Difference between revisions of "Hybrid Ventilation System"

Revision as of 17:18, 13 June 2009

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What is It?

Hybrid ventilation is a building ventilation system that integrates natural (i.e. passive) and mechanical (i.e. active) ventilation components to create an high efficiency and healthy ventilation system for a building. An hybrid system can include:

• Earth Tubes
• Energy Recovery Units (ERU)
• Natural Ventilation
• Moisture Control

The rationale for a hybrid type system is to allow the earth's natural systems (e.g. the ground or wind) to do as much of the ventilation work as possible and only when they can not meet the required ventilation levels to resort to high-efficiency mechanical systems to complete the job. We expect the assistance of mechanical system(s) would only be needed in more severe weather situations.

Highly efficient ventilation systems are an essential element for reaching net-zero energy goals in buildings, as evidenced by the work of the Passive Haus Institute and ZED Factory.

• Also known as: hybrid ventilation, passive ventilation, natural ventilation, geo-heating.

Why is it Important?

A hybrid ventilation system is important to a building's sustainability because it:

• Directly connects the building and its occupants to the earth's natural systems (e.g. natural ventilation.)
• Demonstrates an "appropriate" use of technology (only after the natural systems are unable to meet the needs).
• Includes renewable, high-efficiency, low-carbon components (e.g. earth air tubes).
• Has been shown to result in healthier indoor environments when used properly.

When to Use It?

It is appropriate to use hybrid ventilation systems when:

• The building has access to the earth's natural systems. (e.g. area of land large enough for placing earth tubes).
• The building location has extreme weather conditions (i.e. heat and/or cold).
• While easier to do in new construction, it is possible to do this in major renovations to existing buildings.

Green Garage Use of Hybrid Ventilation System

Sustainability Goals

The sustainability goals for the Hybrid Ventilation System are:

• Fully integrated natural-mechanical system to meet the air exchange requirements with 50% of the required energy coming from natural sources and 50% from high efficiency mechanical sources.
• Meet 20 cfm per person or 800 cfm for the building.
• Maintain the indoor relative humidity at 45% +/- 15%.
• Connect the building and the occupants to the natural systems.
• Ensure healthy indoor environment.
• Allow components of the system to be bypassed when they don't contribute to these goals.
• The system should be simple to maintain, adapt and control.

Strategy and Conceptual Design

Hybrid Strategy

The major elements of our ventilation strategy were:

• The hybrid ventilation system at the Green Garage has three main components:
  • Earth Tubes / ERU - Air Exchanger
  • Natural Ventilation
  • Moisture Control
• Put the passive and active components in a series with the passive first in line. Only after the passive component cannot meet the needs does the active turn on and meet the "net" remaining requirement.
• Reduction in size requirements for the active equipment. It also runs less frequently since it's second in line to the natural system. Both of these reduce energy usage.
• Select the highest-efficiency active methods available.
• Adopt a "Topping Off" strategy where we keep the indoor temperature in a tight range and the system is always running and very low levels and just topping off the heat when needed and cooling when needed. This basically eliminates any night time setbacks, because they lead to heating surges (sometimes followed by cooling surges when the sun comes out and people occupy the building.)
• Make sure we are addressing moisture in every component (i.e. latent energy).

Earth Tube / ERU - Air Exchanger Design

The Earth Tubes and Energy Recovery Unit are tightly integrated into the air exchange system. In general, the Earth Tubes and Energy Recovery Unit provide the year-round foundation for the building's ventilation. During a normal air exchange, the outdoor air will be brought in through the Earth Air tubes and will be pre-heated or cooled, then enter the Energy Recovery Unit where the outbound air will be used to heat/cool the inbound air, as well as dehumidify or humidify the air.

Winter Example:

Outdoor Air Temp: 0 F

Outdoor Air Relative Humidity: ??

Ground Temp: 45 F

Indoor Temp: 68 F

Earth Tubes - Air Temp Coming In: 0 F

Earth Tubes - Air Temp Coming Out: 23 F (Rule of thumb: air temp gain equals 1/2 of the difference between the air and ground temperatures with 100 ft of tubing.)

ERU - Air Temp Coming In: 23 F

ERU - Air Temp Coming Out: 55 F (assumes 70% efficient)

Summer Example:

Outdoor Air Temp: 90 F

Outdoor Air Relative Humidity: ??

Ground Temp: 57 F

Indoor Temp: 78 F

Earth Tubes - Air Temp Coming In: 95 F

Earth Tubes - Air Temp Coming Out: 86 F (Rule of thumb: air temp gain equals 1/2 of the difference between the air and ground temperatures with 100 ft of tubing.)

ERU - Air Temp Coming In: 86 F

ERU - Air Temp Coming Out: 80 F (assumes 70% efficient)

The powerful aspect of this design is the contributions of the Earth Tubes and Energy Recovery Unit components naturally increasing as as the building's demand for energy increases due to the very hot or cold weather. This is because as the difference between the ambient temperature (e.g. outdoor air temp) and the ground or indoor temperature increases, the energy contributions of the Earth Tubes and Energy Recovery Unit increase naturally.

Another benefit of placing the earth tubes in front of the ERU is that the the ERU is susceptible to freezing if the air temperature drops below 23F for more than two days in a row. It would be very unusual for this to happen with the earth tube pre-heating the air before it enters the ERU.

The attached chart shows the relative contributions of each component.

Natural Ventilation Design

Natural ventilation is expected to be able to assist in about 50 - 90 days per year during the spring and fall months. Clearly in the months of extreme weather, natural ventilation would make no contribution. Natural ventilation is provided by opening the building's windows. More information is available on our Natural Ventilation pattern page.

Moisture Control Design

We have attempted to control moisture in and through every component of the ventilation system. The two main areas are the dew point in the envelope systems and the control of humidity, especially in the summer. We are planning on using a pressure-based strategy to control the humidity. It is an approach that has come from the in-depth experience of one of the professionals who has contributed extensively to our Net-Zero Energy design. For more on moisture control please see our Moisture Control pattern page.

Integration Design

Integrating all these components does require significant thought. Some of the controls will be manual (even behavioral) and some will be automated. The key integration areas are:

• Optimizing the Earth Tube / ERU - Air Exchanger. Automatically determining when ERU should run and when the whole system should be bypassed because the outdoor ambient temperature / humidity is better than taking it through the earth tubes. ??? Redo this sentence.
• Designing a behavioral based system to determine the days / hours that windows can be opened so that we don't have windows opened and the ERU and/or running at the same time.
• Integrating the air distribution system to accommodate air from any source.
• Automating the moisture control with all other components.

Supporting Science

The detailed thermal calculations are shown in pages included here. We thank Laurie Catey for her great contributions to our understanding of how to work with natural systems through a better understanding of the science that describes them.

Proposed Materials / Suppliers

• The material for the earth tubes can be found on the Earth Tubes page.
• Manufacturers of the Energy Recovery Unit that we considered during this phase was SEMCO FV Series ERV.
• The natural ventilation materials can be found on the Natural Ventilation page.
• The moisture control materials can be found on the Moisture Control page.

Development Story

The Hybrid Ventilation System - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Hybrid Ventilation System.

Related Internal Links

• Earth Tubes
• Natural Ventilation
• Windows??
• Moisture Control
• Energy Modeling

Resources

• Ventilation system with heat recovery with ≥ 75% efficiency with low electric consumption @ 0.45 Wh/m3

To Do's

• Laurie's data
• Resources
• In Earth Tubes section, get numbers for relative humidity levels.
• In Earth Tubes - last point - where is attached chart?
• Upload images onto Development Story page
• Image for top of page - Kevin

Peggy has reviewed this page  ;)