harnessing-waste-heat

Harnessing waste heat

Excess waste heat can be used to dramatically lower a building’s energy costs.
Tuesday, May 3, 2016
by Brad White

According to the law of conservation of energy, “energy cannot be created or destroyed”. It can, however, be wasted. In buildings, useful energy is frequently lost because there is no good way to move energy from where it isn’t wanted to where it is needed. This waste heat can be observed whenever there is flue gas coming from a chimney at the same time that water is evaporating from a cooling tower. This is a very common situation that happens every day across different facility types.

Nearly every building has too much heat at some point. In standard buildings like offices, this heat can come from many sources – solar gain, IT server equipment, even people. Specialized facilities like recreation centres may have additional sources of heat such as ice plants and dehumidification systems. By creating connections between heating and cooling systems using a variety of approaches such as heat recovery chillers, desuperheaters and heat exchangers, this excess waste heat can be put to good use and can dramatically lower a building’s energy costs and CO2 emissions. The best part is that these savings can be achieved with very attractive payback periods, even in existing facilities where a significant investment to updating and redesigning systems is required. With all of this potential heat available and proven technologies for capturing it, why then don’t most buildings use this approach for their mechanical systems?

One reason is the common perception that heating and cooling generally occur at different times and there would be minimal use for the excess heat. While this certainly can be true in some cases, in many facilities the overlap is significant. Even in cold climates, there can be a requirement for mechanical cooling during winter in areas that have a lot of IT equipment or are exposed to a lot of solar gain. Conversely, in warmer weather, when there may be no requirement for space heating, other loads such as domestic hot water can be served by recovered heat. The first step to determining if the facility is a good candidate for recovering and reusing heat is to analyze the heating and cooling loads and determine exactly how much overlap there is and whether the potential savings create an attractive business case.

From a technical perspective, the most significant hurdle to a connected systems approach is being able to operate heating systems at a low temperature. This greatly increases the flexibility with regards to the heat sources that can be used, which is essential for being able to get the most out of the waste heat. While low temperature heating systems are common in new buildings, they are rare in older buildings. However, it is possible to operate many high temperature systems significantly below their design setpoints much of the time without compromising occupant comfort. This can be achieved through a combination of better control strategies and other changes such as reconfiguring piping or adding variable speed drives to pumps. Connecting systems also create additional complexity that can make the overall system more challenging to operate. Operating the system effectively requires a complex control system able to analyze and respond accordingly to the building’s conditions. Poorly designed or commissioned controls can make or break the entire project, so optimization and testing of these systems is paramount.

While there are certainly challenges to making these projects work, more and more facilities are realizing the benefits. One example is the recent installation of a heat recovery chiller in Vancity Credit Union’s Vancouver headquarters. Originally designed with separate cooling and high temperature heating systems, the new chiller recovered heat from a data centre that occupies part of one floor. This provided enough heat to heat the entire 12 storey building much of the year. Reduced cooling tower and pump usage partially offset the extra electricity required to operate the chiller and reduced water consumption by 20 per cent. This has reduced Vancity headquarters’ natural gas consumption by 5,000 GJ and GHG emissions by 75 per cent, all with a simple payback of six years. Despite these changes in operation, upgrades went unnoticed by occupants and had no impact on comfort.

The Grand Villa Casino in Burnaby also realized significant benefits from capturing waste heat from their chiller to reduce gas consumption from their heating and domestic hot water systems. Already a facility with significant interconnections for energy sharing, a recent cooling tower upgrade provided the opportunity to do even more. This additional energy sharing, along with a bundle of related control system improvements are expected to annually save 3,700 GJ of natural gas, 1,000,000 kWh of electricity, and 200 tonnes of CO2 with a simple payback of under four years.

Beyond these projects, SES Consulting is currently using this innovative approach to reduce energy use across a diverse array of facilities, including a shopping mall, large regional hospital, and a local municipal recreation centre. In the case of the recreation centre, it is expected that following upgrades, the 40 year old facility will use less energy than brand new rec centres.

By redesigning mechanical systems to harness existing energy within a building, it is possible to make great strides in reducing energy consumption and GHG emissions while providing an attractive return on investment to owners. With new developments in technology and greater awareness of the potential for heat recovery and sharing, we have seen just the beginning of what is possible.

 

Brad White, P.Eng is a principal at SES Consulting. SES, a Certified B Corporation, is a Vancouver based firm that helps building owners reduce energy use and GHG emissions through their expertise in mechanical systems, building automation, and occupant engagement.

 

 

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