First in a new series examining how barriers to this promising approach can be overcome.

Ed Arens and Hui Zhang (Center for the Built Environment, University of California, Berkeley) introduce a series of commentaries that explore the development and adoption of personal comfort systems: decentralized building thermal control, in which occupants control their local environments with personal devices while the amount of central space conditioning (HVAC) is scaled back. This has been shown to improve thermal satisfaction and reduce energy demand. What are the barriers to its implementation?

This approach to control promises to make a greater proportion of a building’s occupants comfortable, while at the same time significantly reducing the energy- and system costs of central HVAC systems, and increasing the resilience and sustainability of buildings. It should be the future of HVAC, most certainly given the urgency that the building sector reduce its massive and increasing contribution to climate change.

At this time, decentralized control in buildings is minimal. It might be surprising that it has taken so long to catch on, but its adoption affects many parts of the building industry, and there has not been the necessary leadership coming from government or a major market player. Manufacturing the necessary technology has been feasible for some time, but has taken off for only one of its possible components, advanced ceiling fans. A confluence of recent developments in knowledge, standards, and technologies (e.g. affordable batteries, the internet of things (IOT), and more open building controls) have reduced some of the past impediments. These developments now provide the building sector with assurance that the approach will work and be economical. So, what can society do to kickstart this promising opportunity?

The acceptance and adoption of decentralized control involves a diverse array of actors (manufacturers, standards organisations, design professionals, client groups, real estate and facilities operators), and it would challenge their existing practices.We need to recognize both the barriers and drivers to widely implementing this technology. Then we need to address each to assist and accelerate innovation. What leadership can these different actors provide for promoting this transition? What aspects and arguments will be compelling to the many different stakeholders: occupants, the real estate market, the design professions, and governmental agencies tasked with combatting climate change?

Background

Although a variety of decentralized control methods were common in the pre-air-conditioned era, they virtually disappeared from practice as designers came to regard space conditioning as a task for automated and centralized machines.The capabilities of HVAC machinery greatly influenced modern building design and created the expectation that comfort requires uniform and static levels of space temperature and humidity. This notion proved to have two important faults:

• It failed to recognize that individuals vary in their thermal needs. Substantial fractions (~40%) of building occupants consider their space to be thermally uncomfortable and less than half are satisfied with the thermal environment at any given thermostat setting (Graham et al. 2021).¹ Buildings are routinely overcooled, this disproportionately affected women, particularly in summer when 76% of complaints of cold temperatures came from women (Parkinson et al. 2021).
• Maintaining narrow temperature conditions in pursuit of an optimal comfort condition is very energy-intensive (Hoyt 2015). Buildings are increasingly expensive to operate and major contributors to greenhouse gas emissions.

Personal comfort systems (PCS) are defined here as thermal systems that heat and cool individuals without affecting the environments of surrounding occupants, and that are under the individual’s control. They tend to be devices locally positioned on, or incorporated in, workstation furniture such as chairs, desktops, or positioned near the feet and legs. Some are readily available (such as DC-powered desk fans), while some are still prototypes (heated and cooled chairs; efficient footwarmers). PCS might also include ceiling fans operating under the control of individuals or small groups. It is also possible to stretch the PCS definition to include wearables such as thermally active clothing and jewelry-like carried devices, although it becomes harder to characterize such things as being part of a building system. Traditional passive adaptive opportunities also border somewhat on the PCS definition, such as relaxing office dress codes and providing access to operable windows.

PCS provides occupants with wider ranges of comfortable temperatures. They accommodate the individual variation in thermal requirements that occur between individuals and even within individuals over the course of a day, week, or month. The heating and cooling effectiveness of different PCS devices has been quantified in terms of corrective power (CP) and are now classified in ASHRAE Standard 55-2020 (Thermal Environmental Conditions for Human Occupancy) for use in environmental quality rating schemes (ASHRAE 2020, Zhang et al. 2015a). A building in which such devices are available to occupants earns a higher environmental quality rating in green- and wellness standards.

PCS users directly adjacent to each other can select widely different personal microclimates without crossover or interference. Each individual should be able to adapt to a comfortable condition within their device’s corrective power range, and the adaptation should take effect within the short time frames associated with changes in location and physical activity level (Zhai 2019). Access to PCS should thereby eliminate complaint calls and create a more acceptable work environment. This has been the experience of most PCS field studies done to date (Zhang et al. 2015, Zhang et al. 2018, Kim et al. 2019).

PCS can save energy. Heating and cooling the occupants directly is inherently much more efficient than conditioning their surrounding air or room surfaces. Per occupant, PCS itself uses around 1% of HVAC heating or cooling energy if done properly.² The savings occur in the HVAC system: widening the thermal comfort zone allows the building central system to work less hours and at lower energy intensity, while the PCS devices maintain the occupants’ comfort (Hoyt et al. 2015). Relaxing thermostat setpoints for heating and cooling saves 10% of total HVAC energy per degree C, either way.The effects of PCS are as relevant to home office work as well as at the office, if not more so. The savings in percent of total HVAC energy by relaxing thermostat setpoints is roughly the same for both residential and commercial buildings. The residential energy savings may be more appreciated as the home worker is paying the utility bills.

Finally, the resilience of building systems has recently become a hot topic, with climate extremes and curtailments of the power grid becoming more common. PCS can help here too, by reducing a building’s power demand, shifting its peak load, and by being able to operate independently of the grid (especially in cooling mode) on batteries and onsite renewable power sources.

Current situation

Extensive groundwork already exists to support PCS technology.  Ideas, designs, understanding of physiological bases of PCS comfort, laboratory- and field testing of prototypes, newly supportive environmental standards; all of these different perspectives have to be examined and point to the feasibility and promise of PCS technology at improving comfort and increasing energy efficiency in buildings.  There are at least three major PCS review papers making the case that PCS is effective at providing comfort within a range of ambient conditions (Vesely & Zeiler 2014; Zhang et al. 2015; Rawal et al. 2020).

As mentioned above, corrective powers have been measured for a variety of PCS devices (Zhang 2015b, Luo et al. 2018, Wang 2019).  For cooling, the main options are fans, powered chairs, desktop surfaces, and thermal wearables.  Fans (desk and ceiling) provide CPs of 3 K at medium speed settings (0.8 m/s), ranging up to 5 K at their higher speeds.  Cooled chairs provide 2-3 K cooling.  Desktop wrist and hand cooling and wearables provide 2.5 – 3.3 K for both heating and cooling.  Thermally active clothing claims 2.2 K.  For heating, the options are foot and leg warmers, chairs, and desktop hand and wrist warming devices, and thermal wearables.  Footwarmers provide 2.2 K heating, chairs 4 K, various hand- and wrist warmers, wearable devices, clothing supplements are yet to be rated.  These devices can be used singly or in combination, though CPs for combinations are not yet available.  Either way, these are substantial temperature correction numbers, capable of enabling major reductions in central heating and cooling.   In the few field studies in which thermostat relaxation was done (generally down to 19.5°C and up to 25.5°C), central HVAC energy use was always reduced, by 30 to 70%, without increasing occupants’ discomfort votes. 

It will however be challenging to transform the environmental control systems now used in the building industry.  At this point, key PCS (particularly heated/cooled chairs) are barely available on the market.  Although PCS devices may soon begin to appear in the furniture- and appliances markets to cater to occupants’ comfort needs, it is likely they will first be used to correct existing discomfort problems and to perhaps allow some seat-of-pants thermostat resetting.  It will take new development to coordinate the devices’ sensing and communicating capabilities with their buildings’ environmental control systems in real time.  In this process, new information will be needed to inform and assist both designers and building operators.  

A variety of further barriers will need to be addressed for PCS to be incorporated into building design, or even being used in retrofitting existing buildings.  Who certifies how well specific products work in practice?  Who accepts responsibility for a building design that relies on them?  Who maintains them once installed? What is their service life?  From whose budget would they come, and who then benefits from the energy and comfort improvements?  On the other hand, because PCS devices will be consumer products manufactured in large quantities, they can be inexpensively fitted with IOT technology making them candidates to act as sensors and possibly actuators for the central HVAC system, expanding its capabilities and reliability. How might such additional capability help PCS become integrated into buildings at scale?

In all this, it is useful to reflect on the extraordinary recent adoption of ceiling fans for cooling occupants in high-end buildings. Ceiling fans are a form of PCS that have existed for more than a century, but were held in low regard after the introduction of air conditioning.  The turnaround seems to have been driven primarily by a few models’ beautiful styling and effective advertising, to a lesser extent supported by large improvements in efficiency in DC motors and airfoil shapes.  At this point the fans can be automatically operated with occupancy and temperature sensors, and though they are not connected to HVAC controls, they are effectively integrated with them by enabling operators to relax the HVAC thermostat setpoints when fans are present.  A certain amount of testing standardization work has been accomplished, and design tools are being developed.   Perhaps this successful experience can guide prospective manufacturers of other types of PCS devices.

Conclusion

Decentralizing thermal control to the occupant level addresses two of the current building industries’ greatest deficiencies:

• the universally high rate of occupant dissatisfaction with commercial building thermal environments
• the more than 20% of total global energy consumption and CO₂ generation that currently go into producing those remarkably unsuccessful indoor environments.  

Fixing these deficiencies ought to be viewed as a challenge, equally important and valuable as the onetime moon program, though taking place (appropriately) in a more decentralized manner.  Technologically, the solution is simple to implement. The key challenges reside in other domains. Can leadership from industry, clients or government create the appropriate conditions and incentives for adoption?

Our hope is that this commentary will provide a better understanding of PCS and its potential, prompting its more widespread manufacture and availability in the market, its adoption by facilities owners and managers, and transitioning mainstream HVAC practice toward more decentralized and occupant-centric HVAC systems.

An invitation

We invite additional commentaries on PCS that further broaden our understanding from multiple stakeholder perspectives and address a series of topics to assist with the transition toward implementation.  Discussion of barriers, opportunities, leadership, professional practice, standards and design tools are welcome. If you are interested, in the first instance please contact the Buildings & Cities editors.  

Topics sought for this commentary thread include:

PCS field studies.  There are at this time few examples of field studies in which PCS has been integrated into the building’s HVAC control system.  There are several field studies of occupant comfort alone, with the central HVAC operated conventionally, which always show improved comfort and satisfaction, though more examples would be welcome.  Studies of comfort together with building energy use are rare.  Reports of more tests with energy savings evidence are needed.

Evidence of PCS capabilities for serving metabolically transient and diverse occupancies.  There is an inherent comfort advantage from fast-acting and decentralized environmental control that cannot be matched by any form of uniform control.  Thermal and metabolic transients happen in retail stores, lobbies, commuter arrivals at the office; spatial heterogeneity in metabolic rates are exemplified in fitness centers with their exercisers and staff, restaurants with waiters and customers, and again retail stores with clerks and shoppers.  A building exhibiting such enhanced comfort capability should be recognized as superior to existing buildings, and be the gold standard.  It would be not only more efficient than current practice, but commanding higher rent.

Crediting passive measures, or personally owned wearables, as components of a building’s control system. There may be creative ideas about how to credit adaptive opportunities provided in a building, or by its tenants’ dress policies, that would have the effect of improving comfort and building energy.  Even if they may not ultimately fit within the concept of ‘PCS’ or ‘decentralized control’, it would be good to learn if and where there are useful overlaps . 

Standards implementation.  In order to systematize credit for PCS and fans, ASHRAE Standard 55 has instituted a new thermal-environment-control classification, intended for adoption into green building standards.  When PCS are available for occupants, the building becomes eligible for a  higher IEQ classification by a defined process.  Questions remain, for example: what would be reasonable PMV comfort zone boundaries for HVAC to target when occupants have PCS available-- +/-0.5, +/-0.7, or +/-1.0?  Such considerations will underlie standardization of practice in policies and codes. 

Commercialization and product cost. Examples of successful products, and also of market failure of previous PCS products would be valuable.  What became of early PCS implementations that appeared in Japan and elsewhere in the 1990s?  What has changed since that time that might make commercialization more promising now?  One hypothesis: it was found that personalized ventilation ducting in outside air was not required for improving perceived or actual air quality, removing a forbidding cost barrier.

Quantifying benefits. Evidence is sought on a variety of benefits e.g. productivity gains under PCS, analyses of first-cost reductions in HVAC under PCS-enabled modes of operation, reduced retrofit costs of decentralized PCS control versus other retrofit options, analyses of operational (energy) cost reductions under PCS operation, quantification of resilience and demand-response benefits, IOT data and communications benefits from having sensors/actuators/wireless features included in PCS.

Real estate / facility management economics.  Who pays for the PCS in a decentralized building HVAC system, and who then reaps the operational benefits?  PCS and fans increase the resilience and survivability of buildings especially under warm conditions; how might those be quantified?  How might PCS-enabled design accelerate building retrofits, since it can be carried out in parallel or independently from upgrades to building and system?

Transforming professional A/E practice.  Aside from standards provisions that enable designing with PCS, what other measures would be needed to encourage architects and engineers to use PCS in a decentralized building system?  What would reduce their professional liability for specifying systems with downsized mechanical HVAC equipment and ductwork at the design stage?

Design tools. New tools are becoming available to model transient thermophysiology and comfort at the local body segment level, to predict local occupant-centered radiant temperatures and within-room airflow patterns for fan layouts, and for designing ceiling-fan-integrated air conditioning (CFIAC).  There will be need to develop openly available control sequences for decentralized HVAC operation as enhanced by sensory feedback from IOT-enabled PCS and other devices. Reports about such developments would be valuable.

Notes

1. This is not only the effect of some people being ‘colder’ or ‘warmer’ than others; when people enter a space and sit down there is a period when their comfort requirements will often be very different from what they will need 15 minutes later; adjusting a central HVAC control cannot produce sufficient response in the room in the time frame that is needed, and tends instead to create a condition that is inappropriate for the occupants in the longer term.

2. Portable space heaters do not qualify as PCS. Their typical power draw of 800-1500W causes significant space heating and uses, per capita, similar amounts of energy as the equivalent amount of central HVAC heating.

References

ASHRAE (2020). ASHRAE 55-2020: Thermal Environmental Conditions for Human Occupancy. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Graham, L.T.; Parkinson, T. & Schiavon, S. (2021). Lessons learned from 20 years of CBE’s occupant surveys. Buildings and Cities. https://doi.org/10.5334/bc.76

Hoyt, T.; Arens, E. & Zhang, H. (2015). Extending air temperature setpoints: simulated energy savings and design considerations for new and retrofit buildings. Building and Environment, 88, 89-96. https://doi.org/10.1016/j.buildenv.2014.09.010

Kim, J.; Bauman, F.; Raftery, P.; Arens, E.; Zhang, H.; Fierro, G.; Andersen, M. & Culler, D. (2019).Occupant comfort and behavior: high-resolution data from a 6-month field study of personal comfort systems with 37 real office workers. Building and Environment, 148, 348-360.

Luo, M.; Arens, E.; Zhang, H.; Ghahramani, A. & Wang, Z. (2018). Thermal comfort evaluated for combinations of energy-efficient personal heating and cooling devices. Building and Environment, 143, 206-216. https://doi.org/10.1016/j.buildenv.2018.07.008

Parkinson, T., Schiavon, S., de Dear, R. & Brager, G. (2021). Overcooling of offices reveals gender inequity in thermal comfort. Scientific Reports, 11: 23684. https://www.nature.com/articles/s41598-021-03121-1

Rawal, R.; Schweiker, M.; Berk Kazanci, O.; Vardhan, V.; Jin, Q. &Duanmu, L. (2020). Personal comfort systems: a review on comfort, energy, and economics. Energy and Buildings, 214. https://doi.org/10.1016/j.enbuild.2020.109858

Vesely, M. & Zeiler, W. (2014). Personalized conditioning and its impact on thermal comfort and energy performance: a review. Renewable and Sustainable Energy Reviews 34, 401-408.

Wang Z.; Warren, K.; Luo, M.; He, X.; Zhang, H.; Arens, E.; Chen, W.; He, Y.; Hu, Y.; Jin, L.; Liu, S.; Cohen-Tanugi, D. & Smith, M. J. (2019). Evaluating the comfort of thermally dynamic wearable devices. Building and Environment, 167, 106443

Zhai, Y.; Miao, F.; Yang, L.; Zhao, S.; Zhang, H. & Arens, E. (2019). Using personally controlled air movement to improve comfort after simulated summer commute. Building and Environment, 165, 106329

Zhang, H.; Arens, E.; Taub, M.; Dickerhoff, D.; Bauman, F.; Fountain, M.; Pasut, W.; Fannon, D.; Zhai, Y.C. & Pigman, M. (2015a). Using footwarmers in offices for thermal comfort and energy savings. Energy and Buildings, 104 (3), 233 – 243.

Zhang, H.; Arens, E. and Zhai, Y. (2015b). A review of the corrective power of personal comfort systems in non-neutral ambient environments. Building and Environment, 91, 15-41. https://doi.org/10.1016/j.buildenv.2015.03.013

Zhang, H.; Bauman, F.; Arens, E.; Zhai, Y.; Dickerhoff, D.; Zhou, X. & Luo, M. (2018). Reducing building over-cooling by adjusting HVAC supply airflow setpoints and providing personal comfort systems. Proceedings of Indoor Air 2018, July 22 – 27, Philadelphia.

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