Difference between revisions of "Geothermal System"

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What is It?

Geothermal systems use the constant temperature of the earth, (about 52F in Detroit, MI), to provide renewable heat in the winter when our our outdoor temperatures are much lower, and renewable cooling in the summer when our outdoor temperatures are much higher. This greatly improves the efficiency of the systems. Typically, they are 3 to 5 times as efficient as a normal air conditioner and heat pump. Long loops of tubing placed deep under the ground with a fluid flowing through them are what allow the geothermal system do this "free" heat exchange with the earth.

The main components of a geothermal system are:

• Ground loops...horizontally or vertically installed. They are closed loop in a city situation.
• Heat pump...provides any additional heating or cooling that is needed over and above that provided by the ground energy.
• Heat exchangers...for distributing the heating/cooling via water (radiant) or air (forced air).
• Connection to the HVAC distribution system...radiant tubes or forced air ducts.

Geothermal units can also help preheat water that goes into the domestic hot water supply.

• Also known as: ground source heat exchanger, ground source heat pump

Why is it Important?

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

• Directly connects the building and its occupants to the earth's underground relative warmth and coolness. It works with this natural state, not against it.
• Demonstrates an "appropriate" use of technology (only after the natural systems are unable to meet the needs).
• Includes renewable, high-efficiency components that reduce carbon footprint.
• Reduces energy operating costs because of ultra low energy usage.

When to Use It?

It is appropriate to use geothermal systems when:

• The building envelope has already been improved to reduce the heating and cooling demand.
• While easier to do during new construction, it is possible to do this when renovating existing buildings.
• You have some open land to place the ground loops in the ground. Minimum of 50 ft x 50 ft.

Green Garage Use of Geothermal System

Sustainability Goals

The sustainability goals for the geothermal 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 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

Geothermal Strategy

The major elements of our geothermal strategy are:

• Geothermal is the heating and cooling source of last resort. All passive means will be used first.
• We selected geothermal over other options because one piece of equipment can meet heating, cooling and dehumidification needs.
• Connect the geothermal system directly to the mass thermal storage system. This eliminates the need for sizing the geothermal system for peak hours because the energy will be drawn off the thermal storage.
• Geothermal is connected to both the heating and cooling system and the moisture control system. It's used both for heating and cooling.
• Put the passive and active components in series with the passive components first. Only after the passive components (e.g. solar thermal collectors) cannot meet the needs does the active turn on and meet the "net" remaining requirement. This reduces the size requirements for the active equipment (e.g. geothermal system). It also runs less frequently since it's second in line to the natural system. Both of these reduce energy usage and environmental impacts.
• Select the highest-efficiency active methods available (e.g. geothermal system)
• Make sure we are addressing moisture in every component (i.e. latent energy).
• Running the system in a "Top it Off" mode. Not letting the temperatures fluctuate by more than +/- 2F day and night. This keeps the geothermal unit from running the heat on resistant heat mode which is very energy inefficient.

Conceptual Design - Heating and Cooling

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 required 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.

Conceptual Design - Moisture Control

The geothermal system is also part of the Earth Tubes - ERU - Geothermal Air Exchange sub-system that is an integral part of the Moisture Control system. The geothermal system is last resort to remove moisture from the system. It would do this via the heat pump / compressor, which acts as a dehumidifier using the forced air component of the geothermal system. Please see Moisture Control pattern page for more details.

Loop Design

• Location of the loops: backyard; the front parking lot is an alternative
• Type of Loop: vertical - closed loop
• Number of Loops: To be determined by further design work

Integration and Controls Design

Overview

Integrating the geothermal system with all these components requires significant design effort. Some of the controls will be manual and some will be automated. The key integration and control 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 changing of the thermal storage from winter heating mode to summer cooling mode would likely be done manually.
• Tying the geothermal into the Hybrid Ventilation System downstream of the ERU.
• Integrating the geothermal into the Moisture Control and Hybrid Ventilation System. Automating the moisture control with all other components.

Key Design Criteria

Heating (Winter) Modes

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

• Occupied - Closed
  • Mass Thermal Storage Tank (Priority 1)
    • During heating modes, when the solar thermal collector subsystem is active and the mass thermal storage tank(s) temperature sensor T-2 minimum heating Btu temperature setpoint is not met, the heat pump system packaged controls are activated and the system is allowed to operate to reach that minimum heating Btu temperature setpoint (90 deg F, adj.) It starts it heat cycle at start time (4pm, adj) every day and runs until minimum heating Btu setpoint is reached.
    • Hot water is circulated from the heat pump system to the mass thermal storage tank during heat pump system operation by pump PP-2 with valves to tank open.
    • When the mass thermal storage tank temperature sensor T-2 reaches minimum heating Btu setpoint (90 deg F, adj.), the heat pump system is deactivated.
    • Heat pump system only runs when required to meet the "minimum heating BTU" demand.
    • The strategy is to meet only the demand that the solar thermal collectors can not meet. To achieve this the heating cycle for the tanks has a set start time (4pm, adj) to allow for the solar thermal collectors to do as much work as they can. It also allows the heat pump to use some of the heat from the day to help it create heat (versus running in the middle of the night when ambient temperatures are lowest.) It continues to run until the mass thermal storage tank temperature sensor T-2 reaches minimum heating Btu setpoint (90 deg F, adj.)
  • De-humidification Coil (Priority 2)
    • During daytime occupied hours, there should be a capability to have the heat pump send heat to the de-humidification coil to heat the air to a minimum air temperature (68F, adj) when the ventilation system is activated by occupancy requirements.
  • Note these sequence of operations require the Earth Room
    • On days when it is anticipated that the heat pump will operate and outside air conditions are more favorable than Earth room temperature or when stratification within the Earth room is adversely affecting heat pump efficiency, chimney fan will operate to condition Earth room and thermal mass in room to improve heat pump operation (efficiency). Outside air will be drawn in through the Earth Tubes, pulled across the Earth room thermal mass elements, and exhausted through the chimney.
    • Chimney fan will activate based on predictive weather forecast, outdoor air temperature, and Earth room temperature.
    • Chimney fan will deactivate when Earth room mass representative temperature sensor equals Earth room temperature.
    • In heating mode, air will be drawn from low openings into the room to draw heat down from the ceiling to replace the colder stratified air along the floor.
    • Refer to Hybrid Ventilation System Integration and Control Design for control of ERU booster coil for ventilation system and moisture control.

• Occupied - Open
  • Same as Occupied - Closed heating mode...except the de-humidification coil would be deactivated.

• Unoccupied
  • Same as Occupied - Closed heating mode.

• Emergency
  • By design, the heat pump system is an emergency backup system to the solar thermal collector subsystem in the event that it is not able to provide enough heating. Where it may be necessary, flexible piping arrangements are provided so that the heat pump system may be operated with a direct connection to the radiant floor system in a manual emergency operation mode to prevent the building from freezing.
  • If pump is called to run and it does not run as indicated by its status monitoring point (current sensor), an alarm is generated in the controls system.
  • Refer to manufacturer's operation and maintenance information for emergency information on controls.

Cooling (Summer) Modes

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

• Occupied - Closed
  • De-humidification Coil (Priority 1: from 5am - 7pm on-peak, adj)
    • Heat pump system packaged controls allow the heat pump, in cooling mode, to send cool water to the de-humidification coil when the indoor humidity is higher than maximum setpoint (50%RH, adj.) The heat pump turns off when the indoor humidity setpoint is reached. Should allow for a timed manual override.
    • Refer to Hybrid Ventilation System Integration and Control Design for control of geothermal coil for ventilation system and moisture control.
  • Mass Thermal Storage Tanks (Priority 1: from 7pm - 5am off-peak, adj)
    • During cooling modes, when the mass thermal storage temperature sensor T-5 minimum cooling Btu temperature setpoint is not met, pump PP-2 cycles on and the heat pump system packaged controls allow the heat pump to operate for one cycle in cooling mode during off-peak hours to reach that minimum cooling Btu temperature setpoint (68 deg F).
    • When mass thermal storage temperature sensor T-5 reaches the minimum cooling Btu temperature setpoint (68 deg F, adj.), the heat pump system and pumps are deactivated by the packaged controls and are not allowed to run again until the next off-peak period, if needed.
    • Heat pump system only runs when required to meet the "minimum cooling Btu" demand.
    • Priority is to meet this demand in off-peak hours. Only when it cannot be met in off-peak hours is it allowed to run during on-peak periods.
  • These sequences only apply if the Earth Room is available
    • On days when it is anticipated that the heat pump will operate and outside air conditions are more favorable than Earth room temperature or when stratification within the Earth room is adversely affecting heat pump efficiency, chimney fan will operate to condition Earth room and thermal mass in room to improve heat pump operation (efficiency)similar to Occupied - Closed heating mode.
    • In cooling mode, air will be drawn from high openings into the room to draw stratified warmer air from the ceiling to replace it with the cooler air from along the floor.

• Occupied - Open
  • Same as Occupied - Closed except the de-humidification coil capability is disabled.

• Unoccupied
  • During Unoccupied hours when on-peak electrical rates apply, heat pump subsystem is not enabled to run.
  • During Unoccupied hours when off-peak electrical rates apply, heat pump subsystem is activated for one cycle, if necessary, to bring mass storage tank to minimum cooling Btu temperature setpoint. After this, the heat pump is not allowed to cycle again until the next off-peak period begins.

• Emergency
  • Refer to manufacturer's operation and maintenance information for emergency information on controls.

Shoulder (Spring - Fall) Modes

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

• Occupied - Closed
  • During the shoulder season, the mass thermal storage tanks are manually set so that the mass thermal storage tank system switches from heating mode to cooling mode in the spring or cooling mode to heating mode in the fall.
  • Heating mode-to-Cooling mode change in Spring Shoulder
    • The triggers associated with the mass thermal storage tanks are deactivated.
    • The de-humidification coil operates as in Cooling Mode above.
    • The heat pump is capable of meeting cooling or heating demand from the radiant floor system directly by by-passing the thermal storage tanks. This is done using the heat pump package control system connected to the thermostats.
  • Cooling mode-to-Heating mode change in Fall Shoulder
    • The triggers associated with the mass thermal storage tanks are deactivated.
    • The de-humidification coil operates as in Cooling Mode above.
    • The heat pump is capable of meeting cooling or heating demand from the radiant floor system directly by by-passing the thermal storage tanks. This is done using the heat pump package control system connected to the thermostats.

• Occupied - Open
  • Same as Occupied - Open hours Cooling Mode.

• Unoccupied
  • Same as Unoccupied hours Cooling Mode.

• Emergency
  • Same as during Emergency Cooling Mode.

Controls - Open Design Points

1. Need to add underslab PEX loop to sequence somewhere.
2. Confirm strategy for on-peak / off-peak electrical rates. Idea is to only run geothermal during on-peak when actual load demand exists and do thermal storage tank "top offs" during off-peak times. Includes letting temperature float potentially more than the +/- 2 deg F (used 5 deg F max in winter and no limit in summer) during unoccupied on-peak hours (minimal time). Could always change for extreme weather if temperature gets out of hand.
3. Determine if Altherma controls are capable of doing this, or if additional controls are required. Need to find out if Altherma has auxiliary contact that could be controlled by Energy Management System (EMS).

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

Proposed Materials / Suppliers

• Geothermal Units
  • Two Units
  • Loops
  • Location and Footprint

• Water Furnace Geothermal Units a major supplier of geothermal units. We have experience with these.
• DTE Metering
  • DTE Rate Book
  • DTE Geothermal Rate

Development Story

The Geothermal 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

• Hybrid Ventilation System
• Moisture Control
• Mass Thermal Storage
• Radiant Floor Heating and Cooling
• Energy Modeling
• Human Comfort Zone
• Super Insulated Building Envelope

Resources

• Wikipedia entry for Geothermal heat pump ... good summary
• Water Furnace Geothermal Units a major supplier of geothermal units. We have experience with these.
• Earthlinked Geo-thermal only need short loops!
• Ground Loop design guide from Water Furnace
• Open loop well systems
• Do it yourself Geothermal Kits

To Do's

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

Joe done

Peggy edited this page :)