Hospitals are some of the most energy-intensive facilities in the U.S. A 200,000 square foot hospital will spend, on average, $680,000 a year on energy expenses. According to the American Hospital Association, there are 5,627 registered hospitals in the U.S. While a majority have made energy efficient upgrades to individual buildings, a significant number of opportunities still exist at most hospital campuses. One of the main energy efficiency opportunities in hospitals is related to reheat, which can account for 42 percent of a hospital’s energy consumption. The following will look at hospital reheat systems and what simple and advanced energy efficiency improvements can be made to reduce wasted energy.
Hospital Reheat
Reheat is the process by which the internal air temperature is heated to maintain comfort conditions throughout the facility. For health and safety purposes, many areas within hospitals are “required to maintain space relative humidity (RH) values between 30% and 60%.” Common use of variable air volume (VAV) with reheat systems for simultaneous heating and cooling helps control both the temperature and the RH in the space. This process, however, can waste a lot of energy. Hospitals contain a diverse set of spaces, including reception areas, patient wings, laboratories, intensive care, operating rooms, kitchens, housekeeping, and more. Each area of a hospital will require different temperature settings and RH levels. An operating room, for example, would require different temperature and RH levels than a patient wing.
Common practice of RH level control in the space consists of overcooling the supply air below its dew point temperature and then reheating that air at the desired setpoints. The air-handling unit (AHU) is signaled to send air at the required discharged air temperature to the VAV boxes. The multiple VAV boxes positioned throughout the hospital will reheat the air based on the space temperature setpoints. This constant work between the AHU, chiller, and VAV units consumes and wastes a significant amount of energy. According to a University of Washington Integrated Design Lab study, in conjunction with Solarc A+E at the Legacy Salmon Creek Hospital in Vancouver, reheat can account for 42 percent of a hospital’s energy consumption.
Image Source: Facilities.net
Minimizing Hospital Reheat
An energy management system will control the heating, cooling, and ventilation processes to satisfy different temperature and RH standards throughout the hospital. A variety of measures to minimize hospital reheat are available, such as reconfiguring or optimizing different mechanical equipment and processes. Solving inefficiencies may seem complex, but more often than not, simple improvements can significantly reduce energy waste. Let’s examine some ways to minimize hospital reheat.
Desiccant Systems
A desiccant is a hygroscopic substance: a substance that absorbs moisture from the air. Desiccant systems help improve hospital energy efficiency by extracting the moisture from the air and heating it before it reaches the AHU. Air with lower dew point levels require less energy to heat and cool. Desiccant systems can save between 5-10% of the energy required to bring in fresh, outside air.
Here’s how desiccant systems work in hospitals. Fresh, outside air is brought in and enters the desiccant wheel, which is broken into two or more separate sections. The air is dried as it passes through the desiccant and then can be heated by evaporator coils or heat exchangers. When the section of the wheel that absorbs moisture has filled its capacity, the wheel rotates into a separate stream of heated air to induce evaporation, allowing it to release the absorbed water vapor. Meanwhile, a separate section will dry new air. Once all processes are complete, the desiccant wheel rotates back into its original position and repeats.
This process helps avoid overcooling the air, thereby reducing the amount of energy required to reheat the air to the temperature setpoints.
Image Source: Solair
Desiccant systems are ideal for hospitals because they are more efficient when the facility requires simultaneous heating and cooling. As previously mentioned, the size and complexity of a hospital requires different temperature setpoints and RH levels throughout its various rooms, requiring simultaneous heating and cooling. During all energy-consuming processes, there will always be excess heat or cold produced. Rather than waste the excess heat and cold, it can be rerouted and used for different functions. Where applicable, a heat recovery system can be tied into the heating/cooling air supply to reduce the heating/cooling load on the equipment.
In short, energy that was once wasted can now be recaptured from other essential high energy-consuming processes rather than using more energy to fuel the dehumidification process.
Chiller Energy Efficiency
A chiller can account for up to 50 percent of the HVAC system’s electricity costs. A chiller’s purpose is to remove heat from the occupied space. Converting a chiller with a variable speed drive (VSD) can improve the efficiency of the unit by reducing the speed of the motor based on load requirement demands through temperature setpoints. This process can reduce costs by up to 30 percent and provide an attractive return on investment.
HVAC & Indoor Air Quality
ASHRAE Standard 62.1 & 62.2 and ASHRAE Standard 170 are the recognized industry standards for healthcare indoor air quality (IAQ) and ventilation, respectively. For health and safety purposes, all air within a healthcare facility must be fresh, filtered outside air; the existing air cannot be recirculated. Similar to the reheat process, different spaces within the facility will call for different air exchanges per hour (ACH), making air recycling a continuous process that will, in effect, put a continuous load on the HVAC system. A poorly configured HVAC system could be bringing in more ACH than necessary. HVAC systems should bring only the amount of fresh air needed based on the occupancy of the space at any given time. An HVAC retrocommission can identify and solve this and other inefficiencies.
HVAC Retrocommissioning
Retrocommissioning is the process by which an aging mechanical system is analyzed for any inefficiencies and ways of improvement are provided. Once a retrocommission report is provided, a hospital may take action by remediating the deficiencies suggested. In a previous article, we shed some insight into aging and inefficient HVAC systems. HVAC systems have moving parts and are susceptible to wear and tear. According to a 2009 Lawrence Berkeley National Laboratory study, the top building energy inefficiency issue is duct leakage, and it can be very difficult to ascertain if a zone in a hospital has a duct leak without undergoing commissioning. The more energy lost through these deficiencies will prompt the HVAC system to call for more unnecessary heating and cooling cycles. This wasted energy could increase HVAC operating expenses between 20-40%. It is also worth noting that dirty HVAC ducts can be problematic for maintaining proper hospital IAQ.
Continuous Commissioning
Building commissioning is a process by which existing equipment is checked and verified to ensure its systems, such as heating/cooling, electricity, running water, etc. are working efficiently and as originally intended. Equipment efficiency tends to be sacrificed to meet the required air quality standards by building operators due to the nature of the facility. Continuous commissioning of this equipment through an energy management system (EMS) upgrade and review, combined with rigorous preventative maintenance and operating parameter analysis, can optimize the efficiencies of the equipment. Continuous commissioning is important for hospitals due to 24/7 operations and can identify and solve existing problems or future issues that may arise.
Another important consideration is that areas within a hospital may expand or change functions. When a space that was one used for one purpose changes to another, that can adversely affect the heating and cooling and IAQ requirements for that space. Continuous commissioning is necessary in order to identify problem areas and make adjustments.
Bottom Line
A combination of both simple and complex energy efficiency measures can be designed and implemented to optimize hospital reheat and reduce energy costs. Conducting a comprehensive facility audit can help identify opportunities to optimize heating, cooling, ventilation, and indoor air quality. Conducting regular site audits and performing continuous commissioning will enable better facility energy management and control, helping identify existing issues to reduce energy expenses, or discover problems for preventative maintenance.