|How it works [1,2,3]|
There are two forms of thermal storage:
Sensible heat storage consists in storing energy is by raising the temperature of a medium with high heat capacity, for instance water or rock. The most common form of sensible heat storage in dwellings is the use of thermal mass materials in the building structure to act as a heat store.
- Sensible heat storage
- Latent heat storage
High thermal mass standard construction materials for daily storage are discussed further in the passive design section.
The use of high thermal mass materials (most commonly water) is discussed below and was evaluated as part of this project.
Phase change materials (PCM)
Latent heat storage consists in storing energy by through phase change of the storage medium, which is usually solid-liquid.
Phase change materials have been known and investigated for several decades. The basic principle is based on the high latent heat of some specific material which allows them to store or release heat during the phase change, assuming it occurs at the right temperature.
The appropriate temperature varies according to the application.
Thermal mass can be used in dwellings to reduce the daily temperature variations by either storing heat during the day for release during the night or by cooling off the inside of the dwelling at night to reduce daily peak temperature.
The use of thermal mass is particularly effective in climates when there is a fairly large temperature difference between day and night. Usually dry climates are more appropriate for the use of thermal mass than humid climates where night temperature does not drop as much. See passive design section.
The use of excess electricity to create ice for day cooling is also a form of daily heat storage which is used in a number of place, for instance in Japan.
Night T must drop below the MP of the PCM which reduces the range of materials which can be used.
Reports in literature mention potential reductions of internal peak temperatures of 3 to 4 degrees in best cases, however attention should be paid to what the reference construction was made of, some PCM studies used standard construction reference design which is not necessarily the most efficient.
It is not clear at present whether PCM based construction materials would provide a major benefit as compared with the most efficient thermal mass options based upon sensible heat storage. We did not model the use of PCM in this project as it was considered complicated, we limited our simulation work to the use of standard thermal mass construction materials.
Seasonal thermal storage refers to 'long term' heat storage. By long term we mean the period of the whole process of storing and releasing heat will be one year.
The main benefit of heat storage is to have the ability to either gain access to heat when you normally would not have it, or store excess heat that would otherwise be lost, or both. For instance, solar water heating systems tend to generate a lot of excess heat during summer months in particular, and in a large proportion of European climate the surplus heat can exceed the actual heat utilised immediately. There are obvious benefits to store (part of) that heat as it would make the heating system more efficient and if used in large scheme it would reduce the global fossil fuel consumption for heating.
Seasonal heat storage however is only useful if the solar heating system (or other renewable source in place) generates a reasonable proportion of the heating demand required in the winter.
Seasonal storage could be possibly achieved using phase change materials (see information on PCM below). The main problems are the costs of these materials as well as some properties regarding stability.
Sensible heat storage can be used using water as a storage medium (high heat capacity) or large amounts of concrete. Underground well insulated tank would be the most efficient option but may be cost prohibitive in most cases.
It is also possible to use natural rock formations as was done in a number of locations in Sweden for instance. These are usually expensive in terms of installation unless they are performed on large scale.
Generally speaking, seasonal thermal storage is not common for individual dwellings whether using sensible or latent heat properties, or both.
Benefits of PCM versus sensible heat storage materials
There are several disadvantages with sensible heat storage:
PCM on the other hand have some key requirements:
- The energy cannot be stored or released at a constant temperature.
- The method tends to be also less efficient because it takes usually more energy to change a solid or crystalline structure into a liquid as compared with raising the temperature of a material by a reasonable and practical amount. PCM allows reducing significantly the storage high temperature requirement and limit heat losses.
- Significantly larger quantities of storage medium may be required for sensible heat systems in comparison to latent heat systems to store the same amount of energy.
- As an example, the heat capacity of concrete is approximately 1kJ/kg/K compared with the latent heat of calcium chlorine, which during phase transition, can store or release 190kJ/kg. You therefore need a 190K temperature variation to store the same energy in the concrete for the same mass of material, and without taking into account losses.
The material must have the right temperature of phase transition.
The MP must be high enough so that heat recovered at (or below) MP can be used efficiently. In the case of space heating, heat release needs to take place at a temperature within comfortable range.
The output of the heating system must also be higher than the MP of the material used for thermal storage.
Water for instance crystallizes at 0C where it will discharge a given quantity of heat. Heating ice to melt it will absorb the same amount of heat. Despite the high latent heat (335 kJ/kg) and density (917 kg/m3) of ice, latent heat storage and discharge for water / ice at 0° C is not adequate for heat storage in dwelling as this temperature is too low.
Sodium sulphate is a known PCM, it melts at 32C, its latent heat is 252 kJ/kg and the density is 1495 kg/m3.
The main advantage obviously here is the melting point. A temperature of 32C is closer to the typical summer temperature, slightly higher than the comfort range of about 24-28C. The material could be used for use within building materials as thermal mass or for seasonal storage, assuming the application would require heat back from storage at a temperature no higher.
There are various reports of the use of PCM for seasonal storage but none of these technologies are currently mature and the costs are high in comparison with the potential benefits.
Types of PCM
Typical phase change materials can be divided into three categories:
- Organic compounds
- Inorganic compounds
- Mixed compounds (eutectics)
These are mostly paraffin type materials and the melting temperature of the material depends on its composition.
- Wide range of melting points available
- No toxicity
- No corrosive effect
- Non hygroscopic
- Chemically stable
- High latent heat
- Congruent melting
- Negligible super-cooling
- High cost which has led some researchers to investigate technical grade organic
- Low density
- Low thermal conductivity
- Density variation upon melting (the material detaches from the container when freezing, which affects heat transfer efficiency).
- Octa-decane, which melts at 28C and has a latent heat of 244 kJ/kg
- Eicosane which melts at 36.7 C and has a latent heat of 247 kJ/kg
- Most types of paraffin which melt at 45-64 C and have a latent heat of ~ 190-210 kJ/kg (Source- CIBS Guide)
- Paraffin RT25, RT30, RT40, RT50 (Rubitherm GmbH)
These are mainly salt hydrates or molten salt. Some of the better known examples include:
Sodium sulphate decahydrate which melts at 32.4 C and has a latent heat of 252 kJ/kg
Calcium chloride hexahydrate melts at 29 C and has a latent heat of 187 kJ/kg
(Solid and liquid heat capacities are respectively 1.5 kJ/kg.K and 2.1 kJ/kg.K)
Salt Hydrate S27 (Cristopia)
Salt Hydrate STL47 and STL52 (Mitsubishi Chemicals)
- Low cost (in comparison to organic)
- High latent heat
- High thermal conductivity
- Wide range of melting points from 7-117° C
- High specific density, typically 1.5-2.5
There is a lack of validation data over long periods and multiple cycles for inorganic materials. A number of the literature reporting work on storage using inorganic materials has assumed there was no decomposition.
- Must be sealed (to prevent loss of water when subjected to long-term thermal cycling)
- Long term stability is a requirement for any thermal storage system. A large majority of the materials that have been investigated historically are not stable due to mostly 2 reasons:
- Chemical decomposition
- Decomposition over time due to incongruent melting leading to separation into an aqueous phase and a solid phase which settles at the bottom of the container and the process is irreversible.
- Problems of corrosion with container
- Cost issues:
- Material cost
- Encapsulation cost
In comparison, paraffin generally do not exhibit either of the stability issues.
Mixed materials - Eutectics
A eutectic material is a combination of two or more compounds (either organic or inorganic).
The main problem with these compounds is the cost, actually some two or three times greater than organic or inorganic.
Most have adequate properties with a MP of 50-70C and latent heat of 180-210 kJ/kg
Examples include Palmatic acid (organic), Mystiric acid (inorganic), Stearic acid (organic/inorganic)