Chapter 14B: HEAT PUMPS AND PART LOAD ANALYSIS

Chapter 14B: HEAT PUMPS AND PART LOAD ANALYSIS
Agami Reddy (rev- May 2017) Air-source heat pump systems Operating characteristics of heat pumps: interaction with building heating loads Concepts of supplemental heating, balance point of the heat pump, and excess capacity Factors reducing the efficiency of air source heat pumps Other types of heat pumps: water-source, ground/geothermal AHRI rating standards (steady-state performance) EER and SEER, IPLV rating Part-load performance Part load performance for Unitary equipment: Degradation factor Part load analysis using bin methods- Heat pump example Medium to large chillers: simple polynomial and DOE models HCB 3-Chap 14B: Heat Pumps and Part Load

HCB 3-Chap 14B: Heat Pumps and Part Load
Air-Source Heat Pumps Typically rooftop units (packaged complete or split) Range of unitary HPs: 1.5 – 20 Tons Best suited for mild weathers (South East United States) and where electric price is low compared to gas HP sized to satisfy peak cooling loads Supplemental heating needs backup heating source Fig HCB 3-Chap 14B: Heat Pumps and Part Load

Air-Source Heat Pumps Fig System can be used either as an A/C or a HP simply by reversing refrigerant flow This is done by a Reversing valve HCB 3-Chap 14B: Heat Pumps and Part Load

Fig COP of ideal Carnot, Carnot (with heat exchanger penalty), and real heat pumps Fig Heating performance curves as a function of ambient temperature for four different nominal heat pump units HCB 3-Chap 14B: Heat Pumps and Part Load

Table Sample manufacturer performance data for a unitary residential split heat pump unit. The tests have been conducted with indoor conditions: 80 oF dry-bulb and 67 oF wet-bulb for cooling, and 70 oF dry-bulb for heating. HCB 3-Chap 14B: Heat Pumps and Part Load

Example 14.5 Given: Ktot = 1110 Btu/(h · °F), Tb,summer = 70°F and Tb,winter = 60°F, To,design summer = 105°F, To,design winter = 20°F Figure: See Fig HCB 3-Chap 14B: Heat Pumps and Part Load

39,000 Fig HCB 3-Chap 14B: Heat Pumps and Part Load

Factors Reducing Efficiency of Air Source Heat Pumps
Frosting: ice build up outside coils: at low Tdb (20o F to 40o F) and high RH (>60%) Reduces evaporator airflow Decreases heat transfer Decreases capacity and COP Fig Performance degradation due to frosting HCB 3-Chap 14B: Heat Pumps and Part Load

Defrosting: Required to remove frost buildup Two initiation schemes used: * time/temperature * demand (pressure drop across outdoor coil or temp. diff Tout – Tref Unit operates as A/C for 6-10 min, resistance heat serves as backup to heat air to building - Typically h cycles - Large energy penalty (8-15%) Undercharge Large fraction of units suffer from refrigerant undercharge which reduces performance Cycling Losses: Occurs because unit has too much capacity under part load Some heat pumps require 15 min to reach steady state - COP (cycling) < COP (steady-state) Heating capacity degradation with refrigerant undercharge HCB 3-Chap 14B: Heat Pumps and Part Load

Improving Heat Pump Efficiency Multi-Speed / Variable Speed - Reduces backup resistance - Reduces cycling Gas Furnace for Supplemental Heat Reduces cost (electricity expensive) Better Defrost Cycle - Improves efficiency Different Heat Sources Water Ground HCB 3-Chap 14B: Heat Pumps and Part Load

Decentralized water loop system with several unitary heat pump
Fig Decentralized water loop system with several unitary heat pump Use water instead of air to transfer energy between building and outdoors During hot weather- cooling tower removes heat from water loop During cold weather- boiler heats water Loop allows for simultaneous heating and cooling by individual units (resulting in higher system efficiency and comfort) HCB 3-Chap 14B: Heat Pumps and Part Load

Central Closed-Loop Water Source Heat Pump
System has separate loops for: - conditioning fresh or outdoor air - meeting heating/cooling building loads Note: separate primary ventilation system is not shown Heat Pump terminal unit HCB 3-Chap 14B: Heat Pumps and Part Load

Fig. 14.25 Different ground-source heat pump configurations (downloaded from Geothermal website)
-Becoming popular - Increasingly competitive HCB 3-Chap 14B: Heat Pumps and Part Load

TABLE ADVANTAGES AND DISADVANTAGES OF AIR AND WATER SOURCE HEAT PUMPS Type Advantages Disadvantages Air source Indoor distribution permits air conditioning and humidity control Defrost required Outdoor air source readily available Low capacity at cold outdoor temperature Simple installation Lower efficiency because of large evaporator ΔT ≈ 30°F Least expensive Indoor air distribution temperature must be high for comfort reasons Established commercial technology Reliability at low temperature is only fair, due to frosting effects Must keep evaporator clear of leaves, dirt, etc. Water source Multiple family and commercial installations as central system Needs water source at useful temperature In commercial installations, good coupling to cooling towers Efficiency penalty due to space heat exchanger ΔT No refrigerant reversal needed; reverse water flow instead HCB 3-Chap 14B: Heat Pumps and Part Load

Window AC Systems Design criteria: HCB 3-Chap 14B: Heat Pumps and Part Load

Thru-The-Wall Conditioning Systems
This is a larger air-cooled unit designed to be mounted thru a wall (can provide both heating and cooling) Complete refrigeration system: DX cooling coil, heating coil (electric, hot water, steam) and an air-handler with packaged controls PTAC and PTHP (Packaged Terminal Air-Conditioner Heat Pumps) Packaged A/C with side discharge and return HCB 3-Chap 14B: Heat Pumps and Part Load

Unitary Split Systems For residential and small commercial Air-source
heat pumps with electric heat can also be used Both heating and cooling DX cooling coil Same ducting used HCB 3-Chap 14B: Heat Pumps and Part Load

EER and SEER COP is the scientific measure of performance for VC chillers Performance Factor (PF) used for heat pumps under heating mode HVAC industry uses a dimensional performance measure based on Tons and kW. Energy efficiency ratio (EER) is the ratio of cooling capacity (Btu per hour) to the electric input rate (Watts)- conversion factor EER = COP x Btu/(W. h). In 1978, the U.S. Congress mandated the use of SEER (Seasonal EER) index which takes into consideration how certain variables affect the performance of the unit in different climatic zones Current minimum federal SEER is 13 though commercial units are available with SEER values as high as 23. Since the SEER reflects performance in conditions milder than those for peak or rated conditions, SEER generally > EER by 2-3 points. Go to for up to date info HCB 3-Chap 14B: Heat Pumps and Part Load

Air-Conditioning and Refrigeration Institute (ARI)
Trade organization representing North American air-conditioning and refrigeration equipment manufacturers Publishes standards for rating equipment performance Certifies performance of equipment through independent testing ARI standards related to chilled water systems 550/590: Standard for Water Chilling Packages Using the Vapor Compression Cycle 560: Absorption Water chilling and Water Heating Packages HCB 3-Chap 14B: Heat Pumps and Part Load

Table 14.7 AHRI Testing Conditions for Rating Unitary Equipment
AHRI- Air-conditioning, Heating and Refrigeration Institute OAT- outdoor air temp. EAT- entering air temp. HCB 3-Chap 14B: Heat Pumps and Part Load

Integrated Part Load Value (IPLV) Weightings
Fraction at each load based on distributions from simulations of typical buildings in 129 US Cities Gives little weight to rated full load performance, heavy weight to part load IPLV typically > rated performance because of condensing temperature relief HCB 3-Chap 14B: Heat Pumps and Part Load

IPLV Calculation COP or EER IPLV = 0.01 x A x B x C x D A, B, C, D are COP or EER at 100%, 75%, 50%, 25% load, respectively Note: This metric should not be used for multiple chiller installations HCB 3-Chap 14B: Heat Pumps and Part Load

Part-Load Performance Unitary equipment: - Degradation factor method Medium chillers: - Simplified regression model - DOE model Seasonal and /or annual energy estimation: - Bin method - Modified bin method HCB 3-Chap 14B: Heat Pumps and Part Load

Degradation Factor Method for Unitary Equipment
Part Load Ratio: Fig Modeling part load of unitary equipment using the PLR-PLF method Part Load Factor: Compressor power Simplified relationship assumed Cd values: 0.15 – 0.25 HCB 3-Chap 14B: Heat Pumps and Part Load

Example 14.7. Part Load Modeling
500 HCB 3-Chap 14B: Heat Pumps and Part Load

Recap - Bin Methods Simplified Multiple Measure – Basic Bin Method Gets its name from the way the weather data are put together Weather data are broken into temperature "bins" Each "bin" has the cumulative number of hours it has been at a given temperature range over the whole year Conventional bins are 5 °F 35 – 39 °F 40 – 44 °F 45 – 59 °F… Fig. 10.8 HCB 3-Chap 14B: Heat Pumps and Part Load

Seasonal HP Performance Using The Bin Method
Example 14.8: Seasonal HP Performance HCB 3-Chap 14B: Heat Pumps and Part Load

Solution Process HCB 3-Chap 14B: Heat Pumps and Part Load

Example 14.8: Heat Pump and Building Load Data (Example 14.8)
Table 14.8 Manufac. supplied data HP balance point HCB 3-Chap 14B: Heat Pumps and Part Load

Example 14.9: Heat Pump Energy Calculations (Example 14.8) Contd. Table 14.9 HCB 3-Chap 14B: Heat Pumps and Part Load TOTAL 6415 98.36 75.8 43.5 22.56 66.1

Fig Heat pump energy use by the bin method, Example The numbers at each bin temperature indicate the number of hours of occurrence in each bin. HCB 3-Chap 14B: Heat Pumps and Part Load

Simple Black-Box Performance Models for Medium to Large Chillers
(14.32) (14.33) HCB 3-Chap 14B: Heat Pumps and Part Load

DOE-2 Chiller Model for Medium to Large Chillers
For accurate building energy simulation models, the DOE model approach is used We start from the performance map covering the full range of operating conditions. (a) It is mandatory to acquire the cooling capacity and power draw under rated conditions: Then several polynomial correlations are developed from manufacturer data. HCB 3-Chap 14B: Heat Pumps and Part Load

DOE-2 Chiller Model It consists of several sub-models involving: b) Model for capacity at full load under off-design conditions: CAP_FT = f( Tch,out and Tcd,in) c) Model for Energy Input Ratio (EIR) at full load under off-design conditions: EIR_FT = f( Tch,out and Tcd,in) d) Model for Energy Input Ratio under part load EIR_FPLR = = f( PLR) e) Finally, power drawn by compressor is (14.37) Based on 15 model parameters data points necessary for regression HCB 3-Chap 14B: Heat Pumps and Part Load

Example of DOE-2 Model HCB 3-Chap 14B: Heat Pumps and Part Load

Outcomes Understanding of how heat pumps differ from VC cooling systems Understanding the functioning and components of an air-source HP Be able to solve problems involving selecting air source HPs for residential applications Familiarity with the various factors effecting efficiency of air source HPs Understanding the advantages offered by water source and ground source HPs and some of the system configurations Familiarity with the different types of AC systems for small scale applications Understanding the definitions of EER, SEER, and IPLV Familiarity with the different rating standards Knowledge of the different methods to model part-load performance degradation of small and medium chillers Be able to apply the bin method to analyze the seasonal performance of HP systems Knowledge of the DOE-2 chiller model and be able to apply it to practical problems HCB 3-Chap 14B: Heat Pumps and Part Load

Chapter 14B: HEAT PUMPS AND PART LOAD ANALYSIS

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Chapter 14B: HEAT PUMPS AND PART LOAD ANALYSIS

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