Is it possible to reduce the footprint of our emissions incorporating a better design?
Just a topic for these modern days, when everything is within our reach.
The following description and scenario are for different type of audiences and represent a general introduction to building heating sources with the scope of increasing overall energy performance and decreasing emissions of greenhouse gases, by changing the source of fuel.
Most of the UHN hospitals and research buildings are heated by steam as the source of energy. Heating represents a great proportion of UHN GHG emissions, even though most of the emissions are not produced locally on sites (direct emissions), but at the main source, Enwave district system (indirect emissions).
A typical scenario shows the heating energy distribution within a buildings’ campus, which applies to UHN sites at hypothetical level. The scenario makes it easy to visualize and understand. First, I will talk about steam heat, and then focus on hot water.
Steam is used a great deal in hospitals: it permits high energy densities and enables large amounts of energy to be moved around easily. Steam can be easily generated from water, a plentiful resource here.
Furthermore, steam installations are relatively easy to regulate. Steam is used for many applications in hospitals, including sterilization (surgical instruments and more), humidification, heating and providing hot tap water.
However, heating with steam is not always the best solution, as a heating system that operates at lower temperature is more suitable for space heating.
SITES DISTRIBUTION ANALYSIS – STEAM
- 10 Buildings
- ~ 1 Mile of Distribution Steam Piping
- 150-200 psig Distribution System and Inside Buildings Main Hubs (pressure needed to transport the steam)
- 250-350 °F Steam Temperature
- 15-30 psig at Building Level (Inside Buildings)
- Steam-to-Hot Water Heat Exchangers at Building Levels (Mechanical Rooms)
- Steam equipment inside the building
SITE DISTRIBUTION ANALYSIS – HOT WATER
Note: The 200F temperature for the supply water above is a high temperature, and not the typical temperature of 140F-160F used in a Low Temperature Hot Water heating system.
- 10 Buildings
- Distribution Hot Water Piping, same network where possible or alternative routes.
- (With provisions for alternative routes due to infrastructure constrains)
- 150°F at furthest building.
To take advantage of this energy-saving potential, engineers and designers must change their thinking on heating-water temperatures. An example is the recent engineering design for several different types of large commercial buildings using a 130°F low-temperature hot-water supply with a 20° to 30°F delta T, as opposed to the traditional 180°F water supply.
Advantages of low temperature district heating systems versus steam are:
- Substantial reductions in heat losses in the distribution system (3-5% for water v. 15-20% for steam). A hot water system eliminates the heat losses from steam traps, valve stems and packing gland leakage, blowdown and flash losses.
- Superior heat storage characteristics. In a hot water system, a large volume of water provides a continuous heat reservoir available to handle peak demands or swings in load.
- Flexibility of piping distribution systems. Since a hot water system uses forced circulation of a liquid heating medium, the piping lines can follow the contours of the ground or the building. Conversely, steam and condensate return lines require a certain degree of pitch for drip legs, traps, etc.
- Closed hot water systems have practically no make-up requirements. The only unavoidable losses are minor leakage at pump glands and valve stems;
Common Reasons to Convert from Steam to Hot-Water Heating
Converting from steam to hot-water heating is not a trivial process. It will require a significant investment in planning, system modification and new equipment, including a boiler. There are three major factors that usually drive the decision to change to hot-water heating:
- Cutting annual operating costs: Hot-water heating can be much cheaper and more efficient than steam heating.
- Uneven heating: Steam heating can be inconsistent and uneven. This results in areas that are either hotter or cooler than expected and desired. Adjustments at the thermostat may not be able to resolve the inconsistencies.
- Safety for building occupants: Steam is a potentially dangerous source of heating. Steam radiators can get extremely hot, often over 200 degrees. Accidental contact with these radiators can cause serious burns, especially in children or the elderly. Hot-water heating is, in general, a safer alternative.
When upgrading a building’s heating system from steam to hot water, there are two possible approaches:
- Adapting the existing installation to use hot water. This is the most cost-effective option if there are no plans to renovate the building in the short term. However, part of the efficiency of a hot water installation is lost when using equipment originally sized and specified for steam.
- Replacing the heating system completely. This option is prohibitively expensive in existing buildings, since it involves opening walls and floors to replace piping and its associated fixtures. However, it is cost-effective when a building is already slated for a major renovation.
Alternative to Conversion: Full System Replacement
Replacing a steam heating system with a brand-new hot water installation is feasible, but the cost can be prohibitive in existing facilities due to the need to open walls and replace piping. However, if a major renovation is already planned, it’s an excellent chance to remake the heating system completely.
- Piping can be sized specifically for hot water, eliminating the need to use valves on oversized lines that were originally calculated for steam. Balancing the supply and return of water is much simpler if piping is adequately sized.
- Radiators can be replaced with more efficient alternatives, such as hydronic floor heating systems or water-source heat pumps.
- Automation can be deployed for the entire hot water system, achieving the lowest possible operating cost.
HOT WATER SYSTEM ADVANTAGES – GENERATION AND INTEGRATION WITH RENEWABLE SOURCES
- Increased System Efficiency
- Solar Thermal Heating Integration
- Geothermal Energy Source Integration
- Cogeneration Integration
- Thermal Storage Integration
- Heat Recovery Chillers as Heating Source or Integration
- Waste Heat Recovery as Heating Source or Integration
- Electrification of the Heating Source
- Phasing by section of building conversion and installation of hot water distribution
- Phasing of district hot water temperatures may facilitate reduction of capex by taking advantage of:
- Retrofit or replacement of buildings and/or building systems
- Construction of new buildings designed for low temperatures
- Careful planning to minimize disruption to campus operations.
So, all these being said, it is just a matter of understanding and looking forward the way we move in our green and friendly environment, are we interested or maybe not, is it possible or maybe not, do we have the resources to make it happen, or this is too difficult? How these decisions are taken, we can discuss in another story. Until next time, be well.