RoughDesigns

Ground Source Heat Pump Design

Conditions Temperatures - Assumptions

Ground Source Heat Pump Design Conditions - Criteria - Assumptions
Based on research / similar systems / geographic conditions / site conditions
2/2/2011
Rah

 

Systems under consideration
1.  Open Loop Indirect System Primary thermal source spring water, secondary earth coupling of thermal storage pond
2.  Closed Loop Earth Coupled


Soil / Ground Water Temperatures
Standards Based (Guidelines found for area varied, highs and lows below)
Ground Water Temp                              High    Low
                                                                 51       47
Adj for loop depth    Reduction in winter    -12      -10
(10 to 12 f diff)             Increase in summer   12       10
          Design Ground Temp                                            
                                                       Loading           Evaluation of GSHP loop
                         Winter          35       2/3 htg load      Ground loop LESS efficient than water loop
                         Spring / Fall  41       1/3 htg load      Ground loop LESS efficient than water loop
                         Summer       59       clg 50% htg     Ground loop MORE efficient than water loop
 Spring / Pond Water Temperatures
Observed (Note: Repairs to thermal storage pond should increase winter and lower summer temp)
                                             Spring          Pond                  
                        Winter               38                36                      
                        Spring / Fall       48                47       
                        Summer            55                78                      
        Design Water Temp
                                                        Loading          Evaluation of GSHP loop
                         Winter          36       2/3 htg             Water loop MORE efficient than ground loop
                         Spring / Fall  47       1/3 htg             Water loop MORE efficient than ground loop
                         Summer       78       clg 25% htg      Water loop LESS efficient than ground loop

Required Loop Temps
Standard / Example Based - Informational to use with above to select design temps                       
                                            For Efficient        For standard         HP Shut            Delta Loop 

                                            operation            operation              down                 to Dist   
        Winter       Loop to HP   36                      32                        28                     10         
                         HP to Loop   26                      no data                22                     10         
        Summer     Loop to HP   70                      80                        no data             20         
                         HP to Loop   90                      100                      no data             20         

 

Design Earth Coupled Loop Supply Temp                               
Standard / Example Based
                                        Low         Median       High      Delta    Required vs Design LoopTemp

        Winter      Loading     66%        23%           10%                 
                        Heating     32           45              50         10        Heatg will wk at higher efficiency
        Summer    Loading     10%        10%           80%                 Cooling load to be 25% htg load
                        Cooling      60           70              80         20       Cooling will wk at lower efficiency

 

Design Distribution Loop Supply Temp
Standard / Example Based (Low temp distribution maximizes HP efficiency, max radiator / duct sizes)
                       Low         High         Delta Dist Supply to Return      
         Heating    65            95            15                                                             
         Cooling    50            80            15    

                                                         
Distribution Temps                         
< 95 f   Req highly efficient radiant floor, tile / conc, w / min floor covering.
   95 f   Large / add radiators or low performance radiant floor system
 114 f   Low range standard radiators, some additional radiators may be needed
 120 f   NA Will degrade HP life

 

Design Temps - Indoor / Outdoor
                                      Outdoor    Indoor
                                                      Low         High
        Heating                  68             off            off        Heating system off except for

                                                                                manual dehumidification coord with AC req.
                                       60            70            70     
                                       50            70            70     
                                       20            65            70     
                                       0             55            65     
                                       -20           50            60      Boost temp with supp gas fired ? system

                                                                                to provide backup during power outages also
                  Power Out      -20           40            40      Gas ? fired backup to run off emergency

                                                                                generator
       Cooling                   100          Dehumidify Only                 
                                       90            75            80     
                                       80            70            70     
                                       75            65            65     
                                       70            off            off      Dehumidification under 75 f assumed to be

                                                                               more efficient using heating
                  Power Out         No Cooling Provided

Flow Rate for Spring / Pond
System Standards based
GPM / ton max load         3
System size                    5     tons
Required GPM / Max       15
Partial load (useful for imbalanced systems in climates where heating or cooling dominate) 25%      3.75 tons.
                       
Available flow
                           Min                   Mean             Max
                           July-Sep            Nov-Feb         Mar-June
Upper Spring        1.5                    10                  20
Lower Spring        1.5                    5                   20
Total Supply         3                      15                  40
                           Insufficient         Sufficient        Large Flow
Full load               Flow                 Flow               Highly Eff Op
                           Insufficient         Large Flow      Large Flow
Half load               Flow                 Highly Eff Op   Highly Eff Op

Carbon Impact               
Baseline               Gas                              1     Min Carbon Impact
                            Oil                                2     Double Carbon Impact of other systems
                            GSHP                           1     Min Carbon Impact
                            GSHP with Green Elec   0     No Carbon Impact
I simplified this a bit. The data was amount per equivalent KW of heat or BTUs of heat. While the dataset I used supported the above magnitudes of difference, there are differences in sources, so I won't debate the exact quantities, but will support the ranking, as all sources supported the rankings. While gas once again show itself to be clean, this does not take into account the full carbon effect of using gas to heat, then installing a full size air source heat pump for AC. This doubles the major equipment being installed and the carbon it takes to make, as well as the metals, plastics, etc. used in it.
Note that all sources take into account the transmission losses for electricity by quantifying carbon impact by "primary source". This effectively cuts HP COP's in half.

Assumptions
1. Earth or Pond loops will be sized to ensure loop return to HP will match earth / water temperature.