Leveraging energy efficiency to finance HVAC system retrofits

Geothermal underground heat exchanger at the Centre hospitalier universitaire de Laval (CHUL), one of the four hospital project sites.
Geothermal underground heat exchanger at the Centre hospitalier universitaire de Laval (CHUL), one of the four hospital project sites.

Key measures implemented

A variety of energy conservation measures was implemented at the four CHU sites.

Steam-to-hot water conversion of the heating system

Steam requirements for heating were removed from all four sites. Switching the heating system from steam to hot water reduced thermal losses and eliminated certain inefficiencies associated with steam networks, such as steam trap leaks, boiler blowdown, and flash steam from atmospheric condensate tanks.

Extensive engineering was deployed to understand the heating loads of the buildings before sizing new piping and network components.

The conversion required changing coils in some ventilation systems and radiators. At CHUL, 900 aging steam radiators were replaced with new hot water units. These were spread over six floors and across various departments. Communication with the technical services department and medical staff was key to prevent service disruption for patients.

The Hôpital Saint-François d’Assise (HSFA) underwent a similar conversion for more than 350 radiators. In the grand scheme of things, planning and executing these upgrades was less risky and disruptive than doing nothing and fixing leaks or network failures as they arise.

Planning these interventions involved many meetings with the hospital’s technical services staff, Ecosystem, department directors, health staff, and the infection control committee. It was critical to minimize the amount of relocation time for each patient and to reduce the risk of infection. On average, each patient room was completed within five days and many precautions were taken, such as doing the work with a negative pressurized hatch and wiping clean the wheels of the contractors’ carts every time they exited the hatch.

For the new hot water heating network, some existing steam piping was reused—depending on its condition—and new piping (especially returns) was installed to create new networks. Both the hot and warm water networks focused on using the lowest temperature possible to meet the heating loads. To achieve this, various coils and radiators were sized to provide enough heat with a temperature in the range of 49 to 65 C (120 to 150 F).

On the Hôpital Saint-Sacrement (HSS) site, the chilled water network is now used as a warm temperature heating network during winter, using a switch-over control sequence and warm water supplied by the heat pumps.

One of the most interesting design intricacies was to connect as many loads as possible in series, from the hottest water temperature requirement to the lowest. This enabled a greater temperature differential. Having the return temperature as low as possible allowed the design team to take full advantage of new heat recovery pumps to supply the major part of the heating load during shoulder seasons, and still a fair part during the cold, winter months.

Steam requirements for other needs, such as humidification, sterilization, food services, and laundry, were addressed by a smaller steam network or independent equipment.

Figure 1: This diagram helps to illustrate cascade heat pump system configuration.
Figure 1: This diagram helps to illustrate cascade heat pump system configuration.

Heat recovery and geothermal heating

On all four sites, heat pumps were installed to maximize heat recovery and take advantage of Hydro-Québec’s clean electricity. In many areas, chilled water loops were unified to maximize heat recovery potential. Heat recovery coils were also installed in some ventilation exhausts and boiler chimney stacks. Existing direct contact heat exchangers on chimney stacks were optimized by connecting them to the chilled water loop rather than the heating network return. All these improvements served to maximize the recovery loads for the heat pumps and ultimately increase their output.

On all the sites excluding HSFA, dedicated geothermal heat pumps were also installed and properly sized for the geothermal underground exchanger. The design team opted for a horizontal underground heat exchanger for one site, while the other two sites saw vertical boreholes drilled 183 m (600 ft) deep in their parking lots. With all three sites combined, this underground network adds up to around 53 km (33 mi) of piping. The geothermal system was not employed at the HSFA site due to limited space for drilling boreholes.

In some buildings, the design team opted to connect two heat pumps in a cascade system configuration. The dedicated geothermal heat pumps’ condensers were connected on the evaporator side of the building’s main heat recovery chiller (Figure 1). This configuration made it possible to run the geothermal heat pump at a low discharge temperature on the condenser side (in the 7 to 15.5 C [45 to 60 F] range), improving its co-efficient of performance (COP), while providing an additional warm temperature heat source recovered by the main heat recovery chillers.

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