Combined heating & cooling - Heat pump - Ground-source - School / Canada

Case study assignment
You are a consulting engineer who specialises in energy efficiency and innovative building heating, ventilation and air-conditioning (HVAC) systems. You will design the HVAC system for a new vocational school. The school will be devoted to agricultural studies and will be composed of two separate sections; one for academic and recreational use and the other for shops and pilot plants. The requirements specify that each of the two sections must be served by an independent system. The school board desires a flexible design that will permit them to adapt the HVAC system to possible future changes in building use. Energy efficiency and occupant comfort are also high priorities.

Given the high volume of fresh air required in the shops and pilot plants, combined with the potential changes in use for this section, you opt for a decentralised forced-air natural gas system for this section. Additionally, the shops and pilot plants do not require air-conditioning.

For the academic and recreational section, you propose using a ground-source heat pump consisting of a water-loop heat pump and closed-loop ground heat exchanger. The school board has asked you to compare this with a conventional variable volume air system with high efficiency chiller and natural gas boiler.

Site information

The vocational school will be located near Montreal, Quebec, Canada; the nearest meteorological station is at St. Hubert Airport. The surface soil is heavy and damp; you expect the rock underneath the overburden to be dense. The mean ground temperature is 7.8 °C and the temperature amplitude is 10 °C. There is a very large area of land available for the ground heat exchanger.

The academic and recreational section has a conditioned floor area of 6,856 m² spanning its two floors. This section is located mostly in the centre of the building and is surrounded by the shop and plant section.

Financial information

Typical financial figures are provided by the school board for your analysis. Forty percent of the cost of the school will be paid for in debt; the interest paid on the mortgage for the school will be 8%, and the term will be 20 years. The current discount rate used at the board is 9%, and the inflation rate is 2.5%. Energy rate increases for commercial customers are expected to be slightly below the inflation rate. The HVAC system is expected to last 25 years.

Natural gas is available at the school site at an average cost of $0.21/m³. The price of electricity is $0.045/kWh, plus a load charge of $11.6/kW per month. You assume that maintenance costs will be similar for the two systems under investigation. The school already has the equipment needed for laying horizontal ground heat-exchanger pipes. The cost for the conventional variable volume air system, including non-extended range heat pumps, a central boiler, pumps, a cooling tower, and flat-plate heat exchangers, has been estimated at $173,130. In addition, the variable air volume system will require perimeter heating costing $35,000.

Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.

Solution

The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.

Teacher's notes

• Because the academic and recreational section of the building is surrounded by the shop and plant section, the insulation level is high while the window area is standard.
• A horizontal ground heat exchanger has been selected because of its low initial cost, the availability of land, the availability of equipment for trenching (it is a school of agriculture) and the dense rock underneath the soil, which would make drilling more expensive.
• The costs of the feasibility study, development, and engineering for a conventional system are credited in the cost page.
• The school does not pay income tax.
• The radiators for the GSHP system can be positioned in such a way that additional perimeter heating is not required. Thus the cost of installing perimeter heating has been credited to the GSHP system.
• The GSHP system has both lower initial costs and lower annual operating costs than a conventional system. Its payback period is therefore immediate, the internal rate of return is infinite, the equity, debt, and debt payments are negative (i.e., less than the conventional system), and, obviously, the net present value of the system is positive. The debt service coverage should be ignored.
• According to RETScreen, the financial viability of the system is sensitive to the heating COP of the heat pump. If it is low, the maximum electric load rises rapidly and incremental electricity load charges become significant. This results in the GSHP system having lower initial costs but higher annual costs than the conventional alternative. In short, this high-efficiency conventional system should be compared with a high-efficiency GSHP system.
• This uses the lowest possible cost for trenching, based on the availability of the trenching equipment at the school. In the real installation, an external contractor was hired, with its equipment, to install the ground heat exchanger pipes. No backfilling was required, however, since the machine buried the pipe as it was laid. This helped reduce the labour cost for the horizontal ground heat exchanger. This approach can be used when soil conditions are favourable (not excessively rocky) and there is little risk of damage to the pipe from rocks.

Real project

Results

The Saint-Hyacinthe vocational school was built in 1994. It is located just east of Montreal. The school board sought a high level of architectural flexibility in order to accommodate any changes in use. Energy efficiency and occupant comfort were also primary considerations. To this end, a number of innovative measures were adopted, including:

• A water-loop heat pump system (WLHP) with a closed-loop horizontal ground-source heat exchanger;
• a thermal storage tank in the WLHP system;
• a 618 m² solar wall to preheat fresh air;
• an efficient lighting system; and
• a building energy management system.

The school energy use is approximately 60% that of comparable schools. This translates into annual savings of $4/m², or $41,000.

A feasibility analysis revealed that the horizontal ground-source heat pump (GSHP) system would cost less than the conventional, energy efficient alternative of natural gas boiler, chiller, and cooling tower. The cost of the GSHP system came to $115/m² compared to the estimated $155/m² for the conventional system. The availability of the school's farmland and low cost of laying the horizontal pipe played a significant role in making the GHSP system the financially preferred alternative. The payback period is immediate since the conventional system cost was estimated to be higher than that of the GSHP system.

System description

The ground heat exchanger is made-up of 32 loops measuring 365 m each. Half of the ground heat exchanger consists of a 1-pipe configuration buried 1.8 m deep while the other half is a staggered stacked 2-pipe system buried 1.2 and 1.8 m deep. The circulating fluid in the ground loop is a solution of 21% methanol and water. The WLHP comprises 35 heat pumps ranging from 12.3 kW to 19.9 kW. The total installed capacity is approximately 490 kW.

Lessons learned

• Maintenance costs for GSHP systems are comparable to those of other systems but special training for personnel is required.
• The GSHP system proved to be much easier to modify than a conventional system, making it more adaptable to changes in building use.
• Horizontal GSHP systems are not inherently limited to small commercial and residential applications. When land is available at low or no cost, it may be a better choice than a vertical system.
• Different renewable energy solutions can be combined with excellent results. In this building, a solar wall provides a portion of the make-up air heating for the GSHP system. Preheating of outside air is usually required in cold climates.
• Designers should always be looking for specific site or project characteristics that create opportunities to implement renewable technologies. For example, the large available land area around the vocational school permitted the implementation of a low-cost, efficient horizontal GSHP system.

The big picture

It is rarely optimal to use a single system in a building with a complex configuration and usage. This case study showed how the right combination of conventional and innovative technologies can lead to efficient and financially attractive installations.

Photo

Vocational school - Heat pump - Ground-source, Quebec, Canada

References

• CADDET Brochure R289, Heat Pump System Coupled to a Geothermal Heat Exchanger, November 1997.
• Caneta Research Inc., Learning from Experiences with Commercial/Institutional Heat Pump Systems in Cold Climates, draft report prepared for CEDRL, 2000.
• Soumis, Jean-François, "Personal communication," Commission Scolaire Saint-Hyacinthe, March 2000.
• Thibault, Guy, "Personal communication," Commission Scolaire Saint-Hyacinthe, March 2000.