HVAC
By Philip Haves
Applications
The main areas of opportunity to improve the performance of heating, ventilating and air-conditioning systems are alternative methods of cooling of occupied spaces, and central plant design. Alternatives to the conventional method of cooling spaces using overhead mixing ventilation involve use of hydronic systems, including radiant systems, natural ventilation, and exploitation of temperature and contaminant stratification using underfloor air distribution or displacement ventilation. Choices in central plant design include energy source - fuel vs. electricity, with its implications for carbon emissions now and in the future - and the use of thermal storage to shift electric load to off-peak periods or to match heating and cooling loads in time, particularly in milder climates, to enable use of heat recovery chillers. Related topics are covered under the sections on District-level Energy Services, Waste Heat Recovery, HVAC Controls, Commissioning, and Envelope and Lighting.
Radiant slab heating and cooling is a strong candidate to be considered best practice in new construction for larger buildings or campuses, particularly in climates with little or no latent load, including the western part of the US. In a side-by-side comparison in a large office building in Hyderabad, India, which has a summer climate similar to that of Fresno, CA, radiant slab cooling used 1/3rd less HVAC energy than a conventional variable-air-volume (VAV) system (see Infosys flagship project) (Sastry and Rumsey 2014). The top surface of the slab is exposed to conditioned space above or the bottom side is exposed to the space below, or both. Typically construction is cross-linked polyethylene (PEX) tubing placed in the structural slab or a topping slab before concrete is poured, as illustrated in Figure 1. Because of the large surface area of the slab, the chilled water supply temperature is relatively high (~65oF), allowing good use of water-side free cooling. Similarly, the hot water supply temperature is relatively low (~80oF), allowing the use of low grade waste heat. The slab smooths and shifts load but is, consequently, more difficult to control and size than radiant panels or air systems. Ventilation can be provided by a dedicated outdoor air system (DOAS) in which outside air is tempered and/or dehumidified, as required using an air handling unit designed for the purpose. Ceiling fans can be used as necessary to extend the ASHRAE comfort zone and enhance convective transfer from the slab. There are number of examples of radiant-slab buildings, in locations ranging from Cupertino (Roberts and Nqvi 2010) and Palo Alto (Center for Global Ecology n/d) to Hyderabad (Sastry and Rumsey 2014).
Figure 1. Radiant slab tubing, prior to concrete being poured (Photo courtesy of Infosys Limited)
Radiant cooling and heating can also be implemented using light-weight panels, typically mounted at ceiling height. It is much easier to retrofit radiant panels than a radiant slab, although they lack the thermal storage capability of slabs and hence cannot be used to shift and smooth load without some other form of thermal storage, e.g., chilled and/or hot water tanks.
Air systems that exploit vertical stratification in occupied spaces, such as underfloor air distribution (UFAD) systems and displacement ventilation (DV) systems can also use less HVAC energy than VAV or other systems that are designed to mix the air in the space. DV systems introduce the supply air at very low speed. The air is then entrained into buoyant plumes, which are exhausted near the ceiling as shown in Figure 2. UFAD systems introduce air into the space through swirl diffusers that induce a modest amount of mixing near the floor. Savings arise in two ways: (i) the supply air temperature is higher (~68oF for DV and ~64oF for UFAD, compared to 55-60oF for VAV or other mixing systems), allowing greater use of air side free cooling and correspondingly less chiller use, and (ii) the fan power requirement is reduced because the air is supplied at lower speed. The effect of the plumes is to remove contaminants as well as heat from the occupied part of the space, improving air quality. However, both systems have their challenges. Displacement ventilation systems can be difficult to set up correctly in normal height spaces, although they work reliably in higher spaces, such as auditoria, airports and industrial buildings. UFAD systems use the voids below raised floors as pressurized supply plena and careful construction is required to ensure that they do not leak. Free cooling can be increased by using indirect evaporative cooling in air handling units and roof-top package units, at the cost of greater pressure drop and, hence, increased fan power. Simulation-based analyses are required to identify the lowest energy solutions for a given cost, aggregated over a typical year.
Figure 2. Mixing ventilation (left), displacement ventilation (right) (Images courtesy of Price Industries)
Further energy consumption reductions can be obtained through the use of chillers with magnetic bearings, which have significantly higher efficiencies than conventional chillers, particularly at lower part loads (these were applied by the Infosys flagship project ). A Navy study found savings in chiller plant efficiency of 40 to 50% (Naval Facilities Engineering Command 2012). Chillers whose performance is optimized for low lift, i.e., relatively small difference between the temperature at which heat is removed and the temperature at which that heat is rejected to the environment, are starting to emerge onto the market. Such chillers would be well matched to applications, such as radiant cooling, that require relatively high chilled water temperatures.
Depending on the level of internal heat gains, it may be possible to achieve comfort using natural ventilation, with air flowing in through operable windows or other openings and exhausted through other windows or openings in occupied spaces or at the top of an atrium or draft tower. Simple methods for assessing the viability of natural ventilation for different building types in different climates are described by McConahey (2008). Flow can be driven by wind or by buoyancy ("stack effect"). In general, natural ventilation using operable windows requires a shallow floor plan. One additional advantage of a shallow floor plan is the increased opportunity for daylighting and corresponding reductions in energy use for artificial lighting. In 'cross-flow' ventilation, air may flow through multiple spaces between entering and exiting at different sides of the building. In single sided-flow, air enters and exits through openings in the same facade, typically in the same space. Flow can be wind or stack driven - usually flow rates are lower than under cross flow ventilation but easier to implement in terms of space planning, particularly for existing buildings. A further advantage of natural ventilation using operable windows is that thermal comfort can be based on the empirically derived Adaptive Comfort extension to the ASHRAE comfort standard (ANSI/ASHRAE 2013), in which the range of acceptable indoor temperatures is related to a recent average of the mean daily outside temperature. One explanation for this relationship is that occupants modify their clothing level based on recent weather.
'Mixed mode,' or 'hybrid' systems combine natural ventilation and mechanical cooling, using natural ventilation when and where possible and mechanical cooling when and where necessary (Center for the Built Environment 2015). A particular synergy is possible when the mechanical cooling is provided by a radiant system (Roberts and Nqvi 2010; Center for Global Ecology n/d). Since the design provides for outside air distribution that is adequate to provide the required sensible cooling under mild weather conditions, natural ventilation should be more than adequate to meet fresh air requirements when the sensible load is met by the radiant system. Such designs avoid the various costs (and space requirements) associated with the provision of a duct system as long as the outside air is never so hot that it cannot be sufficiently tempered by contact with interior surfaces or mixing with the air in the space.
There is not a complete consensus that hydronic systems are superior in performance and cost to conventional variable-air-volume (VAV) systems (Stein and Taylor 2013), though here the comparison is with active chilled beams rather than radiant slabs. Careful design of VAV systems can enable zone air flow rates to be turned down by as much as 10:1, significantly reducing the reheat required when there is substantial diversity in zone loads. There is a trade-off between increasing the depth of the coils to increase the difference between the supply and return water temperature; this increases chiller COP and reduces pumping costs but increases air pressure drop and hence fan energy use. As with radiant slabs, though to a lesser extent, increasing the coil depth allows higher chilled water supply temperatures, increasing the opportunities for water-side free cooling, and allowing lower hot water supply temperatures, increasing the opportunities to make use of waste heat.
The solutions discussed so far all assume 'built-up' mechanical systems based on chilled water and hot water from a central plant or boilers and chillers in each building or groups of buildings. An alternative approach is to use packaged units with direct expansion (DX) cooling, typically installed on the roof. Roof-top package units are effectively restricted to low-rise buildings and cannot make use of chilled or hot water thermal storage, though they do not incur the pumping and thermal losses associated with district heating and cooling systems, in particular.
All the technologies discussed above, with the exception of DV, are well established and are starting to be accepted as best practices for commercial new construction, at least in the Western US.
Economics
Infosys Limited has performed a study of the construction costs and operating costs for radiant slab and VAV systems in two identical halves of a building in Hyderabad, India (Sastry and Rumsey 2014) (see case study). The construction cost for the radiant slab system was slightly lower and the operating cost was consistently ~34% lower over a period of several years. The economics of natural ventilation vary significantly. The most favorable case is new construction where the use of natural ventilation can avoid the need for mechanical cooling. In retrofit, replacement of fixed windows with operable windows is typically not cost effective unless the windows need to be replaced for other reasons. Mixed-mode systems may not be cost effective unless the use of natural ventilation allows the mechanical system to be downsized significantly. A more favorable case would be the combination of radiant cooling and natural ventilation if the natural ventilation system can obviate the need for a mechanical ventilation system.
As regards magnetic bearing chillers, a study by the US Navy indicated payback periods ~1 year, depending on the utility rate and the hours of use (Naval Facilities Engineering Command 2012). Maintenance costs are substantially reduced because of the lack of need for lubricating oil and the reduced weight of the compressor.
Other considerations
Hydronic systems typically have lower fan noise due to the lower air flow rate in the DOAS compared to conventional systems. However, systems with exposed ceiling slabs require careful incorporation of measures to reduce sound generated in the occupied space. Noise problems are a major source of complaints in many commercial buildings (Jensen and Arens 2005).
Institutional requirements & capacity
Because they incorporate thermal storage, radiant slab systems are more difficult to size, control and operate and so care is required to select mechanical designers and controls contractors that have previous experience with successful radiant slab systems. Special training for operators is even more important, to ensure that they understand how such systems operate, in particular, the long response times and how to deal with them.
References
ANSI/ASHRAE Standard 55-2013. 2013. Thermal Environmental Conditions for Human Occupancy. ASHRAE.
Center for the Built Environment. 2015. Mixed Mode Buildings http://cbe.berkeley.edu/mixedmode/index.html
Stein, J. and Taylor, S.T. 2013. "VAV Reheat Versus Active Chilled Beams & DOAS." ASHRAE Journal, vol. 55, no. 5, May https://www.ashrae.org/resources--publications/periodicals/ashrae-journal/features/vav-reheat-versus-active-chilled-beams--doas
Center for Global Ecology. no date. https://dge.stanford.edu/about/building/Sus_features.pdf
Jensen, K. and Arens, E. 2005. "Acoustical quality in office workstations, as assessed by occupant surveys). Proc. Indoor Air 2005, Beijing, China. http://escholarship.org/uc/item/0zm2z3jg
Naval Facilities Engineering Command. 2012. "Naval Techval Program" presentation. http://www1.eere.energy.gov/femp/pdfs/ntwg_052012_kistler.pdf
McConahey, E. 2008. "Mixed Mode Ventilation: Finding the Right Mix." ASHRAE Journal, September, pp 36-48.
Roberts, C. and Naqvi, A. 2010. ASHRAE Technology Awards: Learning by Doing. ASHRAE Journal, vol. 52, no. 5, May 2010.bookstore.ashrae.biz/journal/download.php?file=ASHRAE-D...pdf
Sastry, G. and Rumsey, P. 2014. VAV vs. Radiant Side-by-Side Comparison. ASHRAE Journal 56(5) May. https://www.ashrae.org/resources--publications/periodicals/ashrae-journal/features/vav-vs--radiant-side-by-side-comparison