HyPak Ultra High Efficiency Rooftop Packaged HVAC System

Introduction


HyPak incorporates a variety of energy efficiency measures normally available only on larger unitary equipment (>50 tons) into a 10-30 ton range of rooftop packaged units (RTUs). Foremost is an innovative evaporative module that combines condenser water cooling and ventilation air pre-cooling, the latter without the addition of moisture to the conditioned air space.

The HyPak project is a partnership between Davis Energy Group and Munters/Des Champs of Buena Vista, VA with significant support from the U.S. Department of Energy. This project is administered by the National Energy Technology Laboratory in Morgantown, WV.

Statement of Problem


More than half of U.S. commercial building space is cooled by packaged HVAC equipment, most of which are rooftop units with less than 50 tons of cooling capacity. Existing rooftop HVAC units consume more than 1.3% of total U.S. energy annually. Despite their modest levels of efficiency, RTUs are popular because they are inexpensive, provide zonal control, are easy to install, can be serviced without disrupting occupants, and are familiar to the industry. Central systems with chillers and cooling towers are more efficient than RTUs, but have been losing market share due to higher first costs and greater complexity.

Today’s rooftop units are inefficient for a host of reasons. Their location exposes them to elevated temperatures that increase ventilation air cooling loads and reduce efficiency. Air-cooled condensers lose capacity at high ambient conditions. Many are oversized to handle peak conditions, increasing cycling losses in more typical conditions. Undersized evaporator coils reduce compressor efficiency and waste capacity over-drying indoor air in dry climates. Single-speed blowers run for ventilation during all occupied hours, using about half of annual rooftop unit energy. Also, rooftop units often fail to maintain adequate indoor air quality; a California field survey found that only 16% of rooftop units met minimum required ventilation rates. But given their advantages and momentum, their large market share will likely continue. Improving their efficiency is essential to enhancing overall commercial building energy performance.

Project Objectives


The key objectives of the HyPak project are:

  • To design, develop, and test a high efficiency hydronic rooftop unit capable of delivering 100% recirculated air, 100% ventilation air, or any mix of the two.
  • To develop and test a low-cost, high performance plate-type evaporative heat exchanger that efficiently cools water and air.
  • To demonstrate a substantial improvement over current rooftop units, reducing annual electrical energy consumption by 50%, and reducing peak load by 40%.

Technical Approach


RTU efficiency can be significantly improved via evaporative condensing and indirect evaporative pre-cooling of ventilation air. Evaporative cooling of condenser water lowers condensing temperature, increasing compressor capacity and efficiency. Indirect evaporative cooling of the ventilation air can eliminate much of the energy penalty for ventilation air, particularly in dry climates where high dry-bulb temperatures heavily tax conventional air-cooled RTUs. This process can also recover cooling energy from exhaust air. Evaporative cooling ties unit efficiency to wet-bulb rather than dry-bulb temperatures; significantly improving performance in dry conditions. However, previous work by the HyPak team showed that evaporative condensing can cost-effectively reduce energy consumption even in humid climates.

In addition to its innovative evaporative cooling configuration, the HyPak design uses the following strategies to increase efficiency:

  • Application of the indirect evaporative heat exchanger to recover heat from building exhaust air in heating mode
  • Variable-speed supply and exhaust blowers to match airflow requirements
  • High effectiveness, low pressure drop pleated filters with long replacement intervals
  • Oversize evaporator coil with innovative refrigerant circuiting to make full use of heat exchange surface area even during part-load conditions
  • A high-efficiency tankless gas water heater coupled to a hydronic air coil for heating
  • Environmentally-friendly R410A refrigerant

The HyPak project is now in a second funding round. The plate-type evaporative heat exchangers developed in the original project were made first from plastic-laminated paper and then from thermoformed plastic plates. Considerable efforts in the second project are focused on developing an automated process that builds heat exchangers as the plastic is thermoformed.

Status


The original HyPak project was completed in May, 2004. The initial project prototypes demonstrated EERs of 16 to 20 depending on outdoor conditions. However, they were quite large and only capable of delivering up to 40% ventilation air. The second HyPak project began in June 2005 and will be completed in 2009. The goal of the second project is to develop a unit with similar performance, but capable of delivering to up to 100% outdoor air to take advantage of the many hours during cooling season when the enthalpy of the outdoor air is below that of the return air. Major emphasis is also on developing an automated process for production of the plastic plate-type heat exchanger to bring down costs on this critical component. An offshoot of this focus area is the IEHRV project, which extends the low-cost evaporative heat exchanger technology to a more universal configuration.

For more information on HyPak please contact Eric Lee.