Geo-Solar Hybrid Heating and Cooling

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What is It?

Geo-Solar Hybrid Heating and Cooling is a building HVAC (Heating, Ventilation, Air Conditioning) system that integrates passive and highly efficient active systems to create an ultra-efficient and healthy HVAC system for a building. The rationale for a hybrid type system is to allow the earth's natural systems (e.g. the sun and earth) to do as much of the heating and cooling work as possible, and only when they cannot meet the required heating and cooling levels are high-efficiency mechanical systems required to complete the job. We expect the assistance of mechanical system(s) to be needed in only the most severe weather situations.

A geo-solar hybrid system can include:

  • Solar Thermal Panels ... space heating and domestic hot water
  • Mass Thermal Storage ... essentially a thermal battery used for both heating and cooling
  • Geothermal system ... for backup heating and cooling
  • Radiant Floors ... used both for heating and cooling

This should only be used after the building's heating and cooling needs have been reduced through an insulated envelope. This is further discussed in our Super Insulated Building Envelope pattern. It also needs to integrate with the building ventilation system, which is explained in our Hybrid Ventilation System pattern.

  • Also known as: solar hot water, geothermal, solar heating, passive heating, passive cooling

Why is it Important?

A geo-solar hybrid 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. sun panels and earth loops)
  • 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. solar panels, earth loops).
  • Reduces energy operating costs because of ultra low energy usage.

When to Use It?

It is appropriate to use geo-solar hybrid systems when:

  • The building envelope has already been improved to reduce the heating and cooling demand.
  • While easier to do in new construction, it is possible to do this in major renovations to existing buildings.
  • The specific design of the geo-solar hybrid system components would change based on local climatic conditions and the building's heating and cooling loads. It should be noted that the design presented here is for a commercial building in a Michigan climate.

Green Garage Use of Geo-Solar Hybrid System

Sustainability Goals

The sustainability goals for the Geo-Solar Hybrid Heating and Cooling system are:

  • Meet the Green Garage heating and cooling loads per Energy-10 modeling results.
  • Our heating and cooling energy usage would be only 10% of an equivalent commercial building (per ASHRAE data.)
  • 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, and should position the Green Garage for a net-Zero energy future.

Strategy and Conceptual Design

Hybrid Strategy

The major elements of our geo-solar hybrid strategy were:

  • Solar Thermal Panels ... space heating and domestic hot water
  • Mass Thermal Storage ... essentially a thermal battery used both for heating and cooling
  • Geothermal System ... for backup heating and cooling
  • Radiant Floors ... used both for heating and cooling
  • Put the passive and active components in series with the passive components first. Only after the passive components cannot meet the needs does the active turn on and meet the "net" remaining requirement. This reduces the 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 (e.g. geothermal.)
  • Use thermal storage to:
    • Better match energy production with energy demand (e.g. solar heating).
    • Eliminate the peak requirements by allowing peak demands to be met by stored energy, versus needing to be generated on-demand.
  • Make sure we are addressing moisture in every component (i.e. latent energy).
Overall Conceptual Design

Geo-Solar Hybrid Heating and Cooling - Shown in Heating Mode (01/22/10

Solar Thermal Collectors

The Solar Thermal Collectors component captures the energy from the sun and uses it to heat water running through the panels. The panels are connected directly to thermal storage tanks, so the sun's energy is stored in the water in the tanks and drawn on by the radiant floor system when needed. Many of these collectors would be covered in the summer when the collectors generate far more heat than is required for space heating (which would be nearly zero) and domestic hot water.

A complete discussion of our solar thermal panel design is available in our Solar Thermal Panels pattern.

Mass Thermal Storage

The mass thermal storage component stores energy in water when it is generated and then releases it when needed. It is sometimes referred to as a "thermal battery." The energy input comes from the Solar Thermal Panels and the geothermal system. The output goes to the radiant floor system. In the winter the temperature of the water would be maintained above the indoor temperature requirement and in the summer it would be below the indoor temperature. We use water because of it's extraordinary capacity to hold thermal energy.

A complete discussion of our mass thermal storage is available in our Mass Thermal Storage pattern.

Geothermal System

The geothermal system is the highly efficient backup to our passive approaches to heating and cooling the building. When the natural systems are unable to meet the demands of the occupants, the geothermal system will supply the remaining 'net' heating and cooling demand. We plan to connect the geothermal system directly to the mass thermal storage. In the winter when heat is needed, it would warm the water in the mass thermal storage only when it falls below a set minimum temperature. This minimum temperature would be determined based on the amount of energy required to heat the building for, say, three days. In designing it this way, the temperature is low enough so that if there is a sunny winter day the heat energy from the solar thermal panel system can heat the water, but high enough so that if it's a completely cloudy day there's enough energy already in the water to heat the building via the radiant floor system. In the summer, we plan on having the geothermal system cool the water in the mass thermal storage system to 68 degrees so it can be used to cool the building if necessary. This would be done in the off-peak hours when the electric power is lower and there is excess capacity. Also, by shifting our demand to off-peak, our demand could not be used as a justification for building a new power plant.

Connecting the geothermal to the mass thermal storage greatly reduces the size of the geothermal system and the number of ground loops because the system does not need to be sized to meet the peak heating and cooling load in an on-demand mode. It only needs to be sized so it can produce the needed heating and cooling energy over a longer period of time (e.g. three days), not at just peak hour. This sizing of the geothermal system is more on an average hour basis versus a peak hour. This makes a huge difference, as the peak hour is typically 20 times the size of the average hour. We're currently planning on having the geothermal sized to meet the 'peak week' hour (the energy required for the peak week divided by 168 hours in a week). Heat from the geothermal system would be stored in the mass thermal storage tanks and would be drawn upon when needed by the radiant floor system.

A complete discussion of the geothermal system is available in our Geothermal System pattern.

Radiant Floors

The radiant floor system would be used to heat and cool the Green Garage building. The system would be integrated into the floor system in zones sized to meet the heating and cooling demands of the zone. A current ASHRAE study shows that these radiant floors can be used for cooling if they are kept no cooler than 68F.

For more details see the Radiant Floor Heating and Cooling pattern.

Integration and Controls Design

Integrating all the components of the geo-solar hybrid heating and cooling system does require significant design effort. Some of the controls will be manual and some will be automated. The key integration areas are:

  • The integration of the geothermal system with the mass storage system. This would be temperature controlled with the geothermal coming on only when needed.
  • The drainback system of the solar panel system is temperature controlled.
  • The changing of the thermal storage from winder heat mode to summer cooling mode would likely be done manually.
  • Covering most of the exposed solar panels in the summer to reduce the heat buildup is needed.
  • The supply-demand mixing valve for the radiant floors will control the temperature of the fluid in the radiant floor tubing (pex).
  • A small (25 gal) water heater will be installed in the domestic hot water supply line to insure proper temperatures can be maintained. Water entering this system will already be preheated by the thermal storage tanks.
  • Integrating the air distribution system to accommodate air from any source.
  • Automating the moisture control with all other components.
Supporting Science / Experience

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 Geo-Solar Hybrid Heating and Cooling 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


To Do's

  • Laurie's data
  • Resources
  • Short Video
  • Upload images onto Development Story page
  • Image for top of page?

Peggy has reviewed this page  ;) me too- Joe