Over the last several years, UHN’s Bickle Centre has slashed it’s direct greenhouse gas (GHG) emissions nearly in half! Many projects, including major equipment replacements have contributed to this success story at the Bickle Centre, which provides complex continuing care to rehab patients. The central chiller plant was replaced with an efficient variable flow system, the central boiler plant was replaced with a more efficient system, the entire building envelope was reconstructed, and most recently nine air handling units (AHUs) have been replaced. The Energy and Environment team worked closely with the FM-PRO project management team on all of these projects. This blog post will focus on the AHU project and our efforts to ensure effective heat recovery to reduce GHG emissions. In addition to heat recovery, we also included standard energy savings measures such as VFDs on fans (see example of VFD project at TGH) and energy valves on chilled water coils (see example of energy valve project at TGH).
What is Heat Recovery?
Heat recovery aims to make use of energy that would otherwise be wasted. In healthcare, Heating, Ventilation, and Air Conditioning (HVAC) standards from CSA dictate the amount of air changes required for each type of space. In order to achieve a given number of air changes, we must exhaust air from indoor spaces and bring in fresh air from outside. In winter, this means we are exhausting warm air and bringing in cold air that needs to be heated. In summer, we are exhausting cool air and bringing in warm moist air that needs to be cooled and dehumidified.
The most effective type of heat recovery is mixing some of the air from the building with the incoming air, called “mixed air systems.” Most of the AHUs being replaced at Bickle were already mixed air systems, so we did not target those for heat recovery. Here is an example of a mixed air system at Bickle, where some air is exhausted and some is mixed with the fresh outside air.
Another common type of heat recovery system for AHUs is called an enthalpy wheel. An enthalpy wheel is made up of a porous material with some thermal mass that slowly rotates between the exhaust and supply air streams. The wheel holds on to some heat from the exhaust air and transfers it to the supply air. In order to install an enthalpy wheel, the exhaust duct and supply duct must be stacked on top of each other. Here is an example of an AHU with an enthaply wheel at Toronto General:
Many older systems that supply 100% fresh air have been designed with a type of heat recovery called a “run around loop,” which places a coil in the supply stream and exhaust stream and pumps liquid in between to transfer energy. These systems aren’t typically installed anymore because they cause an increase in electricity consumption (running pump, increase static pressure on the supply and exhaust fans) and don’t transfer heat very effectively. The added cost of electricity makes the installation/operation of run around loops uneconomic. Two of the AHUs at Bickle, S7 and S8, were originally designed with run around loops that had been decommissioned. Here is an example of a system with a run around loop at TRI University Centre:
UHN Energy and Environment had been investigating reinstating the heat recovery run around loops at Bickle previously but we kept coming across the same problem that the cost to install/operate was high compared to the small heat recovery benefit. With the AHU replacement project happening, we put on our thinking caps to try to devise a way to make better use of this energy:
Unfortunately, due to the layout of the existing mechanical rooms, it was not possible to stack the exhaust and supply air ducts. Therefore, we were unable to design the new systems with enthalpy wheels. We were also unable to convert the AHUs to mixed air because the rest of the air distribution for S7 and S8 is designed for 100% outside air.
Heat Pump Boost
The main reason that run around loops don’t work very well is because the exhaust air typically isn’t vastly warmer than the outside air, especially after efficiency losses in the coils and pipe work. For example, exhaust air of 24C could maybe produce a glycol temperature of 10-15C in the run around loop (typical heating water temperature is 70-80C). On a cold winter day of outside temperature -10C, the glycol is only 20-25C warmer than the air. Compared to a regular heating coil, the run around loop just isn’t doing that much.
Working with the consultant on the project, Quasar Consulting Group, we came up with a plan to use heat pumps to boost the temperatures between the exhaust and supply air streams. In winter, the heat pump would pull heat from the exhaust stream, boost the temperature using the refrigeration cycle, and reject the heat to the supply stream. Using the heat pump, we can supply water to the air stream at 40C providing a much greater heat transfer potential. After a lot of design work and analysis the project team agreed to move forward with the plan.
I could try to explain how the system works, but sometimes it’s easier to just use a diagram :
As you can see, in the exhaust air stream (top of graphic) the temperature is dropping from 24.6C to 17.8C before leaving the building. The heat pump (red block labeled “Heat Pump 2”) moves the heat over to the supply air stream (bottom of graphic) and lifts the temperature using the refrigeration cycle. At the time of this screenshot, the outside air is heated from -4C to 20C, with almost no heat being provided by the boiler plant (all the heating valves are at 1% or less)! The final supply temp is up to 25.6C after heat is picked up from the fan motor and humidification. So when the outside air is -4 degrees Celsius, this system is able to provide almost 100% of the heating load without fossil fuels. The heat pump does consume extra electricity, but the electric grid in Ontario has a low emissions factor and I will calculate the impact of that below.
If you refer back to the previous diagram showing the run around loop at TRI University Centre, you can see that the glycol in the heat recovery loop is only 10 degrees Celsius despite similar exhaust air conditions to Bickle Centre (about 25C). This 10C glycol can still transfer some heat to the incoming air on a cold day, but you can see the valve on the heating coil is open about 50% to pick up the heating load. This screen shot was on a warmer day than the Bickle example. So, on a day with above freezing temperatures, the run around loop heat recovery system can’t come close to meeting the heating demand, but on a day below freezing the heat pump heat recovery system is able to provide 100% of the load.
As an added bonus, the heat pump system can operate in the reverse direction in the summer, taking load off of the central chilled water plant and providing a small reduction in electricity and water consumption in the cooling tower. This feature adds extra resiliency to the site because we are now able to provide cooling to patient rooms even if the chilled water plant is down.
This little box is one of the heat pumps installed at Bickle Centre
These heat pump units were just commissioned towards the end of winter, so we don’t have much data on winter operations other than a few screen shots from the building automation systems. We did this project on a relatively small site in order to see how well the idea works and to help us decide whether this approach can be implemented on a larger scale at UHN to cut GHG emissions. We will be monitoring performance continuously through BAS trend logging, but for now estimated savings based on system design, manufacturer performance data, and initial observations will have to suffice.
Overall, we are expecting the following savings:
- Reduced Gas consumption: 62,583 m3
- Increased Electricity consumption: 79,541 kWh (heat pump and related pumps consume electricity)
- Reduced Water consumption: 196 m3 (reduced load on chiller plant means less water consumed in the cooling tower)
These savings yield utility cost savings of approximately $8,000 per year.
GHG Emissions Reductions
UHN recognizes that climate change is a top public health threat and we are looking for new and innovative ways to reduce our GHG emissions. Accounting for combustion of natural gas only, this project is estimated to eliminate 117 tons of CO2 emissions at the Bickle Centre. If you also account for emissions during production and transport of natural gas, the total reduction becomes 283 tons CO2 equivalent. Both numbers include an increase of 2.6 tons of emissions due to increased electricity consumption. Put another way, this project is similar to taking 61 typical gas cars off the road. Overall, through this and many other projects, emissions from natural gas combustion at the Bickle Centre have decreased 40% since 2010! Here are the 61 polluting cars off the road:
Utility cost savings aren’t the only consideration when it comes to GHG reductions. GHG emissions impart a cost on society, known as the Social Cost of Carbon. This cost is usually borne unequally, with the most vulnerable people typically being the hardest hit. Canada is not immune many of these impacts. If we assume a social cost of carbon of $170 per ton, this project is saving the general public an additional $56,000 per year. The $170 per ton value is on the higher end of current estimates, but the Federal Government has indicated that the carbon tax will eventually rise to this level so I think it is fair to use.
Heat pumps are a key technology that UHN is leveraging to reduce GHG emissions, including the massive Wastewater Energy Transfer project at Toronto Western Hospital that was announced two weeks ago. I’ve also used heat pump technology to eliminate fossil fuels from my house! We expect that heat pumps will continue to form a major part of our decarbonization plan.