Solar radiant heating with heat storage phase change materials

[Abstract] In today’s environment of paying attention to energy conservation, environment protection and comfort, solar radiant heating floor is gradually accepted by people for its comfort and safety. This paper mainly discusses the form, advantages and disadvantages of radiant heating floor, and phase change heat storage material etc. It shows that the energy storage technology of solar radiant heating floor has obvious economic and social benefits and broad application prospects.

[Key Words] Solar energy, Radiation heating floor, Shapely phase change materials, Phase change thermal storage

Introduction

Low-temperature floor radiation heating is a more advanced and comfortable form of heating, which has been widely used in western developed countries and is more widely used in China. With the development of social economy and the improvement of people’s living standards,  more comfortably radiant heating floor will be accepted and used by people. Solar radiation heating floor is a kind of heating system which uses the collected solar energy as heat source and heating the ground through coils laid in the floor.

In the process of phase change, phase change materials can absorb the heat (cold) in the environment and release heat (cold) to the environment when needed, so as to control the ambient temperature. The phase change material is combined with the building envelope’s structure to form the phase change energy storage envelope structure, which can be used to control the indoor temperature of the building. Phase change energy storage envelope structure can greatly increase the thermal storage effect of the envelope structure, reduce the fluctuation range of heat flow between indoor and outdoor, and delay the effect time (as shown in Figure 1), so as to improve the building’s temperature self-regulation ability and polish up indoor environment, realize the purpose of energy saving and comfort.

1. Solar radiant heating floor

1.1 Structure and Principle of Solar Radiant Heating Floor

Solar radiant heating floor system usually consists of solar collector, water tank, underground heating coil, water divider, water collector, circulating water pump, auxiliary heat source, etc. Among them, underground heating coil and its supporting radiant heating floor are unique in solar floor radiant heating system.

Figure 2 Solar heating system diagram

1.2 Advantages of Solar Floor Radiant Heating

First, the most comfortable heating method (feet warm and head cool)

Second, high efficiency, energy saving and low operating costs (heat transfer in low temperature, indoor temperature is low)

Third, environmental protection, hygiene and health care (radiation transfer heat, reduction of dust and flow of bacterial)

Fourth, no redundant occupied area (compared with heating with radiator)

Fifth, long service life (more than 50 years)

Sixth, reduce floor noise (insulation of insulation layer)

Seventh, wide scope of application

Eighth, household control and heat metering charge can be realized

1.3 Other New Types of Ground Heating System

Because the water temperature requirement of low-temperature floor radiant heating system is not high, it can be as low as 30℃. Its heat source can be formulated according to the different energy and economic characteristics of different places.For example, ground source heat pump low temperature hot water energy storage radiant heating floor, using low-cost electricity at night from the ground, through the power compressor working on the circulating working medium, thus providing 40-45 degrees of low-temperature hot water to the radiant floor system, phase change material goes into the ground coil to heat , heat store in the form of latent heat, heat the room in the daytime.(Fig. 3) For example, the radiant heating floor with electricity and thermal cable energy storage, the standard geothermal energy storage system is based on electricity as energy source, the low-temperature heating cable buried underground as a heating body, and the low-cost electricity at night is used to heat the phase change material through the cable, so that all the electric energy can be transformed into heat energy, and the heat can be stored in the form of latent heat, which is implemented by radiation and conduction during the day to heat the surrounding air, objects and human body.

2 Structure and Working Principle of Phase Change Heat Storage Heating Floor

Floor heating uses environmental protection energy as solar energy and low valley electricity, but these two forms of energy are not very consistent with the heating period, such as when the sun is strongest, the heating load is the smallest, and when the electricity price is underestimated, it is often in the early morning, this time only accounts for a small part of the heating period. If we want to make rational and effective use of these two forms of energy, it will inevitably involve the issue of heat storage and insulation. The low-temperature hot water geothermal system generally uses the water tank for heat storage, which requires that the heat transfer medium is water, and occupies more space in the building.Underground heat storage material is the form of heat storage which does not occupy building area.

2.1 Definition and Characteristics of Phase Change Materials

Phase Change Materials (PCM) refer to substances that change their shape with changing temperature and provide latent heat.The process of phase change material from solid to liquid or from liquid to solid is called phase change process. At this time, phase change material will absorb or release a lot of latent heat.Phase change materials have the ability to change their physical state within a certain temperature range. It is this endothermic and exothermic phenomenon of phase change materials that makes phase change materials become the focus of attention all over the world.

The application of PCM in building materials was started from 1982 by the Solar Energy Company of U.S. Department of Energy. At present, there are thousands of phase change materials were found, but not all PCMs can be used for energy storage. Ideal PCM should have the futures of high melting latent heat, high thermal conductivity and heat capacity, small deformation, non-toxic, non-corrosive, less super-cooling or super-heating. Considering people’s requirements for comfort of residential buildings, the phase change materials used in buildings also need to meet the following conditions: (1) PCM’s phase change temperature must be near the comfortable indoor temperature range, such temperature should be 18 ℃~22℃ in winter room and temperature 22℃~ 26 ℃ in summer room. PCM phase change temperature should be within this range. PCM can not be leaked from the wallboard or not deteriorate after long-term cycle , it should be compatible to building materials. Table 1 lists commonly used phase change materials for building energy storage.

2.2 Energy-saving Principle of Phase Change Energy Storage Building Materials

The application principle of phase change materials in building’s energy saving is that phase change materials are accompanied by the release and absorption of phase change heat, that is, in the process of heat transfer, the cold load in phase change materials is stored in the energy storage structure. With the decrease of outdoor temperature, one part of the stored heat is released to the outdoor, which reduces the building cold load; the other part is released to the indoor, and increases the construction’s cold load at night. According to the above theory, taking phase change energy storage structure as an example, the application of phase change materials to buildings can greatly increase the thermal storage capacity of building structures. A small amount of materials can store a large amount of heat. Because of phase change energy storage structure’s function of heat storage, the fluctuation of heat flow between indoor and outdoor buildings is weakened and the effect time is prolonged, which can reduce the buildings’ design load of heating and air conditioning system, and achieve the purpose of energy saving.

2.3 Shapely phase change materials

The shape-stabilized phase change material (PCM) is a composite energy storage material composed of phase change material (PCM), polymer support and packaging material. Because of the micro-encapsulation and support function of the polymer capsule, the PCM used as the core material will not flow out when solid-liquid phase change occurs, and the whole composite material can keep its original shape unchanged and have certain strength even after the melting of the core material. The shortcomings of common solid-liquid phase change materials are: (1) phase separation; (2)need encapsulation, which is expensive; (3) high additional thermal resistance of the container; (4) leakage of the container. The shape-stabilized phase change material can overcome these shortcomings, and the shape of the material remains unchanged during the phase change process.This kind of material has the advantages mentioned above, which reduces the cost and difficulty of packaging, and reduces the thermal resistance between phase change materials and heat transfer fluids. This kind of material has broad application prospects in the field of building HVAC and building materials.

2.4 Classification of Phase Change Materials

Phase change materials can be divided into four categories according to the change ways of phase state: solid-liquid phase change materials, solid-solid phase change materials, liquid-gas phase change materials and solid-gas phase change materials. Although the latent heat of phase change accompanied by liquid-gas and solid-gas transformation is much larger than that accompanied by solid-solid and solid-liquid transformation, it is difficult to use in practical engineering for the generation of gas during liquid-gas and solid-gas phase change process makes volume change a lot.

Organic solid-liquid phase change energy storage materials mainly include aliphatic hydrocarbons, fatty acids, alcohols and polyenols etc. Their advantages of it are not easy to occur phase separation and super-cooling, less corrosiveness, high latent heat of phase change, the disadvantage is easy to leak. At present, aliphatic hydrocarbons and polyols are widely used. Phase change materials have high thermal storage capacity and good thermal stability, but they are easy to leak when they reach phase change temperature, so they need to be encapsulated in containers.

Organic solid-solid phase change energy storage materials store and release energy through the transformation of material crystalline form. They have the advantages of small volume change, no leakage, no corrosion and long service life in the process of phase transformation. They do not require high technology for containers’ materials and manufacturing, and their latent heat of phase transformation is in the same order of magnitude as solid-liquid phase change materials. At present, there are three kinds of solid-solid phase change materials with economic potential: polyols, macromolecule and layered perovskite.

2.4.1 polyols

Polyols phase change materials mainly include pentaerythritol (PE), 2.2-dicarboxyl-propionic acid (PG), neoglutaric acid (NPG) tricarboxymethyl alkane (TMP), tricarboxymethyl aminomethane and so on. It is generally believed that the solid-solid phase change materials of polyols are layered and low symmetrical crystals at low temperatures. The layers are connected by hydrogen bonds formed by hydroxyl (-OH). When the temperature rises, the solid-solid phase transition occurs. At the same time, the intermolecular hydrogen bond is broken, and the disorder of molecular rotation and vibration is introduced to absorb energy. Two or three polyols phase change materials can be mixed in different proportions to form a "co-fusion alloy" to adjust the phase change temperature and latent heat.

Polyols phase change materials have long-term chemical stability, higher phase change heat and less under-cooling. Moreover, the binary system of polyols can effectively broaden the application scope of polyols and make them become good phase change energy storage materials at low temperature. However, when polyols are heated above solid-solid phase change temperature, they will become plastic crystals with large vapor pressure and easy to steam. It needs to be encapsulated in containers, which does not reflect the superiority of solid-solid phase change materials.In addition, polyols are generally very soluble in water at high temperature and thermal stability is not good, which requires the use of polyols materials to consider the waterproof measures, processing temperature should not be too high, which also limits the application of polyols as a phase change material.

2.4.2 Layered perovskite

Layered perovskite-like phase change materials are organic metal compounds. These compounds have layered crystal structures. Because they are similar to mineral calcium crystals, they are called layered perovskite. Solid-solid phase transition of these phase-like materials is reversible. Different layered perovskite-like materials are synthesized in different molecule proportions to regulate phase transition temperature and enthalpy.

"Layered perovskite" as a phase change material has its unique advantages: good reversibility of solid-solid phase change, stable chemical properties, high temperature resistance of more than 200℃. Among them, n-decanolamine acid (C10M) undergoes phase change at 30.8℃. It is expected to become a solid-solid phase change material for phase change fibers in apparel. However, the phase change heat of polyols and "layered perovskite" is much smaller than that of paraffin, the relatively high price is still a factor restricting the development of organic solid-solid phase change materials. The thermal properties of polyols can be improved by nanomaterials obtained from compounding polyols and their binary systems with montmorillonite. However, the binary system of "layered perovskite" has not been reported.In addition, the thermal conductivity of polyols and "layered perovskite" is poor, and something with higher thermal conductivity like graphite is needed to improve their thermal conductivity.

summary

Compared with solid-liquid phase change materials and inorganic solid-solid phase change materials, organic solid-solid phase change material has the advantages of stable performance, long service life, convenient use, simple equipment, low under-cooling and no phase separation, and is one of the most promising research fields. The focus of future work is how to develop solid-solid phase change materials with low cost, good thermal and chemical stability, green environment protection, high latent heat of phase change and high thermal conductivity. How to use existing phase change materials to develop products with variable functions and even multi-functions is also an important topic.