The Energy Wiz: Be Careful What You Ask For
Lighting and HVAC: The LinksMany lighting efficiency vendors like to include a credit for A/C savings in their cost analyses, but some have not considered just how those savings will occur. In the early days of major lighting efficiency retrofits (i.e., the 1980s), some assumed that each watt of heat removed from a conditioned space would automatically reduce the load on a chiller. Well, maybe. But what happens when we are dealing with constant volume reheat systems which are still common in many older buildings?
In one installation, the reduction in lighting wattage yielded impressive electrical savings but resulted in an increase in reheat energy consumption. How come? While the calculated cooling load due to lower lighting wattage had indeed been reduced, no change had been made to the cooling coil discharge temperature.
As a result, the hot water reheat coil (controlled by a room thermostat now sensing a too-cool room) pumped out more heat, countering the cooling savings, and consuming more steam at the hot water heat exchanger. The return air temperature remained about the same, so the cooling coil (and thus the chiller) felt about the same load as before the lighting upgrade.
Had the reheat coil been electric, it could have countered much of the lighting wattage savings, a truly nasty surprise. If perimeter radiation had been electric, most lighting wattage savings during the heating season would disappear as the baseboards, trying to maintain constant room temperatures, consumed power roughly equal to that saved by the lighting upgrade.
Raising the chilled water (and thus cooling coil discharge) temperature eventually yielded some cooling (and reheat energy) savings but, in some climates, could result in humidity complaints when less moisture was removed by the now warmer cooling coil.
Less Wattage May Mean Less Air FlowTwenty years later, a new (and more sophisticated) problem has arisen where VAV systems exist. Highlighted in a recent paper delivered at the World Energy Engineering Congress (WEEC) by Mark Heizer of Interface Engineering, Inc., a significant reduction in internal loads resulted in "dumping" (i.e., poor distribution) of air from VAV boxes and diffusers.
As explained by Heizer, lighting upgrades, Energy Star computers and office equipment, and the switch from cathode ray tube to LCD monitors have occasionally resulted in such a large drop in internal loads that VAV systems remain at very low flow much longer than envisioned by their designers. Where all-air systems prevail (vs. interior air plus perimeter systems such as radiation or fancoils), the situation is especially noticeable with marked variations in temperature, flow, and comfort from one side of a room to another. Even though room temperatures near windows read a normal 72°F, for example, the chronically reduced VAV flow ceased to counter the radiative effect of heat through glazing, thus making such perimeter spaces uncomfortable for some occupants. Maintaining flow at minimum may also, depending on design and settings, result in insufficient fresh air supply, resulting in stuffiness and IAQ complaints.
To avoid such situations, it may become necessary to raise the minimum VAV flow rate, thus reducing some of the fan savings that lighting vendors may claim will result from their upgrade work. Other possible fixes may include converting to slot diffusers at perimeter locations, and similar measures to improve air distribution at low flow.
Tomorrow's HVAC DesignsMark's analysis found that all-air systems "will become less popular as the interior loads drop to the 2.5 W/sq/ft level and below. [Under such conditions] the minimum outdoor air quantity will be over 50% of the interior supply air quantity." In his view, this leads to a preference for DOAS, combined with radiant slab heating/cooling (and perimeter systems in cooler climates). His calculations show significant savings for such designs over conventional VAV systems. He notes that such systems have been operating in North America "and have been common in Europe long enough to have good data on their ability to provide comfortable conditions for less energy and, in most cases, equal or less first cost."
A version of Mark's paper may be found at: www.ieice.com/AEEPaper.pdf. ES