Fonte: Consulting – Specifying Engineer

Autor: Chris Rush, Arup, New York City

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By considering the first principles of radiative energy, engineers can determine how to balance daylight, electric light, and HVAC systems, particularly considering various options for daylight control and the advent of LEDs.

Learning objectives:

• Understand key principles that affect the integration of lighting and HVAC systems.
• Understand the effects of and design strategies for daylighting with regard to lighting and HVAC considerations.
• Understand the ways in which electric lighting impacts interior HVAC internal heat gains,and the impacts of LED lighting.

What is the most fundamental connection between lighting and HVAC? Daylight and the sun both impact lighting and HVAC. Both require the support of electrical systems for power. But the physics of radiation could be considered the most fundamental link between lighting and HVAC,and of intrinsic importance for building integration possibilities.

Most building occupants probably would link lighting (vision) and sound (hearing), rather than making an obvious connection between lighting and HVAC-where temperature and humidity would be linked to our skin’s sense of touch. Most people do not easily connect lighting and HVAC, partly because temperature and humidity are more intuitive concepts to us than are sound and light, which seem more abstract.

It’s when balancing or coordinating both the lighting and thermal environments of a building design that the importance of radiative energy transfer is more obvious. Both the lighting designer and the mechanical engineer for any building project are accounting for radiative energy transfer in their designs, but the importance of the synergies between the two are not always apparent.

With visible light, radiation is essentially the only method of energy transfer in light generation,reflection, absorption, and our perception. Visible light and infrared radiation operate on relatively similar scales of wavelength and frequency compared to sound or other radiated energy categories. It’s somewhat incidental that our eyes perceive wavelengths near 550 nanometers as visible light, but not ultraviolet (UV) wavelengths shorter than 400 nm or infrared wavelengths longer than 700 nm. But in considering design integration and synergies of lighting and HVAC,radiation at multiple wavelengths must be a paramount consideration. Thermodynamics and the thermal environment involve more than just radiative considerations; they also involve convection and conduction. However, those have very little to do with lighting in basic principle and are outside the scope of this article.

The sun’s radiation is clearly important to us on Earth-providing the light that we see arriving on the ground, and the heat that we sense on our skin (as well as UV radiation, which we recognize when we’re exposed for too long). So whatever you call it-natural light, daylight, or solar heat gain-solar radiation is a clear priority for both lighting and HVAC design.

Also, electrically energized light sources (or artificial light sources, electric light sources, or simply”lights”) emit light by radiative transfer, and have historically emitted most of their waste heat by radiative transfer in the same manner and direction as their light output.

Daylight

Before electric lighting or control systems, daylight was the chief integration opportunity for the design process and a successful outcome of HVAC and lighting. Consider that daylight includes directional sunlight, diffused sky and cloud illumination, and at the same time the visible, infrared,and UV components from these sun and sky sources. For the best potential results, daylighting must be considered very early in the design process for any building.

The direct sun primarily impacts glare and thermal comfort, while the diffuse sky primarily impacts useful natural illumination and some additional infrared radiation. The windows and glazing systems that are so important to occupant enjoyment, and provide daylight inside to offset electric light source use, also admit solar heat into buildings. With more attention paid to well-insulated and airtight building envelopes, the heat of the sun radiating onto a building has a greater chance to accumulate and overheat a building-or to cause notable loads that the building cooling system must offset, with resulting energy use.

So maximum possible daylight is not a win-win solution. Daylight within the limits of useful interior quantities is a partial “win,” allowing us to turn off or dim electric lights and reduce some internal heat gains from those lights. But every hour of the year that indoor daylight exceeds occupant lighting needs, there is a risk for solar heat gain greater than needed, and HVAC energy spent in cooling the building more than needed.

So how do we maximize the daylight for the sake of electric light savings (and occupant appreciation, happiness, enhanced productivity, sleep-wake cycles, property value, merchandise sales rate, etc.), while also minimizing the problematic side effects? In addition to solar heat gain associated with daylight and glazing, there’s a fine line between allowing appropriate amounts of daylight and minimizing hours that occupants close blinds for glare. At first, one may begin to think that shading from glare is probably simultaneously shading from excess heat gain, but this is not necessarily the case.

For glare sources, people are most sensitive to brightness directly in the center of their line of sight, as indicated by the use of the Guth position index in many glare analysis metrics (such as CIE 117). Additionally, research ongoing since 2006 related to a relatively new metric called daylight glare probability shows that the illuminance received at one’s eye particularly affects a person’s perception of glare. And the light sources most significantly impacting illuminance atone’s eye are those sources central to one’s line of sight. So for glare, the sun at the horizon is often the most problematic.

Considering solar heat gain instead of glare, the sun at the horizon is typically not a problem during morning and evening hours. Correlating solar heat gain to external temperature, the most important times to shade a building from the sun are most likely the times that outdoor temperature is high-also the same midday hours that the sun is high in the sky. People are much less sensitive to glare from sources high above their line of sight. Also, sun high in the sky is less likely to reach more than a few workstations that might be intentionally close to the façade.

So the higher the sun gets in the sky, the more important it is to shade the building from solar heat gain. The lower the sun is in the sky, the more important it is to shade the windows from potential glare-to give a chance that window blinds stay open and useful light can get through.