Mass Thermal Storage
What is It?
The mass thermal storage subsystem allows energy to be stored when it can be generated naturally, then allows it to be used at the time it is needed. A specific example: The solar collectors will produce more energy in October than the building can use, so the extra heat energy is stored in the mass thermal storage system, where it can be used weeks later when there is less sun energy available and a higher demand for heat in the building. This is sometimes referred to as a "thermal battery". The energy input comes from the Solar Thermal Collectors and the Geothermal System. The output goes to the radiant floor system. In the winter the temperature of the water in the mass storage (for space heating) would be maintained above the indoor temperature requirement and in the summer it would be below the indoor temperature. Water is used because of its extraordinary capacity to hold thermal energy. Other common storage materials are rocks, sand and concrete.
A typical mass thermal storage subsystem includes:
- Insulated storage tanks - typically one, but multiple tanks are possible and sometimes preferred. The tanks can hold from one hundred to several thousand gallons of water.
- Domestic hot water tank and heater - this allows for the potable water to be heated to the temperature needed for hot water use in the building.
- Piping carrying the water to and from the mass thermal storage - this is often copper or pex.
- Heat exchangers (optional) - they are expensive and introduce inefficiencies, but are a common component when dealing with potable water. The heat exchanger allows for the separation of the mass thermal storage water and the potable water.
The size and number of heat exchangers is determined by the amount of energy that is needed by the building and the means for transferring the heat from the source to the use.
- Also known as: thermal storage, thermal battery
Why is it Important?
A mass thermal storage subsystem is important to a building's sustainability because it:
- Creates harmony between a natural system production of heat and the human need for heat by solving the timing difference between when heat is generated and when it is needed. The mass thermal storage system allows solar energy that is abundantly available in October, but not needed, to be used in December to heat the building.
- Allows more of the energy demand to be met through passive/ renewable sources.
- Reduces the size of the heating and cooling equipment...in this case, the geothermal system.
- Reduces the operating costs of the building by supplying the energy to heat the building and the hot water.
- Reduces the Green Garage carbon footprint.
When to Use It?
A mass thermal storage subsystem is best suited for situations where:
- The space heating demand has already been reduced through insulation of the the building envelope and domestic hot water needs have been reduced through use of low-flow shower heads and efficient appliances.
- There is adequate access to direct sun light in a location on or near the building for locating the solar thermal panel subsystem.
- There is an adequate demand for space heating and domestic hot water.
- There is significant timing differences between when the natural energy is available and when it is needed. The larger the difference in time (e.g. one month), the larger the thermal storage.
Green Garage Use of Mass Thermal Storage
The sustainability goals for the Mass Thermal Storage subsystem are:
- Meet the Green Garage space heating loads for the winter season per Energy-10 modeling results and our estimate for domestic hot water.
- Hold average winter month's heat energy = 575K BTU based on 23 Million BTU per heating season (Nov15 - Mar 15). See GG Solar Thermal Workbook
- Provide > 90% of the domestic hot water requirements.
- Design for a 50 year life of the mass thermal storage subsystem.
- Our active heating energy usage would be only 10% of an equivalent commercial building (per ASHRAE data).
- Select the most efficient mass thermal storage system = highest Heat Storage / Total Life-cycle Cost.
- Help connect the building and the occupants to the natural systems.
- 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 position the Green Garage for a net-Zero energy future.
Strategy and Conceptual Design
Our overall strategy for the mass thermal storage system is:
- Thermal Architecture
- All heat/cooling generators (i.e. solar and geothermal) connected to the storage system
- All heat/cooling consumers (i.e radiant floor and domestic hot water) can draw heat from storage. Radiant floor to draw directly from mass storage, and domestic hot water via heat exchanger. An auxiliary water heater will also be installed after the domestic hot water pre-heat storage tank to 'top off' the water temp should the mass storage temperature fall below what's needed. This is the most efficient operating design for tankless water heaters- preheating the water.
- Use one tank for space heating and cooling and one tank for domestic hot water to keep it simple and less costly for tanks and plumbing (and heat exchangers if used). May use two tanks for space heating and cooling to increase comfort and efficiency during changeover from heating to cooling season (shoulder seasons in Spring and Fall).
- Minimize the use of heat exchangers (due to cost, maintenance and inefficiencies).
- Store heat energy in the winter and cooling energy in the summer
- Size and construction of the tank must allow it to be brought into the building (obvious but sometimes overlooked).
- Place indoors versus outdoors, because heat loss indoors helps heat the building and cooling loss in the summer helps cool the building.
- Place on the ground floor slab in the side building. The tanks can enter in a large truck door (10ft x 10 ft). This is under the location of the Solar Thermal Collectors so the pipe that runs between them will be short. It is also very near the bathrooms, where most of the domestic hot water is needed.
- Should consider a floor drain near the tank.
Storage Tanks Size and Construction
- Space Heating and Cooling Tank(s)
- Storage Fluid: Water - no additive
- Thermal Capacity: > 4,000 gallons
- Winter: 3500 gals = 470cf = 2.3 mBTU: equivalent to approx 13 average winter days (almost two weeks)
- Summer: TBD
- Example: 3,500 gal = 10 ft dia x 7ft tall
- Materials: stainless steel if possible
- Insulation = R30+
- Sealed Non-pressurized
- Maximum = 160F Sustained temperatures above this significantly shorten the life expectancy of the liner.
- Minimum = 33F to prevent from freezing
- Flow Rates
- Flow Rate: Max 15g/m ... geothermal connection
- Heat Exchange - Use direct exchange - lowers cost and raises efficiency. This applies to:
- Solar Thermal Collectors System
- Geothermal System
- Radiant Floor System
- NOT - Domestic Hot Water
- Typically $2 / gallon to custom build
- Domestic Hot Water Tank
- Size: > 40 gallons (recommended to hold 2+ days of domestic hot water need)
- Materials: Standard hot water tank (design still in process)
- Insulation = R30+
- Potable water
Integration and Controls Design
The key integration and control design areas are:
- Manually converting the mass thermal storage from storing heating energy to cooling energy.
- Integrating the domestic hot water storage with the space heating mass thermal storage.
- The integration of the solar thermal collector array with the mass storage system using a drainback design.
- Monitoring and controlling the maximum and minimum temperatures for the tanks.
Key Design Criteria
- Multiple mass thermal storage tanks are connected with a flexible piping and valving arrangement to act as one storage tank system. To the extent possible, it should accommodate the future possibility for the tanks to act individually or in series depending on mode of operation; heating mode, cooling mode, or shoulder season mode (i.e. heating and cooling.)
- The tanks are not pressurized. One (1) water-to-water heat exchanger is used to connect the unpressurized tank to the pressurized floor radiant floor distribution system.
- The tanks will support either heating or cooling, but not at the same time. The tanks will have either hot or cold water in them. It is anticipated that the duration of the modes will be:
- Heating Mode: October - April
- Cooling Mode: June - August
- Spring Shoulder Mode: May
- Fall Shoulder Mode: September
Heating (Winter) Modes
- Occupied - Closed
- During heating modes, the mass thermal storage is manually set so that the solar thermal collectors are the main source of heat with the heated water from the panels going into the mass storage tanks.
- Heated water moves through the mass storage tanks, drawing heating radiant floor water supply from top of warmest tank and returning cooler water to bottom of tank, allowing for stratification of water temperature in tanks.
- Solar thermal collector subsystem cycles to maintain the mass thermal storage tanks within the required temperature range (90 deg F to 200 deg F). Refer to Solar Thermal Collector and Radiant Floor Heating and Cooling Integration and Controls Design for subsystems operations.
- Normal heating operation is for load to be met with only solar thermal collectors active.
- If solar collector subsystem is active and the mass thermal storage tank temperature is below its minimum heating Btu setpoint (90 deg F, adj.), heat pump subsystem may supplement the hot water input to the mass thermal storage tanks. Refer to Geothermal System Integration and Controls Design for subsystem operation.
- When mass thermal storage tank temperature sensor reaches minimum heating Btu temperature (90 deg F, adj.), first deactivate heat pump subsystem, and then allow solar collector subsystem to continue under normal heating operation.
- Occupied - Open
- Same as Occupied - Closed hours heating mode.
- Same as Occupied – Closed hours heating mode.
- Mass thermal storage tanks are equipped with pressure relief valves if required by local codes (verify requirement and if setting is dictated by code).
- Sensor S-1 located in the tank sets off an alarm if the water level gets below a minimum setpoint indicating a loss of 1% (adj.) of the water in the tank.
- Refer to Emergency Modes of operation for related subsystems for further detail.
Cooling (Summer) Modes
- Occupied - Closed
- During cooling modes, the geothermal system and solar thermal collectors provide cooling for the water in the mass storage tanks.
- Cooled water moves through both mass storage tanks, drawing cooled supply water for the radiant floor system from the bottom of the tank system and returning warmer water to the top of the tank system, allowing for stratification of water temperature in tanks.
- In-ground Annex Loops (Priority 1)
- When the thermal storage tank temperature is above the ground temp setpoint (55F, adj) pump P-1 is activated, V-1 is open, V-2 is closed and V-4 is opened to the trickle loop (closed to solar collectors) and the water in the tank is circulated in the in-ground annex 1" loops until the temperature difference between water in the tank T-2 and the water exiting the loop T-1C is less than a setpoint (2F, adj) or the tank cooling btu setpoint T-2 is reached. This can be activated any time of the day.
- Solar Thermal Collectors (Priority 2)
- The solar thermal collectors are used to cool the water in the mass thermal storage tanks by circulating the water through the collectors at night. Refer to Solar Thermal Collectors Integration and Controls Design for subsystem operations.
- Geothermal System (Priority 3)
- The geothermal subsystem operates to maintain the mass thermal storage tank at its required minimum cooling Btu temperature setpoint. Refer to Geothermal System and Radiant Floor Heating and Cooling Integration and Controls Design for subsystems operations.
- Occupied - Open
- Radiant floor heating and cooling system is deactivated and locked out from operation while in Open mode.
- During Unoccupied hours both the solar thermal collector and geothermal subsystems can be activated, if necessary, to bring mass storage tank to cooling Btu temperature setpoint (60F, adj).
- Once mass thermal storage tank temperature has reached the minimum cooling Btu temperature setpoint T-2, the geothermal subsystem is not allowed to run again until off-peak hours.
- Same as during Emergency heating mode.
- Trigger point for freezing (don't think this is needed).
Shoulder (Spring - Fall) Modes
- 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 mass thermal storage tank system is manually set to "Spring Shoulder" mode allowing the temp of the tanks to naturally drift down for a specified number of days, after which the system automatically changes to Cooling Mode.
- During this time, the hot water from the mass thermal storage tanks is used to heat the building in afternoons. This continues until the tank temperature is equal to or greater than Spring Shoulder setpoint (75F, adj), at which time the system automatically changes to Cooling Mode.
- As determined by the building operator, solar thermal collector system may be allowed to cycle in heating mode to maintain the mass thermal storage heating setpoint.
- Cooling mode-to-Heating mode change in Fall Shoulder
- The mass thermal storage tank system is manually set to "Fall Shoulder" mode allowing the temp of the tanks to naturally drift up for a specified number of days, after which the system automatically changes to Heating Mode.
- During this time, the cool water from the mass thermal storage tanks is used to cool the building in afternoons. This continues until the tank temperature is equal to or greater than Fall Shoulder setpoint (68F, adj), at which time the system automatically changes to Heating Mode.
- As determined by the building operator, geothermal system may be allowed to cycle in cooling mode to maintain the mass thermal storage cooling setpoint.
- Occupied - Open
- Same as Occupied - Closed mode.
- Same as Occupied - Closed mode.
- Same as Emergency heating mode.
Controls - Open Design Points
- Mass thermal storage geothermal subsystem backup for heating is currently written only to come on if radiant floor heating load is not met. Should it also look at minimum mass thermal storage tank temperature to come on and keep the tank at minimum, or perhaps keep the tank at some minimum temperature instead of looking directly at the load? GEOTHERMAL MAY COME ON ONCE DURING OFF-PEAK HOURS TO MEET MINIMUM BTU REQUIREMENT IN STORAGE TANKS. THAT GIVE IT ENOUGH BTU'S FOR THE NEXT DAY. ONCE IT CYCLES OFF (HAVING REACHED SETPOINT), IT IS NOT ALLOWED TO CYCLE AGAIN UNTIL THE NEXT OFF-PEAK TIME PERIOD.
- How radiant floor heating and cooling is controlled may influence how the mass storage minimum temperature is maintained. If radiant floor is single zone with outside air reset, should tank minimum temperature be equal to reset temperature? If radiant is controlled by zones, then some minimum supply water temperature will need to be established, and tank minimum may be controlled to that.
- Will radiant floor be piped up so that all zones (if there are zones) are either heating or cooling, not a mixture of both? During shoulder season, when various days could fluctuate between heating and cooling, and mass storage tanks have both available, will switchover be handled manually? Will assume yes and indicate that way for now.
- Need to work out temperature conditions during shoulder season when radiant floor return water can go directly to either the heating or the cooling tanks when supplied from the other tank to be used later on. How does this work hydraulically? Will require tanks to co-mingle and possibly some automatic valving and somewhat complex controls?
- When temperatures are favorable on radiant heating days, heating water being returned from the radiant floor system will be discharged into the cooling mass storage tank for use on cooling days. When temperatures are favorable on radiant cooling days, cooling water being returned from the radiant floor system will be discharged into the heating mass storage tank for use on heating days.
- Look at primary/secondary pumping from storage tanks.
- Use geothermal underslab loops directly to circulate with mass storage tank water to get ground / slab economizer cycle. Bypass geothermal heat exchanger unit.
- Consider adding extra ports on tank for future connections and flexibility.
Supporting Science / Experience
There was a tremendous amount of research that went into this design. Our primary sources for information were:
- Alan Rushforth from Rushforth Solar LLC. He has extensive experience developing custom thermal storage tanks for use in Hot Water Systems in apartments buildings, many that he owns. A summary of our conversation with Alan can be found on our Conversation with Alan Rushforth on April 2, 2008 page. His designs are simply elegant. He has expressed a willingness to help us build our tanks if needed.
- Bob Ramlow book ... Solar Water Heating
- Here's a summary of our Communications with Brad @ STSS. They are a manufacturer of thermal storage tanks. He did calculations for helping us with a tank. We've used some of his ideas in our design.
- Build it Solar website. Chris' excellent site on everything solar. Here's the page on designing Thermal Storage.
- Used William A. Shurcliff's - Solar Heated Buildings of North America. You've got to love this man's contribution to our movement to a more sustainable future. This is where we concluded (looking at the example buildings) that the more mass storage the better if we're trying to get to net-Zero energy. A ratio of 10 gallons per 1 sf of solar collector is a good rule of thumb.
Proposed Materials / Suppliers
- Mass Thermal Storage Manufacturers
- Alan Rushforth of Rushforth Solar LLC, designs and constructs his own custom thermal storage tanks and is willing to help others if the situation is right.
- STSS...manufacturer of large tanks in PA. They come recommended by several people.
- Pex Manufacturers
- Use insulated pex-al-pex to run/return between the panels and the storage.
The Mass Thermal Storage subsystem - Development Story page contains many images and videos documenting the process used at the Green Garage to design, build and operate our Solar Thermal Collector subsystem.
Related Internal Links
- Geo-Solar Hybrid Heating and Cooling
- Solar Thermal Collectors
- Geothermal System
- Radiant Floor
- Energy Modeling
- Super Insulated Building Envelope
- Bob Ramlow book ... Solar Water Heating is a great resource if you are starting out. Very clear, down to earth explanation of how to build a solar water heating system.
- Build it Solar website. Gary's excellent site on everything solar for the DIY-ers. Here's the web page for thermal storage suppliers on Build it Solar.
- google data...check link to solar thermal data page...23 million for winter season.
- Laurie...average building requirements
- Short Video
- Upload images onto Development Story page
- Image for top of page (Kevin's)
- email Alan
Peggy edited this page :)