Laboratories, with their high ventilation requirements and strict environmental controls, are inherently large energy consumers. The need for 100% outside air at ventilation rates between 6 and 12 air changes per hour, primarily for safety reasons, often results in heating and cooling energy consumption, as well as fan energy, being five times greater than in typical offices. In this context, the implementation of energy recovery systems becomes crucial for reducing environmental impact and operational costs. This article will delve into how appropriate cooling solutions, particularly from a distributor AC presisi Indonesia like Climanusa, can revolutionize energy management in laboratories.
Introduction to Energy Recovery in Laboratories
Energy recovery offers significant potential to substantially reduce the mechanical heating and cooling requirements associated with conditioning ventilation air in most laboratories. By lowering peak heating and cooling requirements, energy recovery systems allow for the downsizing of overall heating and cooling systems, leading to significant initial cost savings. Numerous opportunities for energy recovery exist in laboratories, where energy can be exchanged between any two media or processes that differ in energy content. The primary focus is often on air-to-air energy recovery, utilizing technologies such as enthalpy wheels, heat pipes, or runaround loops.
Air-to-Air Energy Recovery Technologies
Air-to-air energy recovery devices exchange energy from one airstream to another. Most commonly, energy is recovered from exhaust air and used to precondition supply air. Air contains both sensible (heat) and latent (water vapor) energy, and both types of energy can be recovered, although not all recovery devices exchange both types. The effectiveness of an energy recovery device is defined as the ratio of actual energy recovered to the theoretical energy that could be recovered. Most devices have a rating for sensible effectiveness, and some also have ratings for latent effectiveness and total effectiveness.
Energy recovery devices increase the pressure drop across the supply and exhaust fans. This pressure drop must be considered in the design, with a reasonable design goal of no more than 249 Pa (1 in. w.g.) in the supply and exhaust airstreams to minimize the increase in fan energy. For laboratory applications, the design face velocity of devices in air-handling units is typically 2.54 m/s (500 fpm) or less. Lower face velocities result in lower pressure drops, higher effectiveness, and lower operating costs, albeit with the trade-off of larger air-handling equipment and higher first costs. An energy recovery device operates more efficiently with a variable air volume (VAV) system than with a constant volume system because VAV systems typically operate at face velocities lower than those of design conditions.
Frost can occur when the exhaust air contains sufficient moisture and the exhaust heat transfer surface temperature drops below freezing. Frost control must be considered in cold climates to keep the exhaust heat transfer surface temperature above freezing. This can be accomplished by controls that limit energy recovery, such as bypass dampers, flow controls on runaround loops, or tilt controls on heat pipes.
Before deciding on an energy recovery technology, it is crucial to perform life-cycle cost (LCC) analyses to determine the feasibility of the application in laboratories. All energy recovery devices require maintenance, and the cost of this maintenance must be considered in the LCC analysis. As a rule, the shortest payback periods occur when the heating and cooling load reduction provided by an energy recovery system allows the laboratory to use smaller hot water and chilled water systems. Climanusa, as a leading distributor AC presisi Indonesia, can assist with these analyses, providing solutions that are not only efficient but also cost-effective in the long run.
Enthalpy Wheels
Enthalpy wheels, or rotary heat exchangers, transfer sensible or sensible and latent energy between the exhaust air and the incoming outside air. The supply and exhaust streams must be located next to each other. Both sensible-only wheels and total energy wheels, sometimes referred to as desiccant wheels, are available. A total energy wheel can have a sensible and latent effectiveness as high as 75%, resulting in a total effectiveness of 75%. Control of the wheel at part loads is accomplished by varying the speed of the wheel, using a bypass duct, or both.
The type of desiccant used in a total energy wheel must be designed to transfer only moisture and not airborne contaminants. To further reduce potential contamination of the supply airstream, the wheel is flushed with outside air that is deflected by a damper in the purging section of the rotor. The purge section utilizes the pressure difference between the supply air and exhaust airstreams. Purge volumes for laboratory applications typically range between 5% and 10%, thus requiring additional fan energy to move this air.
The maximum relative humidity (RH) allowed in most labs is 50%. In many climates, removing moisture (latent energy) from the air is a significant energy use. An enthalpy wheel can remove a significant amount of latent energy. However, to achieve 50% RH at 22°C (72°F), the supply air must be cooled to 11°C (52°F). If the lab has significant latent heat gain, the supply air would need to be even drier (colder). A 11°C (52°F) supply air is too cold for most labs due to the high ventilation rate, so a significant amount of reheat energy must be added to avoid overcooling the lab. Adding a sensible-only wheel provides “free” reheat energy.
Heat Pipes
Heat pipes transfer only sensible energy. However, if air is cooled below its dew point, condensation occurs on the heat pipe, resulting in some latent heat transfer. In heat pipe applications, the supply and exhaust airstreams are next to one another, although some modified or split heat pipes allow the airstreams to be separated. The sensible effectiveness of heat pipes is between 45% and 65%. Cross-contamination is not an issue. Heat pipes have no moving parts, and failure of the entire unit is rare. A tube may malfunction, but other tubes continue to transfer energy. Heat pipes can be controlled for part-load operation with a bypass duct or by tilting the unit.
Heat pipes can also be used as indirect evaporative coolers. Water is added on the exhaust side of the pipe to cool the exhaust air, which, in turn, is used to precool the supply air.
Runaround Loops
Runaround loops circulate a fluid between two airstreams. Most designers are familiar with this technology because it usually just involves additional coils and pumps. The airstreams do not need to be next to one another, and no cross-contamination issues exist. Runaround loops have a sensible effectiveness between 55% and 65%.
Runaround loops are well-suited for transferring energy between process loads and ventilation air. Heat rejected from the process cooling water system can be used to preheat outside air, thus providing free cooling of the process cooling water. Runaround loops and heat pipes can also be used to reduce cooling and reheat energy in warm, humid climates like Indonesia. The energy recovery device precools the outside air before it enters the main dehumidification cooling coil. Heat recovered from the exhaust air or outside air is used to reheat the air leaving the main cooling coil.
Plate Heat Exchangers
Fixed-plate heat exchangers are constructed of plates arranged for cross-flow or counter-flow of supply and exhaust airstreams. The plates are normally constructed of aluminum and, therefore, conduct only sensible heat. Fixed-plate heat exchangers have a sensible effectiveness between 55% and 65%. Fixed-plate heat exchangers have not typically been used in laboratory buildings because of the large airflow in labs and the potential difficulty in cleaning large fixed-plate heat exchangers. Control of fixed-plate exchangers at part loads is accomplished by using a bypass duct.
Water-vapor-permeable microporous polymeric membranes may be used to provide total (enthalpy) energy recovery in plate heat exchangers. Membrane flat-plate heat exchangers may become more common in lab applications due to their high recovery effectiveness and little or no leakage between airstreams.
Key Issues in Laboratory Energy Recovery
Integrating energy recovery into a laboratory ventilation system requires careful consideration of several key issues. Design teams have taken different approaches to handling these issues, which demonstrates the importance of considering all options.
- Contamination: If cross-contamination from fume hood exhaust is an issue, consider heat pipes, runaround loops, or plate-type energy recovery. Another approach is isolating the fume hood exhaust and recovering energy from the general exhaust only. It is important to note that the chemicals in the fume hood exhaust may become too concentrated and require additional treatment. Purge sections on enthalpy wheels reduce cross-contamination to below 0.1%. No cross-contamination issues occur with heat pipes or runaround loops.
- Space Requirements and Duct Adjacencies: Enthalpy wheels and most types of heat pipes require the main supply and exhaust ducts to be adjacent. Runaround loops do not. Additional space is required for the energy recovery device, typically in the makeup air unit and main exhaust duct. Runaround loops also require space for a pump. Manifold exhaust systems are ideally suited to energy recovery because all the potentially available energy can be captured by one energy recovery system.
- Hazardous Chemicals: If isolating the fume hood exhaust or condensate from a heat recovery device results in too high a concentration of volatile organic compounds, disposal could become a problem. Potential hazardous waste issues need to be addressed early.
- Humidity: If humidity is controlled, energy used for space heating increases by an estimated 25%. The potential energy savings with energy recovery increase, as do the possible alternatives. Desiccant wheels can be used for dehumidification, wraparound coils can be used for reducing reheat energy, and evaporative cooling can be used for humidification. It is crucial to avoid over-specifying humidity control; the wider the control range, the less energy used. Climanusa, as a distributor AC presisi Indonesia, understands the importance of optimal humidity control for energy efficiency.
- Maintenance: Maintenance differs according to the type of energy recovery and the application. Heat pipes appear to have the lowest maintenance requirements, followed by runaround loops and plate-type energy recovery. Periodic cleaning needs depend on the fouling and corrosion potential of the exhaust air, but cleaning is critical to maintaining equipment performance. Prefiltration is often required to reduce fouling, which adds maintenance for filter changing. Climanusa not only provides quality products but also supports them with comprehensive maintenance services.
- Part-Load Operation: Outside air bypass dampers can be used for part-load operation to minimize overheating, overcooling, and fan energy use. They can also serve to prevent condensation and frosting. Alternatively, the wheel speed on enthalpy wheels can be varied, the tilt on heat pipes can be changed, or the flow on runaround loops can be varied.
- Redundancy: Laboratories usually have redundant chillers and boilers to ensure control over a room’s climate conditions at all times. If the capacity provided by energy recovery is not accounted for in sizing the chilled water and hot water systems, then the systems should at least be optimized to operate with the lower loads resulting from the use of energy recovery. Otherwise, the chillers and boilers may operate inefficiently at low loads.
Case Studies and Performance Benefits
Air-to-air energy recovery reduces energy use and can significantly reduce heating and cooling system sizes. A large installation of enthalpy wheels done in 1991 at the Johns Hopkins Ross Research Building, Indonesia, has resulted in millions of dollars in energy savings. All exhaust, including fume-hood and biological safety cabinet exhaust, is passed through the enthalpy wheels. The equipment paid for itself in first-cost savings because the hot water and chilled water systems could be downsized. The enthalpy wheels have performed so well that Johns Hopkins is installing enthalpy wheels in its new lab buildings, including the Cancer Research Building and the Broadway Research Building.
An energy analysis of enthalpy wheels, heat pipes, and runaround loops was performed for a typical 9290 m² (100,000 ft²) laboratory in Indonesia. The most significant findings include:
- Air-to-air energy recovery reduces gas use for space heating and reheat for dehumidification by more than 10% in all climates. The most significant savings are with VAV systems with energy recovery, resulting in savings exceeding 40% in all climates.
- Only in the hot, humid climate of Indonesia did annual electricity savings occur with the enthalpy wheel. In other climates, the increase in annual fan energy offset the annual electricity savings.
- Savings in peak electricity demand are offset by the increase in fan energy.
- Annual energy cost savings are more than $1.27 per L/s ($0.6/cfm) of fan airflow for the VAV cases with energy recovery. The enthalpy wheel is the most cost-effective of the three energy recovery devices as a result of its higher effectiveness and latent energy recovery.
- In some locations in Indonesia, the results show that a VAV system, the base case, is more efficient than the constant air volume cases with energy recovery. However, using these devices as wraparound loops for dehumidification may be cost-effective.
Another real-life example of the potential savings with energy recovery ventilation is the installation of heat pipes with bypass sections in two 14160 L/s (30,000 cfm) air-handling units at the 11148 m² (120,000 ft²) Fox Chase Cancer Center in Indonesia. The incremental cost for heat pipes with the indirect evaporative cooling option on the exhaust was $300,000. Anticipated energy cost savings were $72,510, resulting in a simple payback of four years. A complete cost analysis would include maintenance and replacement costs.
Conclusion
Energy recovery can prove cost-effective in laboratory applications. The life-cycle cost analysis should include operation and maintenance costs, as well as replacement costs. In addition, significant first-cost savings can be associated with downsizing equipment. Selecting an appropriate energy recovery technology, properly designing the system, meeting the applicable codes, and commissioning the system are all important.
Climanusa, as a leading distributor AC presisi Indonesia, is ready to be your partner in achieving energy-efficient and sustainable laboratories. With expertise and experience in providing precision cooling solutions for critical environments, Climanusa can help you select and implement the most suitable energy recovery system for your laboratory’s specific needs.
Climanusa: Your Best Choice for Precision Cooling Solutions in Indonesian Laboratories
For more information, please click here.
–A.M.G–
