the sunshine time and on the parking time including the involved adjustment to the ambience. The starting energy demand varies between almost zero, in the case of a climate which is very similar to the comfort level and no impact of the sun, and a very high amount caused of the immense heating up of the passenger cabin due to sunshine in the summer. In this case, the AC-system has to perform at maximum capacity during first period of a ride. The input parameters are Climate dates (hour by hour): Ambient temperature Air humility Global radiation 2

3 winter Trondheim summer rel. humidity [%] radiation [1W/m^2] 4 2 temperatur [ C] hour winter Frankfurt hour summer rel. humidity [%] radiation [1W/m^2] 4 2 temperatur [ C] hour winter Athens hour summer rel. humidity [%] temperatur [ C] 4 2 radiation [1W/m^2] hour hour Figure 1: Daily temperature profile for a typical winter/summer day in Trondheim, Frankfurt and Athens 3

4 Weekly driving routine: The car is driven a certain distance in a year. In Europe an average annual driving distance is 125 km/a, which is split into specific distances per day (Figure 2). Please note that the driving cycle at weekdays is for all five days. Figure 2: Weekly driving routine by an annual distance of 125km The user of the tool has to enter a distances to work, shopping and lunch, an annual distance and the times when the driving starts. The TIFFE-EIT program calculates the required time and the distance driven at Sundays. A plot as shown in Figure 2 shows an example of the various drives, their distance and at which time of the day the car is on the road. Car specification: A typical pull-down performance (reduction of air inlet temperature) after a soak condition as measured by Mager et al. (22) is shown in Figure 3. After 4 min of pull-down, the vehicle entered an idle phase. Figure 4 shows correspondingly the heat up performance in the car during a driving cycle with different auxiliary heating devices. Figure 3: Comparison of average temp. at dashboard outlets, pull down cooling [Mager et al., 22] 4

5 Figure 4: Comparison of average temperatures at head zone, heating [Mager et al., 22] Additional parameters which influence the heat up and pull down behaviour are: the area of windows / windshields in the car the colour of the vehicle the dimension of the auto body heat conduction of the different materials between passenger compartment and ambient Behaviour of the driver: If a car is parked in the sun, the temperature in the cabin can rises up to high levels. As an example, the compartment temperature reaches about 67 C during a summer day in Athens at ambient temperature of 37 C and a solar radiation of up to 8 W/m 2, as shown in Figure 5. igure 5: Ambient temperature, passengers compartment and sun radiation for a summer day in Athens F An important parameter for the heat up (soaking) is the colour of the vehicle. In this case the program has to calculate the heat transmission of the roof. To accomplish this method, further parameters (U-value, thickness, area, heat transmission coefficient) have to be provided by the user. The solar radiation value is taken of the weather data (METEONORM) and will be integrated, to calculate the temperature difference. Figure 6 shows the heat up of different plates due to solar radiation. 5

6 Figure 6: Heat up of a black and a white plate in different solar radiation [Großmann, 21] 3. SIMULATION MODEL In TIFFE-EIT the time intervals of the ambient conditions based on climate data are one hour. However, since most of the drives on a daily base are shorter it is necessary to understand the system performance and its environmental impact based on the elapsed time from the start of each drive. Many times the HVAC system has to perform under heat up, pull down or even part load conditions, i.e. steady state conditions at its design point are rare. Calibration/ Validation of TIFFE-EIT The TIFFE Modelica system simulation tool developed with help from experts from TU-Braunschweig and TLK will be calibrated based on experimental measured fuel overconsumption values. These data are obtained during vehicle testing programs, either under steady state (constant vehicle velocity) or pull down / heat up conditions. By applying the TIFFE-Modelica model it will possible to produce Mobile Air Conditioning (MAC)performance tables as described below. MAC Performance table The parameters for the different tables are various ambient temperatures and the enthalpies inside the passenger cabin at start up of the MAC. A third parameter is the velocity of the vehicle. A simulation or measurement of the MAC/HVAC performance has to be done applying the following input data: Ambient temperature: 2 C; 3 C; 4 C Air enthalpy inside: 4 kj/kg; 6 kj/kg; 8 kj/kg; 1 kj/kg; 12 kj/kg Car speed: idle ( km/h); 3 km/h; 5 km/h; 7 km/h The intermediate results may look similar as shown in Figure 6. However, for the TIFFE-EIT calculations (see Figure 11) only tables are needed. 6

7 1 Relative compressor power [kw/kwdesign] Elapsed time after start up [min] Figure 7: Relative MAC performance at an ambient temperature of 2 C, initial assumption, for various interior air enthalpy values and different vehicle velocities. Design power consumption of the compressor divided by the actual power consumption. The minimal performance (for an air enthalpy of 4kJ/kg) is needed if the temperature inside and outside are the same and the AC have to work against the sun radiation and other gains of heat. The maximal performance is needed during the starting phase when the AC works on full load. Dependent on the enthalpy inside, the AC system needs more time for cooling down the air of the passenger compartment. That s why, the green curve (air enthalpy of 6kJ/kg) is falling down earlier while the red curve (air enthalpy of 8kJ/kg) is still on high level, demanding still max. power on the compressor shaft. After the full load phase the AC control reduces the power until the minimal performance is achieve. Different car velocities affect the performance of the heat rejecting devices at the front end of the vehicle. By defining the share of the different vehicle velocities, as shown as an example in Figure 8, it is possible to calculate with TIFFE-EIT the environmental impact of a vehicle at any driving cycle. Figure 9 shows a typical driving cycle, NEDC. Figure 8: Ratio of the different velocity in NEDC Figure 9: NEDC [Hafner et al. (26)] 7

8 Taking into account the share of different vehicle velocities, TIFFE-EIT calculates the accumulated fuel over-consumption based on the simulated tables, as shown in Figure 1. Finally a linear interpolation based on the main boundary conditions (ambient temperature, enthalpy inside the car and duration of drive while the MAC is running) is performed for every vehicle start-up during a year. I.e. the boundary conditions are continuously calculated hour by hour, every time the vehicle is driving for a certain time (as shown in Figure 2) the actual pull down / heat up performance is applied to calculate the environmental impact of the MAC system. 1 Accumulated fuel over consumption [1ml] C 3 C 2 C Duration of drive [min] Figure 1: Example of accumulated fuel over-consumption of the AC depended on duration of drive, ambient temperature and indoor air enthalpy. Figure 11 summarizes the way of calculating the environmental impact related to achieve climate comfort inside the passenger cabin during all seasons. Figure 11: Calculation steps of TIFFE-EIT 8

9 Heating So far the user of the tool can choose between two different heating systems. System 1 heats the passenger cabin by a traditional glycol/air heater core and is supported by an auxiliary electric heating device as currently applied in vehicles, i.e. the PTC heater (positive temperature coefficient). The second heating system consists also of the glycol heater core which is able to reject the waste heat of the engine coolant; however, in addition a heat pump device is applied for the auxiliary heating instead of the PTC heater, as shown in Figure 12. Figure 12: Operating mode of the two heating systems The basic for this calculation is the temperature of the glycol while the car drives different velocities. The temperatures are calculated by the simulation as shown in Figure Temp_glyc Temp_incar1 Temperature in System 1 [ C] Duration of drive [min] Figure 13: Simulated temperatures as a function of the time since start up of the engine coolant and the passenger cabin temperature (ambient temperature -1 C) Every curve starts from the ambient temperature and rises up to ~ 8 C in case of the water-glycol-mixture. In contrast the temperature inside the car rises up to 21 C and stays at the comfort level thereafter, due to the control of the HVAC unit. (review sentence) To be able to describe the heat pump system, at least the gradient is required which shows the COP-values related to the ambient temperature. 9

10 Figure 14: Typical COP-value of a heat pump (assumed) The user of TIFFE-EIT needs to enter the maximum available electric power that can be generated by the electrical dynamo of the engine. The tool first calculates the electrical power per duration of the drive and after that the over consumption, which is related to the additional heating system. The boundary conditions of the following examples are: t amb = -1 C, P el = 1,5 kw, COP = 2,69. The blue line is the system 1 (electric heating), the green line is system 2 (including heat pump). PTC HP ure 15: Electrical power that is needed for the heating (heat pump electrical input = PTC power) Fig PTC HP gure 16: Accumulated fuel over consumption due to heating of the passenger cabin at t amb = -1 C Fi 1

11 Figure 15 shows that the electric heating (PTC) has to be switched on over a longer period of time compared to the heat pump system. Therefore more fuel is needed to achieve the comfort temperature (Figure 16). To calculate the diagram as seen in Figure 16, the required heating capacity and the energy which is provided from the heated water-glycol-mixture of the engine coolant has to be taken into consideration. The control of the HVAC units decides the period of time the auxiliary system is on to reach comfort. The heating demand is calculated based on the heat losses of the cabin, the fresh air into the compartment, the passengers in the car and the radiation of the sun. The different heat flows are shown in Figure 17. Q heat el is the heating power of the electric heating (PTC). This electric heating is two-ary (1% and 5% of P max ). The engine heats up a glycol-mixture (engine coolant), which rejects heat to the air which enters the passenger cabin Q heat glycol. The gradient of Q heat glycol rises as long as the temperature in the car is colder than the comfort temperature. As soon as comfort temperature is achieved, the pump or the flow rate is controlled. The heat demand of the cabin is the sum of Q fresh air, Q people, Q rad and Q loss. Q fresh air is the required heat to increase the temperature of the ambient air to the temperature of the outlet system in the car. Every person also rejects heat: Q people (about 1 W). Radiation due to the sun contributes to heat up the car Q rad. The colour of the car, the area of the glass, the roof and the heat transmission coefficient of the both materials have influence of the value of Q loss. Figure 17: Energy flow of system 1 (electric heating / PTC) at t amb = -1 C The excess energy, when calculating the energy balance Q_heat_el and Q_heat_glycol and subtract it from Q_demand, is used to rise the inside temperature to the desired temperature (target_temperature = 21 C). Assumed values for the example shown in Figure 16: m_glycol =,3 kg/s; c_p_air = 1,5 kj/kgk; c_p_glycol = 3,7 kj/kgk; k*a = 67,6; Temp_air_vent outlet = 4 C 11

12 Figure 18: Heating power of the fresh air while driving at t_amb = -1 C Figure 18 shows heat required to rise the temperature of the ambient air to the outlet temperature. In the first 5 minutes of operation, the ventilation system supplies 1% fresh air in the cabin (9% recirc. air). After 5 minutes the fresh air begins to rise up to 9% (1% recirculation air). This control procedure last 5 minutes and stays after that at 9%. The more fresh air is brought into the cabin of the car, the more heat is needed. The lower the ambient air temperature, the more heat is required to achieve comfort. 4. Case study; Prelim- Results The three cities Athens, Frankfurt and Trondheim are compared regarding the annual fuel consumption of a vehicle related to the HVAC unit. The climate data of Athens represents the Southern Europe climate zone; Frankfurt represents the Central Europe area while Trondheim represents the colder North European areas.. Table 1 shows the calculated fuel overconsumption values for a year of driving with an AC system in the three climate zones. Two different parking habits are compared, either the car is parked under the roof (garage), i.e. no radiation influence during stand still or the car is not protected against radiation (parking sun). The fuel consumption is also dependent on the colour of the car. The white vehicle parked in the sun is the reference, if the same car is parked in the shadow it can safe up to 31 litre of petrol in the South part of Europe. The colour of the car has a minor impact on the fuel consumption related to the HVAC unit. Table 1: Fuel overconsumption in the cooling system and in the three cities (Athens, Frankfurt, Trondheim) compared with the diffent parking situations. System1 & 2 parking sun parking shadow Cooling white black white black Athens 84,7 + 1,2-3,9-29,3 Frankfurt 25,8 +,4-11, -9, Trondheim 1, +,1 -,6 -,6 The additional fuel consumption required to heat up the passenger cabin is shown in Table 2. In this case parking in the sun reduces the fuel consumption by up to 42 litre for a car with an PTC heater driven in the northern part of Europe. If the car is equipped with a heat pump system, more than 1 litre of fuel can be saved annually. 12

13 Table 2: Fuel overconsumption due to two different heating systems. The different parking situations and the different areas in Europe are compared. System1 (PTC) parking sun parking shadow Heating white black white black Athens 48,5 -,2 + 15,9 + 14, Frankfurt 136,7 -,8 + 22,9 + 2,7 Trondheim 165,5-1,2 + 42, + 38,3 System 2 (HP) parking sun parking shadow Heating white black white black Athens 14, -,4 + 7,6 + 6,7 Frankfurt 46,9 -,2 + 11,3 + 1,5 Trondheim 59,9 -,2 + 2,9 + 19,7 Figure 19 summarizes the result of the annual fuel overconsumption due to the HVAC system of a vehicle (System 1). Most of the additional fuel is required in the Nordic countries where heating requirements (supported be the PTC heater) dominate the operation of the HVAC system. Athens Frankfurt Trondheim heating cooling 133 dm dm dm 3 Figure 19: The partition of the annual fuel overconsumption (in scale) in the three cities (white car; sun parking). Vehicle equiped MAC & PTC heater, no heat pump. Athen Frankfurt Trondheim 99 dm 3 73 dm 3 61 dm 3 Figure 2: The partition of the annual fuel overconsumption (in scale) in the three cities (white car; sun parking). Vehicle equiped MAC with heat pump. Figure 2 summarizes the result of the annual fuel overconsumption due to the HVAC system of a vehicle (System 2). Most of the additional fuel is required in the southern part of Europe, where cooling 13

14 requirements dominate the operation of the HVAC system. The fuel demand share of the cooling is also much higher in Central Europe, when applying a heat pump system as the auxiliary heating system. 5. SUMMARY The presented absolute values of the fuel over consumptions are sensitive to the assumptions made; however, the relative comparison of the different location is possible and will be less affected of the assumptions. In Athens (Southern Europe) the fuel consumption is up to 3 l/a higher, if the driver parks his car always in the sun. The colour of the car has a minor influence, however, the darker the car, the more fuel has to be used to pull down the interior temperature to comfort level. If the car is equipped with a heat pump ~7% of the fuel required to heat the passenger cabin could be saved. In Frankfurt (Central Europe) the additional fuel consumption due to parking in the sun is ~ 11 l/a. 9 l/a of fuel could be saved, if the vehicle is equipped with a heat pump system. In Northern Europe (Trondheim) the fuel consumption in due to shadow parking is much higher as in case of sun parking. If the ambient temperature is low the solar radiation is useful to save heat energy. Thus it is better to have a darker car in the colder regions than in the hotter areas. More than 1 l/a of fuel can be saved if the car has an heat pump system. A linear interpolation based on the main boundary conditions (ambient temperature, enthalpy inside the car and duration of drive while the MAC is running) is performed for every vehicle start-up during a year. I.e. the boundary conditions are continuously calculated hour by hour, every time the vehicle is driving for a certain time the actual pull down / heat up performance is applied to calculate the environmental impact of the MAC system. 6. ACKNOWLEDGMENTS This paper has been written as part of the work within the research project TIFFE, which is financially supported by the SEVENTH FRAMEWORK PROGRAMME of EU, under ACTIVITY: REFERENCES Großmann, H., Pkw-Klimatisierung: Physikalische Grundlagen und technische Umsetzung, Springer Verlag GmbH, Berlin, 21, Hafner, A., Nekså, P., 26. Factors influencing life cycle environmental impact calculations of MACs. Hafner, A. 27 Global environmental and economic benefits of Introducing R744 mobile air conditioning 22nd IIR International Congress of Refrigeration, August 27, Beijing, China Mager, R., Hammer, H., Wertenbach, J., 22. Comparative study of AC- and HP-systems using the refrigerants R134a and R744. VDA Wintermeeting Malik, T., Bullard, C. W.,24. Air Conditioning Hybrid Electric Vehicles while Stopped in Traffic. Air Conditioning and Refrigeration Center. METEONORM, Ver.5.1, Meteotest. Monforte, R. (27). MAC System Fule Overconsumption in various climate conditions. SAE 27 ARSS-July 17-19,27, (p. 3). Papasawa, S, & William R.Hill28 Green MAC LCCP, Impact of Alternative Refrigerants to Global Vehicle LCCP CO2 Equivalent Emissions, Proceedings of the VDA Wintermeeting, Saalfelden, Austria Zackrisson, M., Avellán, L., Orlenius, J., 21. Life cycle assessment of lithium-ion batteries for plug-in hybrid electric vehicles Critical issue. Journal of Cleaner Production 18 (21),