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Cover page of Ceiling fans: Predicting indoor air speeds based on full scale laboratory measurements

Ceiling fans: Predicting indoor air speeds based on full scale laboratory measurements

(2019)

We measured indoor air speeds generated by ceiling fans in 78 full-scale laboratory tests. The factors were the room size, fan diameter, type, speed, direction (up or down), blade height, and mount distance (i.e. blade to ceiling height). We demonstrated the influence of these factors, showing that the most significant are speed, diameter and direction. With other factors fixed, the average room air speed in the occupied zone increases proportionally with fan air speed and diameter. Blowing fans upwards yields lower but far more uniform air speeds than downwards. We show that for the same fan diameter and airflow, fan type has little effect on the air speed distribution in the region outside the fan blades. We developed several new dimensionless representations and demonstrate that they are appropriate for comparisons over a wide range of fan and room characteristics. Dimensionless linear models predict the lowest, average, and highest air speeds in a room with a median (and 90th percentile) absolute error of 0.03 (0.08), 0.05 (0.13), and 0.12 (0.26) m/s respectively over all 56 downwards tests, representing common applications. These models allow designers to quickly and easily estimate the air speeds they can expect for a given fan and room. We include all measured data and analysis code in this paper.

  • 9 supplemental PDFs
  • 2 supplemental images
  • 4 supplemental files
Cover page of Codes and standards report

Codes and standards report

(2018)

The goal of this study was to (1) propose changes to Title 24 to support improved modeling capabilities and help achieve significant energy efficiency goals for radiant systems in California, and (2) propose changes, as needed, to relevant ASHRAE Standards, Handbooks, and Guidelines to provide new information and guidance on radiant systems. The current version of California Building Energy Efficiency Standards, Part 6 of the California Building Standards Code (Title 24) does not address factors specific to high thermal mass radiant systems within the body of the Standards. The alternative compliance method references some limited aspects relating to radiant systems but it is incomplete and not practically applicable, and has not yet been implemented in the associated compliance software. In addition, there are some modeling limitations for radiant systems in EnergyPlus, which is the simulation engine underlying the compliance software for the Title 24 performance approach. Updates to the Title 24 alternative compliance method are needed to ensure that modeled performance accurately reflects proposed designs, and to properly allow buildings with radiant systems to take appropriate credit for their performance. This study provided a background and roadmap of the steps needed to provide effective coverage of radiant systems for Title 24 compliance. Listed below are topics that are recommended to be added to the ASHRAE Standards and Handbooks.

• Provide consistent definitions for different radiant system types in ASHRAE Handbook System and Equipment, Chapter 6 (Radiant Heating and Cooling).

• Provide comfort data in real radiant buildings in ASHRAE Handbook System and Equipment, Chapter 6 (Radiant Heating and Cooling).

• Provide revised cooling load definitions and calculations in ASHRAE Handbook Fundamentals, Chapter 18 (Nonresidential Cooling and Heating Load Calculations) and ASHRAE Handbook Fundamentals, Chapter 19 (Energy Estimating and Modeling Methods).

• Provide revisions to account for effect of night cooling for buildings conditioned by radiant system in ASHRAE Handbook Fundamentals, Chapter 18 (Nonresidential Cooling and Heating Load Calculations), ASHRAE Handbook Fundamentals, Chapter 19 (Energy Estimating and Modeling Methods), ASHRAE Guideline 36-2018 (High-Performance Sequences of Operation for HVAC Systems), and ASHRAE Handbook Systems & Equipment, Chapter 6 (Radiant Heating and Cooling).

• Provide new design guidance to account for the impacts of direct solar radiation on chilled radiant floors in ASHRAE Handbook Applications, Chapter 54 (Radiant Heating and Cooling), and ASHRAE Handbook System and Equipment, Chapter 6 (Radiant Heating and Cooling).

• Provide new design guidance to account for the impacts of acoustical ceiling panels and clouds on cooling capacity of radiant ceiling slabs in ASHRAE Handbook System and Equipment, Chapter 6 (Radiant Heating and Cooling).

• Provide new design guidance to account for the impacts of air movement from ceiling and other fans on cooling capacity for both radiant ceilings and floors in ASHRAE Handbook System and Equipment, Chapter 6 (Radiant Heating and Cooling).

Cover page of Comparison of construction and energy costs for radiant vs. VAV systems in the California Bay Area

Comparison of construction and energy costs for radiant vs. VAV systems in the California Bay Area

(2018)

The goal of this study was to perform a design stage cost analysis comparing a selected radiant building against an identical building with a traditional variable air volume (VAV) system. Major findings from the cost estimates include:

• The radiant HVAC design has a total cost of $38.9/ft2 compared to $29.9/ft2 for the VAV design, representing a $9.0/ftpremium for the radiant design.

• The higher costs for the radiant system can largely be attributed to higher piping labor costs for piping and radiant equipment, which itself is $9.8/ft2 higher than that for the VAV design.

• Since labor rates are higher in the San Francisco Bay Area, for the estimated national average labor rate, the premium for radiant is $6.8/ft2, compared to the VAV system. The high installed cost for the radiant equipment is partly a reflection of the current radiant manufacturers’ pricing strategies and the contractors’ bidding practices. The radiant market is relatively small and immature in the United States, especially compared to the well-established VAV market. Alternative design approaches are discussed that may reduce first costs and/or energy costs. Energy models of the two designs (radiant and VAV) were developed in EnergyPlus to evaluate the corresponding energy and comfort performance. In the VAV system model, the controls are generally based on the recently published ASHRAE Guideline 36 (ASHRAE, 2018), which provides high performance sequences of operation for VAV systems. However, for the hybrid radiant slab and DOAS system, there are no well-established control sequences readily available. The annual simulation results show that the total site HVAC energy use is 16.2% higher for the radiant system (2.9 kBtu/ft2) than the optimized VAV design (2.5 kBtu/ft2). The report contains further discussion of opportunities to improve the energy performance of radiant systems. For example, in mild climates, such as the Bay Area in California, radiant designs should take advantage of the benefits of free cooling as much as possible either with airside or waterside economizers.

Cover page of Quantifying energy losses in hot water reheat systems

Quantifying energy losses in hot water reheat systems

(2018)

We developed a new method to estimate useful versus wasted hot water reheat energy using data obtained from typically installed instrumentation that applies to all pressure independent VAV terminal units with discharge air temperature sensors. We evaluated the method using a year of 1-minute interval data for a 11,000 m2 building with 98 terminal reheat units, and found a 14% upper bound for the uncertainty associated with this method. We found that just 21% of gas energy is converted to useful reheat energy in this building. The distribution losses alone were 44% of the heat output from the boiler. The results raise questions regarding the tradeoffs between hot water heating systems, which have significant distribution losses, and electric heating systems, which effectively have zero distribution losses. In this building, and likely many others, an electric reheat system supplied by a small photovoltaic panel system would have a lower operating energy cost and a lower initial cost than the hot water reheat system. Further investigations using this method will be relevant to designers and standards developers in deciding between electric and hot-water reheat, particularly for modern designs using dual-maximum controls and low minimum airflow setpoints.

Cover page of Side-by-side laboratory comparison of space heat extraction rates and thermal energy use for radiant and all-air systems

Side-by-side laboratory comparison of space heat extraction rates and thermal energy use for radiant and all-air systems

(2018)

Radiant cooling systems extract heat from buildings differently than all-air cooling systems. These differences impact the time and rate at which heat is removed from a space, as well as the total amount of thermal energy that a mechanical system must process each day. In this article we present measurements from a series of multi-day side-by-side comparisons of radiant cooling and all-air cooling in a pair of experimental testbed buildings, with equal heat gains, and maintained at equivalent comfort conditions (operative temperature). The results show that radiant cooling must remove more heat than all-air cooling – 2% more in an experiment with constant internal heat gains, and 7% more with periodic scheduled internal heat gains. Moreover, the peak sensible space heat extraction rate for radiant cooling (heat transfer at the cooled surface, not the cooling plant) must be larger than the peak sensible space heat extraction rate for all-air systems, and it must occur earlier. The daily peak sensible space heat extraction rate for the radiant system was 1–10% larger than for the all air system, and it occurred 1–2 hours earlier. These findings have consequences for the design of radiant systems. In particular, this study confirms that cooling load estimates for all-air systems will not represent the space heat extraction rates required for radiant systems.

Cover page of Current practice for design and control of high thermal mass radiant cooling systems, and opportunities for future improvements

Current practice for design and control of high thermal mass radiant cooling systems, and opportunities for future improvements

(2018)

Radiant cooling and heating have the potential for improved energy efficiency, demand response, comfort, indoor environmental quality, and architectural design. Many radiant buildings have demonstrated outstanding performance in these regards. However, there are no well-established best practices for design of radiant buildings and their control systems, and most industry professionals are unfamiliar with radiant systems. This study summarizes interviews with eleven professionals with substantial experience with design and operation of radiant buildings in North America. Interviews focused specifically on high thermal mass radiant buildings, referred to as thermally active building systems (TABS). Interviews revealed a diverse range of approaches for design and control of TABS buildings. While interviewees expressed many similar approaches, they also have manyunique preferences. Examples of consistent themes include the use of dedicated outdoor air systems for ventilation and for supplemental cooling, and the use of a relatively simple control schemes that target a constant slab temperature for all times of the day and night. However, interviewees described unique preferences for space types where TABS should be applied, design and types of valves or pumps used for radiant zone control, the control of changeover between slab heating to slab cooling, and many other design considerations. Preferences appear to be driven by project constraints and by personal experience. Interviewees report that their design preferences are effective, but there is no industry consensus about how alternatives compare for energy performance. This paper outlines opportunities for further research, improvement radiant design and control, and the development of best practices.

Cover page of How High Can You Go? Determining the Highest Supply Water Temperature for High Thermal Mass Radiant Cooling Systems in California

How High Can You Go? Determining the Highest Supply Water Temperature for High Thermal Mass Radiant Cooling Systems in California

(2018)

Cooling demands are a major driver of energy consumption in buildings, and is mostly performed using systems based on the refrigeration cycle, an energy and cost intensive process. To investigate the potential of eliminating the refrigeration cycle from a building design in Californian climates, we created a single zone EnergyPlus model that uses a high thermal mass radiant system as the primary conditioning system, and that meets California’s energy code requirements. On the cooling design day, we randomly selected the start and number of hours of radiant system operation, lighting and plug load power densities, and occupant density for a set of models to determine the supply water temperature (SWT) that maintained comfortable temperatures. About 67% of tested models required SWT at or above 18 °C indicating that high thermal mass radiant systems have a high potential to use less energy and lower cost cooling devices like evaporative cooling towers in most California climates.

Cover page of Adaptable cooling coil performance during part loads in the tropics—A computational evaluation

Adaptable cooling coil performance during part loads in the tropics—A computational evaluation

(2018)

Air conditioning and mechanical ventilation systems may be oversized in commercial buildings in the Tropics. Oversized cooling coils may lead to reduced dehumidifying performance, indoor air quality and thermal comfort and increased energy consumption. In this paper, an adaptable cooling coil design is assessed with a general-purpose coil selection software tool, in which the number of active rows changes as a function of the load. For a 100% oversized coil, it is shown that the adaptable cooling coil is able to provide small but relevant improved humidity control down to 25% of the design load. This was obtained without affecting energy performance in typical variable air volume design and control. For specific applications, where variable air volume systems mainly control space humidity, there are also energy savings. The adaptable cooling coil could be seen as providing additional flexibility in the operation of HVAC systems, particularly in the tropics.

Cover page of Effect of acoustical clouds coverage and air movement on radiant chilled ceiling cooling capacity

Effect of acoustical clouds coverage and air movement on radiant chilled ceiling cooling capacity

(2018)

Thermally activated building systems have the potential to achieve significant energy savings, yet, the exposed concrete may also create acoustical challenges due to the high reflectivity of the hard surface. Free-hanging acoustical clouds reduce the acoustical issues, but also the cooling capacity of a radiant chilled ceiling system. Fan-induced air movement can be used to compensate for the cooling capacity reduction. We experimentally assess the combined effect of acoustical clouds and fans on the cooling capacity for an office room. We installed a ceiling fan between the clouds (blowing in the upward or downward direction) and small fans above the clouds (blowing horizontally) at the ceiling level to increase the convective heat transfer along the cooled ceiling. We tested the different fan configurations against a reference case with no elevated air movement. The tests conducted without fans showed that cooling capacity decreased, but only by 11%, when acoustical cloud coverage was increased to 47%, representing acceptable sound absorption. The ceiling fan increased cooling capacity by up to 22% when blowing upward and up to 12% when blowing downward compared to the reference case over the different cloud coverage ratios. For the variants with small fans, cooling capacity increases with coverage, up to a maximum increase of 26%. This experiment proves that combining fans with acoustical absorbents close to the radiant surface increases cooling capacity while simultaneously providing improved acoustical quality, and quantifies the impact.

Cover page of Evaluation of a cost-responsive supply air temperature reset strategy in an office building

Evaluation of a cost-responsive supply air temperature reset strategy in an office building

(2018)

This paper describes a new supply air temperature control strategy for multi-zone variable air volume systems. We developed the strategy with the intent that it is simple enough to implement within existing building management systems. At 5-minute intervals, the strategy estimates the cost of fan, heating and cooling energy at three different supply air temperatures (current, higher, lower), and chooses the one with the lowest cost as the setpoint. We then implemented this strategy in a seven floor, 13,000 m2 office building and compared the energy costs to the industry best practice control strategy in a randomized (daily) controlled trial over a 6-month period. We showed that the new control strategy reduced total HVAC energy costs by approximately 29%, when normalized to the typical annual climate data for this location and operating only during typical office hours. These findings indicate that the current industry best practice control strategy does not find the optimal energy cost point under most conditions. This new control strategy is a valuable opportunity to reduce energy costs, at little initial expense, while avoiding more complex approaches, such as model predictive control, that the industry has been hesitant to adopt. We describe the new control strategy in language common to the industry (see sequence of operations included as supplemental material) so that readers may easily specify and implement this immediately, in new construction or controls retrofit projects.