Using Heat Recovery To Improve Energy Efficiency In Mission Critical Facilities
What to do with all this waste heat?
Hospitals often need significant amounts of energy with an elevated minimum load due to the critical functions they support. The work of cooling and heating air and water creates waste heat that usually gets rejected back to the atmosphere. This rejection sends useful energy right out the door with no benefit to the owner who paid for it. Are there ways this heat can be reused? How effective are the solutions? In this article, we will review the options owners and engineers can use to capture waste heat from mechanical and electrical systems, review how climate zones affect the viability of the options, and understand which option fits the project.
Available energy resulting from heating or cooling a building comes from many sources. This article will focus on three types: heat recovery from airstreams, water to water heat exchangers, and combined heat and power (CHP) cogeneration. Each of these methods has varying levels of energy production and savings with cogeneration providing the largest and most direct capture of waste heat for energy savings and campus distribution. In the local California market, heat recovery from airstreams does not typically provide return on investment that is short enough, so it’s not often seen, although some 100% outside air systems with high levels of exhaust should still be considered for a financial evaluation.
AIRSTREAM HEAT RECOVERY
Return or exhaust air heated from building occupants, lights, and heat generating equipment or sent through process applications such as laboratory fume hoods, are examples of energy that can be captured through an air-to-air heat recovery wheel for example. This heat transfer can be used for preheating outside air to reduce the need for a central heating hot water or steam boiler to operate. This reduces the usage of fossil fuels that power the boiler, saving the building owner energy costs. When used in 100% outside air systems with high volumes of exhaust air being rejected from the building at elevated temperatures, this type of heat recovery can provide significant levels of heating energy savings during mild or winter ambient air conditions.
WATER TO WATER HEAT EXCHANGERS
Water to water energy savings are available within hospitals via the condensate return system. Return water from condensed steam that is distributed throughout a health care campus to provide heating hot water, domestic hot water, and sterilization functions, have a significant amount of energy remaining. Heat recovery applications route the condensate return through water to water heat exchangers, preheating the domestic hot water return and heating hot water return to minimize the work needed by the steam or hot water boilers, lowering the energy cost of heating the building water. Acute care health care settings are best suited for this heat recovery application, as the water volume used through the building needs to be high enough to justify the added equipment and piping required during the initial construction. High water consuming functions include kitchens, laundry, and sterile processing.
COMBINED HEAT AND POWER
More facilities, including hospitals, are choosing to take full advantage of waste heat on their campus by electing to install CHP cogeneration equipment in their central utility plants. The most popular type of cogeneration are gas turbine plants that utilize natural gas fuel input to power a turbine that creates electricity and captures the byproduct heat through heat exchangers to create hot water for the same uses, as defined above, that are distributed throughout the campus. The combination of power creation and heating functions elevates the efficiency of both needs. Due to the high cost of electrical energy in California, these types of installations are becoming more and more prevalent for institutional facilities where owners are looking to save energy costs over a longer-term period.
CLIMATE ZONE CONSIDERATIONS
The dependence of climate zones across the U.S. for the three heat recovery options presented decreases as the interface with the ambient outside condition lowers. Air to air heat recovery is highly dependent on the ambient climate, due to the direct interface with outside air, and therefore more mild climates such as coastal areas near the Pacific Ocean get less benefit. Water to water heat recovery is slightly dependent on the ambient condition due to mild conditions not requiring as much building heating and therefore processing less water volume during a set period. Cogeneration has a low dependence on the ambient climate due to the ability to modulate heating production and typically being driven in capacity by the electrical energy consumption of a facility.
Now that we have covered what the typical options for heat recovery are, how does one determine which is the best fit for a project?
First, start with understanding details within the project buildings and where the largest energy usage will be. Ask some of the following questions, are the air handlers designed around a 100% outside air configuration? Does the building contain a high level of fume hood exhaust? Does the building contain high domestic hot water load spaces such as a kitchen or a sterile processing department with washers and sterilizers?
After you have determined which approach is the best fit for the internal building elements, it’s time to look at the municipal energy providers landscape at your project site. Is the cost of electrical energy comparatively high? How much less cost is natural gas than electricity? Finally, pay attention to incentive programs local to the project. Does the city, county, state provide incentive programs (California Incentive Program) for cogeneration? Does your project qualify for energy buyback programs?
The above detailed process to narrow down the most viable heat recovery solution for capturing waste heat typically takes the design process through the preliminary stages. Before heading into more detailed design, the next measure is evaluating the cost model for the proposed system. This is an important step because heat recovery is not yet required by local and state codes and therefore it needs to be financially viable to a facility owner. Engineers must develop a life cycle cost analysis to understand at what point the amount of energy saved equates to offsetting the initial increase in construction cost for the heat recovery solution. This crossing point in time is graded differently by each type of owner. Institutional owners are willing to wait longer to see a positive return (approximately five to eight years) while developers looking to sell a facility in a short-term hold scenario want a speedy return on their investment (approximately two to three years).
As the state-wide push towards net zero building mandates continue to pick-up speed, health care facilities are expected to lose more of their exemptions from energy code standards in the coming code cycles. Facing rising utility costs, health care facilities need to come up with innovative and progressive solutions to energy savings. Recovery waste heat provides a sustainable and energy conscience solution to these problems, while benefiting the facility owner for decades to come. There are technological and maintenance challenges to implementing these systems, but as more are seen in the market, qualified personnel to operate the systems will increase as well. We are on the cusp of an exciting time where engineers will have the opportunity to push the limits of energy efficiency in health care facilities. However, engineers need the right partnerships with health care providers and manufacturers of heat recovery equipment to make this happen. Together we can make a substantial difference and bring the goal of carbon neutrality closer to reality.