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A 12-Step method for closed-loop ground-source heat-pump design.

Article Excerpt
INTRODUCTION

The suggested steps for designing of vertical ground-coupled heat pumps (a.k.a closed-loop ground source heat pumps) for non-residential buildings are:

1. Calculate peak zone cooling and heating loads and estimate the off-peak loads;

2. Estimate the heat input and output into the loop field to account for potential ground temperature change;

3. Select preliminary loop operating temperatures and flow rates to begin optimization of first cost and efficiency;

4. Correct heat pump performance from rated to actual operating conditions;

5. Select heat pumps to meet cooling and heating loads and locate units in the building to minimize duct cost, fan power, and noise;

6. Arrange heat pumps into ground loop circuits (unitary, sub-central, or central) to optimize cost and minimize pump energy and demand;

7. Conduct site survey to find ground thermal properties and drilling conditions;

8. Determine one or more optimum loop field arrangements (bore depth, separation distance, completion methods, and header arrangement);

9. Determine the ground heat exchanger dimensions with calculations or software;

10. Iterate to determine optimum operating conditions, loop field arrangement and viable alternatives;

11. Layout interior piping and compute actual loss in critical system path(s); and

12. Select pumps and control method, determine system efficiency, and consider modifying water distribution system if power exceed 8% of the total and air distribution system if fan power exceeds 12% of total GCHP system demand.

Step 1: Calculate Cooling and Heating Loads

Conventional methods are used to determine the cooling and heating load for each zone. Off peak loads should be determined since ground loops can be arranged to take advantage of load diversity among zones that peak at different times. Furthermore, ground loop lengths are affected by off-peak loads because thermal storage effects occur throughout the day. For example, a church with a two-hour cooling peak once a week will not require as long a loop as a building with the same peak cooling load but has high loads 24 hour per day, 7 day per week. It is suggested that loads be calculated once in the morning (8 am-Noon), afternoon (Noon-4 pm), evening (4pm-8pm) and night (8 pm-8 am) as a compromise to hour-by-hour loads calculations.

Step 2: Estimate the Ground Heat Input and Output

Ground loops with insufficient bore separation and length are susceptible to long term ground temperature change. Close separation and shorter lengths reduce the volume and thermal mass in near contact with the ground exchanger. This will increase the thermal drift from the cooling season when heat is added to when it is removed in the heating season. Table 7 in Chapter 32 of the 2007 ASHRAE Handbook--HVAC Applications lists values for the temperature penalty for several combinations of heating and cooling hours. The problem is compounded by the fact that heat pumps will reject approximately 80% more heat per hour in the cooling mode than they will in the heating mode. Figure 1 is a schematic diagram of the heat balance that explains this concept for a heat pump with a COP = 4.0 (k[W.sub.t]/k[W.sub.e]) (EER = 13.6 [Btu/h/[W.sub.e]) in both cooling and heating.

[FIGURE 1 OMITTED]

Table 8 in Chapter 32 of the 2007 ASHRAE Handbook--HVAC Applications presents a summary of ASHRAE TRP-1120 (Carlson 2001) that determined Equivalent Full Load Hours (EFLHs) in cooling and heating for buildings in 26 locations and 4 different occupancy schedules. These values can be used in conjunction with the cooling and heating capacity of the selected heat pumps to estimate the total amount of heat rejected into and removed from the ground on an annual basis (Figure 1). The net quantity of heat stored in the ground on an annual basis must be considered when determining the required length and separation of the vertical bores in Step 9.

Step 3: Select Preliminary Loop Temperatures and Flow Rates

The operating temperature of the ground loop will rise above the local deep earth temperature in cooling and drop below this value in heating. Short loop lengths will increase these variations and lower operating efficiency. Long lengths will increase efficiency but will require greater costs and land area.

Most frequently the optimum entering water temperature (EWT) to the heat pump during cooling mode design conditions is 20[degrees]F to 35[degrees]F (11[degrees]C to 19[degrees]C) above the local deep earth temperature. For buildings in cold climates that have large heating loads compared to cooling loads, the optimum temperature variation will tend to be at the low end of this range. In warm climates the optimum temperature will be near the high end of this range. In heating, the optimum temperatures are typically 10[degrees]F to 15[degrees]F (6[degrees]C to 8[degrees]C) below the deep earth temperature at design conditions. Loops connected to high heating load buildings will tend to have optimum values near the upper end of this range while warm climates loops will likely have optimum values near the lower end of this range.

High liquid flow rates will improve heat pump performance but require greater pump input power. Although economics is a factor, optimum flow rates can be determined by minimizing the combined power input to the heat pump and water pump. For closed-loop systems optimum flow rates are typically in the 2.5 to 3.0 gpm/ton (2.7 to 3.2 Lpm/k[W.sub.t]) range.

Step 4: Correct Rated Heat Pump Performance to Actual Conditions

The current dual unit ISO Standard 13256-1 is a...

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