Strained Molecules & Solar Heat Storage: NBD QC Research

Yes, certain strained molecules, such as norbornadiene (NBD) and its derivatives, can store solar energy as chemical potential and release it as heat later, with research showing storage durations ranging from hours to decades depending on the specific molecular design and catalyst used. This article explains what the research shows, how the storage mechanism works, and what practical limitations remain for using these “molecular solar thermal” (MOST) systems.

Formula

The calculator above simply converts a user-entered energy density (in kJ per kg) into the heat released per kg when the molecule reverts to its ground state. The key formula for a MOST system is:

Heat released (kJ/kg) = Energy density of the strained molecule (kJ/kg)

Where:

• Heat released is the thermal energy given off when the molecule relaxes (kJ/kg).
• Energy density is the amount of energy stored per unit mass in the strained molecular bonds (kJ/kg).

For a real system, the energy density is determined experimentally by measuring the enthalpy difference between the metastable (strained) isomer and the ground-state isomer. The calculator lets you see how much heat a given energy density corresponds to, which is useful for comparing different molecular designs.

Worked Example

Using the default values from the calculator:

1. You enter a storage duration of 10 years (this is just a label; the actual duration is set by the molecule’s half-life, not by the calculator).
2. You enter an energy density of 500 kJ/kg (a typical target for advanced NBD derivatives).
3. The calculator computes: Heat released = 500 kJ/kg.

So if a material has an energy density of 500 kJ/kg, every kilogram of the material will release 500 kilojoules of heat when triggered to revert. For comparison, the theoretical maximum for NBD is around 1,000 kJ/kg, but practical systems are often lower.

Background: What Are Strained Molecules?

Strained molecules are compounds whose atoms are forced into geometries that are energetically unfavorable. The strain is like a coiled spring: energy is stored in the distorted bonds. Norbornadiene (NBD) is a bicyclic hydrocarbon with a strained double bond. When it absorbs ultraviolet light, it undergoes a photoisomerization to quadricyclane (QC), a more strained, higher-energy isomer. This reaction is reversible: QC can revert to NBD, releasing the stored energy as heat.

The NBD–QC pair has been studied since the 1960s as a candidate for solar energy storage. The key advantage is that the storage is chemical, not thermal: the energy is locked in the molecular structure, so it does not leak away as heat loss over time, unlike a hot water tank or molten salt. This allows for long-term, lossless storage at ambient temperature.

How NBD–QC Storage Works

The process has three steps:

1. Absorption: NBD absorbs a photon of ultraviolet light (wavelength around 300–400 nm) and undergoes a [2+2] cycloaddition to form QC.
2. Storage: QC is metastable at room temperature. The energy barrier for the reverse reaction is high enough that QC can persist for months to years, depending on the substituents attached to the molecule.
3. Release: A catalyst (such as a cobalt or copper complex) or heat triggers the reverse reaction, converting QC back to NBD and releasing the stored energy as heat (about 90–100 kJ/mol for the parent NBD).

The energy density of the parent NBD–QC system is about 0.3–0.5 MJ/kg, which is lower than lithium-ion batteries (0.5–0.7 MJ/kg) but higher than many phase-change materials. The advantage is that the energy can be stored indefinitely until the trigger is applied.

How to Choose a Molecular Solar Thermal System

Selecting a MOST system involves trade-offs between several properties:

• Energy density: Higher is better, but often comes with a trade-off in stability or absorption wavelength.
• Storage half-life: The time for half the QC to revert spontaneously. For seasonal storage, you need a half-life of months to years. Parent NBD has a half-life of about 20 hours at room temperature, but adding substituents (like phenyl groups) can extend this to years.
• Absorption wavelength: The ideal molecule absorbs strongly in the visible range, not just UV, to capture more of the solar spectrum. Many derivatives use a sensitizer or modify the chromophore to shift absorption.
• Cyclability: The molecule should survive thousands of charge–discharge cycles without degradation. Some NBD derivatives degrade after a few hundred cycles due to side reactions.
• Cost and toxicity: The starting materials and synthesis should be cheap and non-toxic for large-scale use.

Research has focused on adding electron-donating or withdrawing groups to NBD to tune these properties. For example, a derivative with a dimethylamino group and a cyano group shows absorption up to 450 nm and a half-life of several years, but its energy density is lower than the parent.

Common Problems and Limitations

Despite the promise, several challenges remain:

• UV absorption: Parent NBD absorbs only in the UV, which is about 5% of sunlight. Most research aims to shift absorption into the visible, but this often reduces the energy density or stability.
• Degradation: Repeated cycling can lead to irreversible side products, lowering the usable lifetime. For example, some NBD derivatives undergo cycloreversion to other isomers.
• Energy density gap: Even the best NBD derivatives have energy densities around 0.5–0.7 MJ/kg, which is still lower than many batteries. For a home heating application, you would need a large volume of material.
• Catalyst deactivation: The release catalyst must be efficient and not poison over time. Some catalysts require high temperatures to work, which defeats the purpose.

A common misconception is that MOST systems can store solar heat indefinitely with zero loss. In reality, all molecules have a finite half-life, so there is always a slow spontaneous release. The goal is to make that half-life longer than the storage period, but it is never infinite.

Comparisons with Other Storage Technologies

MOST systems are often compared to sensible heat storage (hot water) and latent heat storage (phase-change materials). Here is a rough comparison:

• Sensible heat (water tank): Low cost, simple, but loses heat over time (hours to days) and has a low energy density (about 0.02 MJ/kg for a 20°C temperature rise).
• Phase-change materials (paraffin, salt hydrates): Higher energy density (0.1–0.3 MJ/kg), but still loses heat gradually and requires insulation.
• MOST (NBD–QC): Energy density up to 0.7 MJ/kg, can store indefinitely at ambient temperature, but requires a UV/visible light source and a catalyst for release. The material itself is reusable.

For seasonal storage (summer to winter), MOST is attractive because it does not lose heat over time. However, the cost of the molecule and the need for a solar concentrator to provide sufficient light intensity are significant barriers.

Costs and Practical Considerations

As of now, NBD and its derivatives are not produced at industrial scale for energy storage. The synthesis involves several steps, often using expensive catalysts. Estimates in the literature suggest that a commercial MOST system would cost several hundred dollars per kilogram of storage material, which is far more than water or paraffin. However, the material is reusable, so the cost per cycle could be amortized over many years.

Another practical issue is the need for a transparent container that allows UV light to reach the molecule, and a separate chamber for the catalyst-triggered release. The heat release is exothermic, so the system must be designed to transfer that heat to a working fluid (like water) without overheating the molecule.

Safety and Environmental Aspects

NBD and QC are hydrocarbons, so they are flammable, but they are not acutely toxic. The main safety concern is the exothermic release: if a large amount of QC reverts suddenly, it could cause a rapid temperature rise. In practice, the reaction is controlled by the catalyst, which can be added in small amounts to moderate the rate.

Environmental impact depends on the substituents used. Some derivatives contain halogens or heavy metals, which could be problematic. Research is moving toward “green” derivatives using only carbon, hydrogen, nitrogen, and oxygen.

One often-overlooked aspect is that the storage molecule itself is a solid or liquid at room temperature, which makes handling easier than compressed hydrogen or molten salt. But the UV light source (sunlight) is intermittent, so a MOST system would need to be coupled with a solar collector that tracks the sun.

Frequently Asked Questions

Can NBD–QC really store heat for decades? Yes, with the right substituents. The parent NBD has a half-life of about 20 hours at 25°C, but derivatives with bulky groups can have half-lives of several years. Some studies report half-lives of up to 10 years or more at room temperature, meaning that after a decade, half of the stored energy is still there. For practical purposes, that is “decades” of storage.

How much heat can it store per kilogram? The parent NBD–QC system stores about 0.3–0.5 MJ/kg. Advanced derivatives have reached up to 0.7 MJ/kg. For comparison, water stores about 0.02 MJ/kg for a 20°C temperature rise. So NBD stores about 15–35 times more heat per kilogram than water over that temperature range.

What triggers the heat release? A catalyst, typically a cobalt or copper complex, lowers the activation energy for the reverse reaction. Alternatively, heating the material to a certain temperature (often above 100°C) can trigger it. The catalyst allows release at lower temperatures, which is more useful for heating applications.

Is the molecule reusable? Yes, in principle. After releasing heat, the molecule returns to NBD and can be exposed to sunlight again to re-form QC. However, side reactions can degrade the molecule after many cycles. Research aims to achieve thousands of cycles, but current lab demonstrations often show degradation after a few hundred cycles.

Why isn’t this technology widely used? The main barriers are cost, low energy density compared to batteries, and the need for UV/visible light absorption. The synthesis of NBD derivatives is expensive, and the energy density is still too low for many applications. Research is ongoing, but commercial deployment is likely years away.

Does this work on cloudy days or at night? The charging step requires sunlight, so it only works during the day. But the beauty is that the stored energy can be released at night or on cloudy days, because the molecule holds the energy indefinitely until triggered. This is the key advantage over thermal storage.

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in t | Storage duration | years | 10 | 0
in e | Energy density | kJ/kg | 500 | 0
out heat | Heat released | kJ/kg | e | 0