Heating, ventilation and air-conditioning systems (HVAC) have always been a major consideration for engineers who sought to find ways to increase occupants comfort levels. The main determination of how HVAC systems have been designed over the last few decades have come down to two main factors, energy costs, and technological advances. In this post, we look at 3 current trends in HVAC design from thermal comfort strategies to decoupling ventilation from heating and cooling and indirect evaporative cooling.
Occupant-Based Thermal Comfort Strategies
Traditionally the central focus of many HVAC systems is to achieve thermal comfort through ambient temperature control. The trend today, however, is to shift focus towards the occupant’s needs. Systems like radiant cooling help to reduce energy use and improve thermal comfort. Thermal comfort can be directly addressed through other systems such as using low-energy systems to provide localized heating and cooling or by controlling the HVAC system through occupant polling.
There is also a resurgence in the use of energy efficient ceiling and desk fans and natural ventilation. This traditional occupant-based approach that provides air movement for cooling is a popular addition to net zero energy (NZE) and other ultra-low energy buildings.
Achieving high energy savings without considering the thermal comfort of users is unrealistic. A simple strategy of setting the temperature setpoint to the ambient temperature would achieve the maximum savings, but could also have a negative impact on the comfort and productivity of people. Using a simple app to poll occupants in order to set HVAC temperatures can significantly reduce energy use caused by overcooling or overheating while also maintaining optimal thermal comfort.
Decoupling of Ventilation and Heating/Cooling
Variable air volume (VAV) systems, which became popular after World War 2, serve as a sort of all-in-one system providing heating, cooling and ventilation. The industry, however, is moving HVAC design away from this system to decoupled systems which either partly or completely separate ventilation functions from heating and cooling functions. The reason for this is that decoupled systems see significant reductions in fan energy and as a result provide increased cost savings.
A prime example of this is a dedicated outdoor air system (DOAS). With a typical DOAS, the airflow provided by the fan system is limited to the ventilation component. The DOAS air handling unit provides heated and dehumidified air for ventilation and is frequently provided with some form of heat recovery component such as enthalpy transfer wheels, “run around” coils or heat pipes to further reduce energy consumption by utilizing building exhaust air to precondition the ventilation air. A DOAS system typically provides 20% or less airflow than what would be provided at peak cooling periods utilizing a VAV system.
With a DOAS system, the heating and cooling requirements for space are met through a hydronic system. Since water has a much higher capacity for energy transfer than air, the amount of energy required to deliver the heating and cooling is greatly reduced, while pump energy is somewhat increased. A side benefit of the reduced air quantity is smaller ductwork, which decreases the cost of the ventilation system and, potentially, the building’s required floor-to-floor height. DOAS systems are typically paired with passive chilled beams, radiant heating and cooling, or fan coils.
An important consideration when designing DOAS and chilled beam systems is how the air is distributed to the set space and how heating and cooling is accomplished. In a building with low cooling requirements, the ventilation air may be enough to provide adequate cooling, while in buildings with higher cooling needs additional cooling systems like the use of radiant cooling may be required.
The second trend in decoupled systems is a sort of hybrid model. Ventilation and both heating and cooling are delivered through the active chilled beams but the chilled beams deliver most of the heating and cooling needed while the air handling unit only delivers a portion of the required heating and cooling. Similar to the DOAS and passive chilled beam system, the active chilled beam system delivers pre-heated and dehumidified air to the set space through the use of an air handling unit which is frequently provided with a means of heat recovery.
The National Science Foundation analyzed a simple 20-story building to compare the DOAS and passive chilled beam system and the active chilled beam system to an ASHRAE standard 90.1 baseline VAV system. The results found that passive chilled beams used significantly less energy than the baseline VAV and active chilled beams.
Indirect Evaporative Cooling
When it comes to green buildings, owners and architects are striving to meet HVAC loads with optimum comfort and minimal energy. Indirect Evaporative Cooling (IEC) offers a highly efficient way to cool an indoor space without raising the humidity.
Long before the introduction of mechanical cooling, evaporative cooling could be found in traditional architecture in hot and dry climate worldwide. The system works rather simply by adding water vapor to the hot air and, then through the process of evaporation, removes sensible heat from the air and effectively lowers its temperature.
Over the years this process has been improved by manufacturers and designers, which lead to the invention of indirect evaporative cooling which eliminates the addition of humidity into the building.
In an IEC system, return air passes from the building directly over a wetted medium to passively remove sensible heat before discharging it to the outside. Outdoor air enters as a secondary air stream, crossing the medium in an adjacent but isolated side and is ‘indirectly’ evaporatively cooled before being delivered to the set space. This system works well when combined with radiant technology for heating and cooling and DOAS for ventilation.
In today’s net zero energy (NZE) and green building industry, the trend is to move away from sole reliance on mechanical cooling and introduce passive methods in order to an optimal indoor climate and significantly reduce energy consumption.