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Saving Energy with Low-Grade Heat
Today we are seeing more building owners willing to pursue options and strategies based on energy performance and environmental responsibility. But the way we heat our buildings remains an area rich with improvement opportunities.
Too frequently, we speak with well-intentioned owners who are looking for efficiency upgrades in their central boiler plants, but are requiring these plants to continue to produce high-temperature heating water or steam*. This real or perceived need for such high-grade heat limits the opportunities available for a central plant since higher-efficiency equipment like condensing boilers and heat pumps cannot effectively produce such high temperatures. The truth is that hydronic (water)-based space heating can almost always be accomplished with supply water that’s below 60°C (140°F) if coils and heat exchangers are selected for these lower temperatures. Given this fact, owners should include retrofits of heating coils and small process loads within their buildings to accommodate lower temperature water prior to, or coincident with replacement of their boiler systems. Often this adds relatively little to the cost of an energy project, especially if the system moves away from steam*.
Supply Water and Energy Efficiency
An example of the inverse relationship between efficiency and supply temperature is seen in condensing boilers. Using natural gas, these boilers can operate with annualized efficiencies well above 92%, but to do this they require relatively cool discharge and return temperatures. Producing 90°C (185°F) supply water from the same boiler typically limits its peak efficiency to about 85%, which is no better than less-expensive, non-condensing equipment.
Further support of this point is seen with water-to-water heat pumps. There are a number of variables at play, but the amount of heat this equipment can supply per unit of electrical energy, the Coefficient of Performance (COP), is reduced significantly as the supply water temperature increases. For example, a specific water-to-water heat pump can produce 60°C (140°F) heating water with a COP of 3.5. The same unit with all other factors equal will have a COP of only 2.2 when supplying 80°C (175°F) water (pretty much the upper temperature limit of this type of equipment). While this is still good when compared to a fossil-fuel fired boiler (COP of 0.92 or so), it’s clear that there’s a big efficiency hit – more than a 35% - to supply hotter water.
More Opportunity through Lower Heating Supply Temperatures
Once we realize we can live with lower heating supply temperatures, we open the door to a world of opportunities to gather and use heat from a variety of sources such as data centers, building exhaust streams, and the surrounding air, land, and water. These sources may be thought of as low-grade heat when compared to the steam and high-temperature water mentioned above.
Application of water-to-water heat pump systems including heat-recovery chillers is perhaps the most effective way to capitalize on the use of low-grade heat. They provide great flexibility in that they can effectively move heat around a building to where it is needed. Additionally, they are able to extract usable heat from nearby thermal sinks such as the air, the ground, or surface water. A 10°C (50°F) pond, for example, might sound cold, but it can serve as a great heat source when combined with water-to-water heat pumps in a ground-source heat pump system. Since they provide the added capability of producing chilled water for cooling, water-to-water heat pumps can help reduce overall HVAC system installation costs.
There are, of course, a great many things that must be considered to design a truly efficient heating system. But by taking a gentler approach and searching for sources and applications of low-grade heat, significant energy and emissions reductions can be realized.
Below are some simple guidelines for selecting and designing a heating system.
- Size hydronic heating loops for supply temperatures no greater than 60°C (140°F).
- Utilize heat pump technology (including heat recovery chillers) where possible.
- Look for internal sources of waste heat that may be reclaimed within the building.
- Evaluate external sources of low-grade heat that may be used in conjunction with heat pumps (the air, the earth, surface water, wastewater, etc.).
- When fossil-fuel-fired boilers and furnaces are absolutely necessary, utilize condensing equipment.
*Steam systems can be especially problematic as they are often poorly maintained. This leads to a relatively low percentage of condensate returning to the boiler plant resulting in not only higher energy use, but very high costs for chemical treatment of make-up water.
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