Heat-Pump Electrification at a Middle School
A school district is deciding whether to replace a 30-year-old natural-gas boiler plant at a 180,000 ft2 middle school with air-source heat pumps. The boiler plant uses 45,000 therms per year at $1.10/therm and has an estimated annual efficiency of 78%. The proposed heat-pump system would deliver the same space heating and domestic hot water with a seasonal coefficient of performance (SCOP) of 2.6, but it would add 170 kW to the winter peak demand. Electricity costs $0.14/kWh and $16/kW-month demand. The local grid emission factor is 0.36 kgCO2/kWh; natural gas emits 5.3 kgCO2/therm. The district's sustainability policy says zero-carbon buildings by 2040. The heat-pump quote is $1.35 million, and the boiler replacement is $650,000. The school has a 250 kW rooftop solar array proposed but not funded. The building envelope is average for its age, and the district has no carbon price or internal shadow price.
1. Calculate annual operating cost and CO2 emissions for the boiler and the heat-pump option. What is the simple incremental cost per tonne of CO2 avoided?
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Boiler fuel cost: 45,000 therms x $1.10 = $49,500/yr.
Delivered heat: 45,000 therms x 0.78 = 35,100 therms. Convert to kWh using 29.3 kWh/therm: 35,100 x 29.3 = 1,028,430 kWh thermal.
Heat-pump electricity: 1,028,430 kWh / SCOP 2.6 = 395,550 kWh/yr.
- Energy cost: 395,550 x $0.14 = $55,377/yr.
- Demand cost: 170 kW x $16/kW-month x 12 = $32,640/yr.
- Total heat-pump operating cost: $88,017/yr.
- Incremental operating cost: $88,017 - $49,500 = $38,517/yr.
Emissions:
- Boiler: 45,000 therms x 5.3 kgCO2/therm = 238,500 kg = 238.5 tCO2/yr.
- Heat pump: 395,550 kWh x 0.36 kgCO2/kWh = 142,398 kg = 142.4 tCO2/yr.
- Avoided emissions: 238.5 - 142.4 = 96.1 tCO2/yr.
Incremental capital: $1,350,000 - $650,000 = $700,000. Simple capital cost per tonne avoided: $700,000 / 96.1 = about $7,280/tCO2, undiscounted. That ignores the higher operating cost and any incentives. If demand is diversified and the full 170 kW is not billed, the operating penalty is smaller, but the abatement cost is still high relative to most efficiency measures.
2. Does the heat-pump option meet the district's zero-carbon by 2040 policy? What role could the proposed 250 kW PV play?
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No. The heat pump reduces emissions by about 40% on the current grid, but it is not zero-carbon: about 142 tCO2/yr remain. Grid decarbonization would lower that over time, but the district cannot assume the grid will reach zero by 2040. The policy claim needs a plan for residual emissions.
A 250 kW PV array might generate roughly 350,000 kWh/yr in a good location. That is close to the heat-pump electricity use of 395,550 kWh/yr, so annual net consumption could approach zero. But PV produces mostly in summer and midday, while heating loads peak in winter and at night. Annual netting does not guarantee temporal matching. Without storage or a contracted renewable supply, the building would still import high-carbon electricity in winter and export surplus in summer.
The PV also competes for roof area and capital with envelope improvements. A better sequence is to reduce heat load first, then size the heat pump and PV to the reduced load. The district should not call the building zero-carbon unless it addresses winter residual emissions, refrigerant leakage, and the difference between annual and hourly matching.
3. Recommend a decision to the board. Consider capital, operating cost, carbon, resilience, and sequencing.
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The board faces a real trade-off, not a simple payback decision. The boiler replacement is $700,000 cheaper and has lower annual operating cost, but it locks in fossil heating for 20 to 25 years and conflicts with the 2040 policy. The full heat-pump option cuts emissions by about 96 tCO2/yr but costs about $38,500/yr more to operate and has a high simple abatement cost near $7,300/tCO2.
A defensible recommendation is a phased hybrid rather than an immediate full replacement:
- Reduce the building heat load with envelope, controls, and setpoint measures. This lowers the required heat-pump capacity and the winter demand charge.
- Install a heat-pump system sized for baseload heating, with the existing boiler retained for peak and backup. This captures most of the carbon reduction at lower capital and preserves resilience.
- Pursue the 250 kW PV and utility incentives to reduce operating cost and cover part of the heat-pump electricity.
- Set a date to eliminate the boiler once the grid is cleaner or on-site renewable supply plus storage can cover winter load.
If the district wants full electrification now, it should first secure incentives or a carbon price to close the $700,000 capital gap and the operating-cost gap. It should also require a low-GWP refrigerant, verified SCOP performance, and a winter demand-management plan. The board should compare this abatement cost with other district options such as solar, envelope upgrades, and controls before committing.