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Seasonal Underground Thermal Energy Storage


Underground Seasonal Thermal Energy Storage (USTES) and Vacuum Tube Solar Collectors Combination

How can Seasonal Thermal Storage save money and reduce the cost of your Solar Water Heating Project for both Space Heating and Domestic Hot Water Heating? Many say it does not work, here's why it does and why and when it does not....!

1- Introduction

Solar Energy is the most abundant renewable energy in our planet, however one of the disadvantages of solar energy is that it's available when it's less needed. We have more sunny hours in the summer than in winter in most Canadian Cities, which make any solar system (Whether PV Panels, Evacuated Tube Solar Collectors, Solar Air Heaters, etc...) oversized for summer, when designed to cover winter demand, and very expensive for a regular middle class Canadian Family.

The ideal scenario will be to store the excess produced energy in the summer (when demand is low) and be able to use it in winter (when demand is high). That was and still not easy to achieve in places like Canada and the North Eastern portion of the US. Daily Storage, whether thermal storage with water tanks or electricity storage with DC batteries is very common in North America, however the stored energy can not last more than couple of days.

2- Renewable Energy Annual Load Profile (KWh generated per month)

To show the discrepancy between renewable energy supply and building's energy demand, we have simulated both the heating demand for a typical Ontario home (an old project that we recently finished designing) and the supply of the thermal heat of Vacuum Tube Solar Collectors that will be installed on the project's roof.

House annual Heating demand (DHW heating and hydronic space heating) is 14 595 KWh. An evacuated tube solar collector, in the project's geographic location will generate 2 051 KWh/year. So by doing simple math, the project's requires 7 to 8 Evacuated Tube Solar Collectors to fully cover the demand.

Our Vacuum Tube Solar Collectors have a winter thermal efficiency of 38-39% and a summer thermal efficiency of 41-42%.

The power output (in KW or Btu/hr) per unit Area of one Panel is pretty steady all year long, it ranges between 0.22-0.28 KW/m² when panel is exposed to the sun at its optimal tilt angle for winter performance (57 degree for Sudbury (ON) as shown in GRAPH 1 on the right).

Power Output tells half of the story, since the longer a panel is exposed to sun's radiation the more thermal energy in KWh or BTU is able to produce.


In most Canadian and Northern US cities, we have more sun exposure between May and September than in the period between October and April.

The Graph on the right is for the Number of Sunny Hours for the city of Sudbury (Ontario).

It's clear from GRAPH 2 on the right, that in the month of July (Peak Sun Exposure) we have 3 times the hour of sun we have in January.

For almost steady power output from an Evacuated Tube Solar Collector we should expect more Thermal Energy (KWh or BTU) in summer than in winter due to longer summer sun exposure.


The Generated Monthly Thermal Energy by a Vacuum Tube Solar Collector is the product of power output per unit Area of one Panel times  the number of sun exposure hours of the month


GRAPH 3 explains why we have a peak for KWh generation in July. However in the months of December to March we have a significant amount of generated thermal energy that could be instantaneously used for Space Heating, Domestic Hot Water Heating or process Heating (Which is not possible with Flat Plate Solar Thermal Collectors since their thermal efficiency in Nordic Winter is close to Zero).

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Vacuum Tube Solar Collector Kit
Vacuum Tube Solar Collector Kit

Vacuum Tube Solar Collector Kit, SRCC Certified, VT58 SERIES - Complete with Manifold, Frame, Tubes and 45⁰ Frame Kit

$ 715.00 CAD
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$ 7,050.00 CAD
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2.1- Vacuum Tube Solar Collectors - 30 Tubes Each - Qty=8

As you can see in the histogram on the left, Vacuum Tubes peak thermal energy generation is between May and September when heating demand is at its lowest. By installing 8 x  Evacuated Tube Solar Collectors, the project will require a source of backup heat in winter to compensate for discrepancy and will be forced to dump heat in summer since demand is almost absent (the only summer demand is DHW heating).

Without Underground Seasonal Thermal Energy Storage, 55% of produced thermal heat will be dumped to the environment and 38% of annual heating demand will have to be procured with conventional source of heat (in this project, it will be gas boiler).

2.2- Vacuum Tube Solar Collectors - 30 Tubes Each - Qty=24

By increasing the number of Panels, from 8 to 24, to eliminate the 38% backup heat required in winter, will exponentially increase the cost of the investment as well as the amount of thermal energy produced in the summer season that will end up being dumped.

As we conclude from the histogram on the left, Without seasonal thermal storage, we require 24 x  Evacuated Tube Solar Collectors instead of 8, to fully cover the winter heating demand. That means that project will cost at least three times with many more heat to throw to the environment during peak summer time.

3- Available Seasonal Thermal Storage Systems

The most commonly know seasonal thermal storage methods are the Sensible thermal Storage, Latent Thermal Storage (Phase Changing Materials) and Chemical Thermal Storage. Chemical Thermal Storage will be addressed in future blog. Latent Thermal Storage (Phase Changing Materials) have promising potential, however the cost is still high and does not justify its usage in light commercial or residential projects. 

When properly designed and installed, Sensible seasonal thermal storage for heating is the most affordable long term thermal storage system. Since Space heating and domestic hot water heating are purely sensible processes, that means energy is transferred from heating source (Solar, Geothermal, Air to Water Heat Pump, Boiler, etc...) by changing water temperature without changing the phase of the water. Selection Criteria for Sensible Storage Method are as follows:

3.1- Storage Medium such as sand, gravel, rock, bricks, water, etc....
3.2- Storage Medium Location relative to heat production and Heat consumption sources.
3.3- Heat Production Source Temperature Constraints.
3.4- Heating system operating temperatures and thermal properties of the building's envelope.
3.5- Heat Exchanger Design and properly calculated Heat exchange area.

3.1- Storage Medium

Gravelly Earth

Storage Medium shall be selected based on its affordability (availability near project's site), ability to absorb heat and dissipate heat. The main purpose of storing heat (when it's less needed) is to be able to use it later (when it's more needed).

Storage medium shall have high Specific Heat (KJ/(KG.K) as well as High thermal conductivity.  Higher Specific Heat will allow Medium to absorb more heat per unit volume and Higher thermal conductivity will allow fast dissipation of heat to thermal storage battery (from Heat Source or to Building's heating system).

Heat Stored in Storage Medium is no more than the increase of Medium's Internal Energy. According to the second law of thermodynamic, Internal Energy is defined by:

Internal Energy Change (KJ) = Density (Kg/m³) x Volume (m³) x Specific Heat (KJ/(Kg.°K) x Temperature Increase of Storage Medium (°K).

The amount of Energy to be stored is the amount of thermal energy produced by heat source and not used (such as thermal heat produced by solar thermal panels in the summer season when heating demand is non existent). The Seasonally stored energy should be equal to the internal energy change per season of the storage medium. Once Building envelope has been defined by architect, Annual Energy Consumption can be accurately calculated by simulation. Temperature increase of storage medium is limited by the undisturbed ground temperature (location dependent) and the maximum fluid temperature the heat source can provide.

When using Liquid Source Heat Pump as a heat source, water temperature produced in the summer (form most residential heat pumps) is limited to 110-120°F. For a place where undisturbed ground temperature is 45°F, Storage Medium temperature increase will be in the range of 55-65°F. But When using Vacuum Tube Solar Collectors are used as a primary heat source, Produced Fluid temperature can go up to 160-170°F which gives us a Storage Medium Temperature Increase of 160-170°F - 45°F = 115-125°F.

The higher the fluid temperature the heat source can provide, the smaller is the Storage Medium Volume. From the internal energy formula, we can conclude that what differentiate a storage medium from another is the product of two constants: Density and Specific Heat, it's simply called Volumetric Thermal Capacity 

MaterialDensity (Kg/m³)Specific Heat (KJ/(Kg.°K)Volumetric Thermal Capacity (10³ x
KJ / (m³ x °K))
Gravelly Earth205018403.77

Source: Norton (1992).

3.2- Storage Medium Location relative to heat production and Heat consumption sources

Since the purpose of seasonal thermal storage is to keep heat from high production / low demand season to be used in low Production / high demand season, Solar thermal Energy Storage pit shall be design to minimize seasonal heat loss.

Underground  (below the Slab of Heated Building) offers a steady temperature for storing heat and having a building on top of the thermal pit minimizes heat loss in winter to the outdoor air. Thermal Pit Walls shall also be insulated as well as it's bottom and top. Insulation thickness and thermal resistance shall be determined by annual energy simulation taking into consideration Below Ground Temperature and Expected Seasonal Energy Demand/Storage in the pit.

Thermal Pit Bottom Level is constrained by Water Table level in the geographic site location. We recommend having at least a foot clearance between the bottom of the pit and the shallowest water table level. Without Clearance and even when thermal pit's bottom is insulated with Styrofoam, Water an still infiltrate and flush out a significant amount of stored heat.

3.3- Heat Production Source Temperature Constraints


Maximum Temperature of Heat Production Source influences the size of  underground thermal pit, heat exchanger design and the most important the available heat transfer fluid temperature for space heating .

When using a liquid source heat pump , that has a 110°F maximum condenser supply temperature, to dump heat into the thermal pit, it requires a thermal pit twice the volume of another pit heated by Vacuum Tube solar collector with a supply fluid temperature of 170°F.

Conclusion: The higher the heat source supply temperature, the smaller is the size of underground thermal energy storage pit.


3.4- Heating System Operating Temperatures and Thermal Properties of the Building's Envelope

Required Maximum Heat Transfer Fluid Temperature (at Peak Outdoor Winter Conditions) determine the minimum Temperature of the Underground Thermal Energy Pit. Highly Insulated/Air tight Building's Envelope require lower supply Air to Water space heating temperatures. Lower Space Heating fluid supply temperature means smaller heat exchange area for the pit embedded underground heat exchanger, smaller storage medium volume and lower project's cost.


Energy Efficiency does matter...!

Chart on the left shows the infloor heating loops average water temperature as a function of heating demand per unit area (Btu/hr.ft²).

The more energy efficient the envelope is, the lower is the heating demand as well as the infloor heating loop temperature.

Underground Thermal Energy Storage Pit Size Does Matter: The smaller the better (Cheaper, more Sustainable and Least Intrusive).

3.5- Heat Exchanger Sizing: Heat Transfer Area, Thermal Resistance and Friction Loss

Referring to the second and third law of thermodynamics as well as Fluid Mechanics momentum conservation, Heat Exchanged between two medium (such as Gravelly Earth and Water or Gravelly Earth and Air) depends on:

1- Heat Transfer Area between the two mediums.
2- Thermal Resistance of Heat Transfer Area.
3- Design Temperature drop between heat source and heat sink (no more than the difference between underground pit average temperature and heat transfer fluid supply temperature).

Heat Transfer is the product of Heat Transfer Area, Temperature Drop between heat source and heat sink divided by the thermal resistance. The greater the heat transfer area is, the smaller is the temperature drop between heat source and heat sink. In order to maximize heat transfer, Thermal Resistance shall be reduced to the minimum. Thermal Resistance is driven by the fluid velocity inside the heat exchanger (air for forced air heating system and water for hydronic heating system). The higher the fluid velocity, the lower is the thermal resistance however higher fluid velocity increases friction loss in fluid conduits. A tradeoff shall be calculated by heat exchanger designers in order to balance heat transfer and friction loss.

Heat Exchange Area is a determinant factor for the success of any underground thermal storage. They are the reason behind the failure of more than two third of Seasonal thermal storage projects. Smaller than needed heat exchange areas lead to a larger temperature difference between heat source (underground thermal energy pit) and heat sink (either space heating water and air). That will lead to non useful low temperature hot heating fluid, which might trigger the start of backup heat.

For hydronic heating system, Heat Exchanger in underground pit could be simply made of PEX or HDPE (High Density Polyethylene Pipes). As explained above Heat Transfer Fluid shall be circulated at the optimal velocity that minimizes thermal resistance as well as friction loss in pipes. In applications requiring low temperature thermal energy storage as well as low temperature hydronic space heating, several layers of PEX or HDPE loops shall be installed in order to maximize heat transfer area and reduce temperature drop between Thermal Pit and Radiant Heating system.

Water has a lower viscosity over glycol, so for insulated underground thermal pit we recommend using glycol as heat transfer fluid for higher thermal efficiency, lower pumping power and lower cost.

Capillary Tube
underground pipe

Some Space Heating systems use air as their primary heat transfer fluid. Here the same laws of Thermodynamics and Fluid Mechanics apply. Since Air thermal conductivity is lower than water thermal conductivity, underground heat exchangers for air require larger heat transfer area than water and more pumping power (since a much larger volume of fluid is required for heating per unit area).

Despite the above, there was many successful forced air heating underground pit that were built.

One of the advantages of underground air heating system, is that in building with large fresh air flow requirements (such as in green houses) fresh air can be mixed with recirculated air prior of entering the underground pit without the need of preheating the outside air.

4- Conclusion

Underground Seasonal Thermal Storage when combined with Medium Temperature Renewable Energy (such as Vacuum Tube Solar Collectors) has a promising potentials. Proper design and knowledge of building's energy demand is a key success factor for seasonal thermal storage project. Criteria such as Annual Heating demand, heat source maximum supply temperature, Storage Medium Choice, Heat Exchanger design skills, etc... are the backbone of any Seasonal Sensible Underground Thermal Energy Storage.

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