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1 Introduction

Energy use in buildings means a large part of global energy demand becoming 30 % of energy-related CO2 emissions (one-third of black carbon emissions) and 32 % of global energy used. Over 60 % of residential, and almost 50 % of commercial building energy, is expended for thermal application as water heating in residential buildings, and as a cooling supply in commercial buildings (IEA 2012; Ürge-Vorsatz et al. 2015).

Thermal energy storage (TES) is also known as heat and/or cold storage (Cabeza 2015; Mehling and Cabeza 2008). This technology allows storing thermal energy to be used at a later time. Implementing TES in buildings allows the use of renewable sources of heat and cold that otherwise would not be possible (Heier et al. 2015). The duration of the storage can be from short-term, daily storage, to long-term storage, usually between seasons and known as seasonal storage.

The application of storage in buildings can have various objectives. One of them is the reduction of the building energy demand, either by reusing the high internal gains or by storing exterior heat or cold to be used in the building. A second objective is “peak shaving” (also called “peak shifting”), as shown in Fig. 16.1; usually, according to this strategy, no energy is saved, but economic advantages are achieved. TES can also improve HVAC systems, increasing their efficiency or reducing emissions.

Fig. 16.1
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Strategies for peak shaving when TES is used in buildings . From left to right load levelling, demand limiting, and full storage

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2 Materials Used for TES in Buildings

TES materials are becoming a key issue for the deployment of thermal heating and cooling technologies in the building sector. Meanwhile, the International Energy Agency (IEA) suggests energy storage as a possible answer to address the energy problems described in Sect. 16.1, focusing on materials and systems for building applications (IEA 2010). As previously stated, thermal energy can be stored following three processes: as sensible heat, as latent heat, or as thermochemical heat . These methods are described in the following sections as well as the desired properties for material selection, as are the main advantages and drawbacks of each technology.

2.1 Sensible Heat

This is the simplest method for storing thermal energy and consists of applying a temperature gradient to a media (solid or liquid); the heat that can be accumulated in this media is the sensible heat stored. Figure 16.2 shows the thermal profile of one substance or material when a temperature gradient is applied. The area under the curve is the total heat accumulated (Basecq et al. 2013).

Fig. 16.2
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Thermal energy storage profile versus temperature when heat is stored as sensible heat

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The most common storage media used to store energy as sensible heat is water (see Fig. 16.2 as ice). Sensible heat storage has two main advantages: it is cheap and is without the risks derived from the use of toxic materials. Moreover, the material used to store energy is contained in vessels as bulk material, thereby facilitating the system design.

Heat capacity comes from either the content of the material used, volume, or mass. Sensible heat is often used with solids, such as stone or brick, or from liquids, such as water (high specific heat).

The materials mainly used to store sensible heat are based on common ceramics (cement, concrete, etc.), or natural stones, such as marble, granite, clay, sandstone, or polymers (PUR, PS, PVC). All of them are inexpensive materials, and sensible heat will depend on the mass and volume used. Therefore, waste materials or by-products from several industrial processes with proper thermophysical properties are becoming suitable candidates to be used for sensible heat TES (Fernandez et al. 2010; Navarro et al. 2011). Figure 16.3 shows the specific heat capacity versus density of all the materials mentioned before to be used as sensible heat media. Note that the waste materials are proper candidates because they have a high enough density and account for suitable specific heat capacity.

Fig. 16.3
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Specific heat versus density of material candidates to be used for sensible heat thermal energy storage (SH-TES)

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The more relevant properties for sensible heat TES are the ones collected in Table 16.1. The physical and technical requirements must be taken into account during the design step of the system to be implemented in buildings .

Table 16.1 Requirement for materials to be used as TES media
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Materials to be used for sensible heat-TES can present two drawbacks. The first is related to corrosion incompatibility, but this drawback is easy to avoid if the material used to contain the storage media is well selected and/or the surface of the vessel material is coated with a material that is compatible with the sensible storage material. The second drawback is the thermal stability over thermal cycles. This behavior must be studied before the implementation under real thermal conditions fitted to the application.

2.2 Latent Heat

TES proposes phase change material (PCM) as materials able to store high amounts of energy as latent heat. PCMs increase the thermal mass of the building envelopes and building systems . Such materials have been extensively studied in the past by several researchers (Baetens et al. 2010; Khudhair and Farid 2004; Sharma and Sagara 2005; Zalba et al. 2003). Among all materials, those that have high storage density for small temperature ranges are considered PCM (Günther et al. 2006), and are classified as different groups, depending on the material nature (paraffin, fatty acids, salt hydrates, etc.).

A conventional profile—when one PCM is submitted to a temperature gradient—is shown in Fig. 16.4. The extremes correspond to sensible heat, and the central part to latent heat stored.

Fig. 16.4
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Thermal energy storage profile versus temperature when heat is stored as sensible + latent heat

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A temperature increment produces a temperature increase on the material evaluated. Then, the temperature remains constant during the phase change but the heat stored is increased. Further storage of heat results as sensible heat again when the phase change is finished.

Buildings can use the latent heat from PCM by two different concept systems: passive latent heat energy storage systems (LHES), or using active LHES systems (Parameshwaran et al. 2012). However, independently of each system, PCM must be encapsulated or stabilized for technical use, and PCMs are introduced in building systems using several methods.

PCM encapsulation in structures is the direct inclusion in walls and ceilings. These building components offer large areas for passive heat transfer within every zone of the building (Zhang et al. 2010).

Impregnation (Khudhair and Farid 2004) is an extensively used method to introduce PCM in porous materials such as gypsum (Nomura et al. 2009), mortar, wood, etc. This technique has been widely studied lately (Barreneche et al. 2013a, b; De Gracia et al. 2011), and is based on introducing liquid PCM inside porous materials using vacuum systems.

A microencapsulation (Tyagi et al. 2011; Zhang et al. 2010) PCM process is projected as one way to incorporate PCM in building materials becoming a TES part of the building structure even for lightweight buildings . Microcapsules consist of small PCM containers using hard shells, although they allow volume changes.

A shape-stabilized PCM is the incorporation of PCM in a polymer as one shape stabilized material (Del Barrio et al. 2009; Li et al. 2012). PCM slurries consist of a microcapsuled PCM suspended on a thermal fluid to be used applying them in several active systems such as heat exchangers and thermal control systems (Delgado et al. 2012). Material candidates to be used as latent heat TES must undertake several required properties. Table 16.1 shows the properties required by PCM for building applications (Cabeza et al. 2011; Tyagi and Buddhi 2007).

The criteria that limited the storage process from the beginning are the segregation defined as the phase separation of the PCM when this material has more than one component, and the subcooling that appears when a PCM starts to solidify at a temperature below its congealing temperature. These two processes must be studied during the material selection step and must be avoided or minimized. In addition, there are other drawbacks that will limit the implementation of PCM but they will appear afterwards. Regarding corrosion problems between PCMs and containers, the compatibility will appear afterwards and it must be studied before the implementation; cycling stability at long term: one substance used as PCM must be stablized over cycles; and finally, the leakage is one of the most important drawbacks because if this effect occurs, the effectivity of the system will decrease drastically. For that reason, the techniques to encapsulate or stabilize PCM are a key issue in PCM implementation.

2.3 Thermochemical Reactions

Thermochemical materials (TCM) are materials that can store energy by a reversible endothermic/exothermic reaction/process and the resulting reaction products are easily separated (usually a gas-solid system or a liquid-solid system) (Cot-Gores et al. 2012). First of all, the charging process is performed: applying heat, the material reacts and is separated into two parts: A + B. Then, when the storing process takes place, the heat is stored because the products are easily separated and the storage could be complete at low temperature. Finally, the discharging process is performed—as a reversible reaction, when products A + B are placed together, and under the suitable pressure and suitable temperature conditions to react, the energy is released again. This process is shown in Fig. 16.5.

Fig. 16.5
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Steps followed by TCM to store energy

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TCM systems are those that are more energy-efficient, but they are being developed (N’Tsoukpoe et al. 2009). There are almost no real applications implemented in the market, but researchers and companies are making a huge effort to bypass the barriers of this technology.

Building systems that include TCM are described for seasonal storage, in order to store the overheating of summers to be used to heat buildings during the winter. Table 16.1 shows the properties required by TCM for building applications; these are the materials that present more technical requirements.

The main drawbacks of TCM storage are due to the corrosion, as with PCM and sensible storage, and due to the exhaustion of the solid-gas chemical reactions because they are poor regarding the heat and mass transfer performance in the reactive bed and present low thermodynamic efficiency of the basic cycle. In order to enhance the performance and well-operation of the storage system, the chemical reaction or the sorption process is the most important issue for storing energy through TCM. For that reason, precise control of the kinetics of the process/reaction as well as the boundary conditions of the system (pressure, temperature, water vapor flow, etc.) are priority issues. This challenge can be overcome by studying the details of the chemical reaction/sorption process.

3 Passive Technologies

3.1 Introduction

Buildings should be designed to provide thermal comfort for inhabitants with the minimal use of energy by the HVAC system (Soares et al. 2013). TES is a technology that can help achieve this objective, both with sensible and latent TES. The use of TES in the building components can significantly reduce the heating and cooling demands by absorbing solar radiation, internal heat gains, and/or free cooling strategies.

3.2 Sensible Passive Systems

3.2.1 Integration in Building Components

Passive storage with sensible heat storage is achieved mainly with high thermal mass materials, able to store bit amounts of energy, that give thermal stability and smooth indoor thermal fluctuations (Kosny et al. 2014). Examples of these materials are stone, rammed earth, concrete, stone, and brick. A comparison of rocks, clays, water and salts was carried out by Rempel and Rempel (2013); the authors compared the patterns of heat uptake of various materials and thicknesses (Fig. 16.6) to give an intuitive understanding of TES to architects and engineers (Fig. 16.7).

Fig. 16.6
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Exterior mass walls. Heat fluxes across outdoor (left axis) and indoor (right axis) surfaces of exterior walls on a typically sunny January day in Denver, Colorado (Rempel and Rempel 2013)

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Fig. 16.7
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Daily heat flux patterns across surfaces of thermal storage walls that would best meet design intents of a residential evening heating; b workplace daytime heating; c sunspace or greenhouse all-night heating; and d cooling (Rempel and Rempel 2013)

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Solar walls can be used for energy savings in buildings (Saadatian et al. 2012). They can be classified as standard solar wall (Trombe wall), solar water wall, solar transwall, composite solar wall, and fluidized solar wall. Only the Trombe wall type and the solar water wall include storage in their operation.

A typical Trombe wall consists of a high thermal mass wall (concrete, stone, brick or earth wall) covered by glass, with an air channel in between (Stazi et al. 2012). Its operating principle can be seen in Fig. 16.8. The energy benefits of Trombe walls as a passive system can be found in the literature (Briga-Sá et al. 2014; Llovera et al. 2010). Solar water walls follow the same principle as the standard ones, but the massive part of it is replaced by water containers forming the wall (Adams et al. 2010).

Fig. 16.8
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Trombe wall configurations: a without ventilation; b winter mode with air circulation; and c summer mode with cross ventilation (Stazi 2012)

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3.3 Latent Passive Systems

3.3.1 Integration in the Building

As previously stated, the most promising methods for incorporating PCM in construction materials are direct incorporation, immersion, encapsulation, microencapsulation, and shape-stabilized (Cabeza 2015; Hawes et al. 1993; Memon 2014).

A first way to incorporate PCM in building envelopes is through wallboards, as shown in many studies recently published, both experimental and numerical (Cabeza 2015; Heier et al. 2015; Kosny et al. 2014). The factors that will influence the final efficiency of PCM-containing wallboards are how the PCM is incorporated into the wallboard, the orientation of the wall, the climatic conditions, the direct solar gains, the internal gains, the color of the surface, the ventilation rate, the PCM chosen and its phase change temperature, the temperature range of the phase change, and the latent energy per unit area of the wall.

Recommended studies are those of Kuznik et al. (Kuznik et al. 2008a, b; Kuznik and Virgone 2009) (Fig. 16.9), with a wallboard with 60 % microencapsulated paraffin melting at 22 °C that reduced the air temperature fluctuations in a room and the overheating effect of solar energy (Fig. 16.9); Shilei et al. (2006), with a wallboard with 26 wt% of fatty acid tested in China; Diaconu and Cruceru (Diaconu and Cruceru 2010; Diaconu 2011), with a three-layer sandwich insulating panel, tested experimentally showing a contribution to the annual energy savings and a reduction of the peak heating and cooling loads (Borreguero et al. 2010), with the inclusion of microencapsulated commercial paraffin (RT27) to gypsum blocks; and, more recently (Lee et al. 2015a, b), with a PCM thermal board, tested experimentally achieving up to 27 % average heat flux reductions and two to three hours of delayed peak heat transfer rate per unit of wall area (Biswas et al. 2014), with an innovative nano-PCM supported by expanded interconnected nano-sheets of gypsum wallboard developed and tested (Lai and Hokoi 2014), with the addition of aluminium honeycomb in a PCM wallboard.

Fig. 16.9
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Dupond PCM composite wallboard, studied by Kuznik et al. (Kosny et al. 2014)

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Zhou et al. (2014) evaluated the factors influencing both interior and exterior PCM wallboards under periodically changing environments, with solar radiation also being considered. Thermal properties, such as melting temperature, melting temperature range, latent heat, thermal conductivity and surface heat transfer coefficients, were qualitatively optimized in two design criteria: inner surface temperature history, and diurnal thermal storage.

Other ways to include PCM in passive walls are the addition of PCM to cellulose insulation (Evers et al. 2010) (with a reduction of 9.2 % of the average peak heat flux), or coupled with vacuum-insulating panels (VIP) (Ahmad et al. 2006), the addition of PCM to a structural insulated panel (Medina et al. 2008) (with a peak heat flux decrease of 62 %, and a daily heat transfer across the system reduction of 38 %, when 20 wt% PCM is added), or in rigid polyurethane foams (Castellón et al. 2010; Yang et al. 2015), or the addition of PCM as a new layer (Castell et al. 2010; Jin et al. 2014; Lee et al. 2015a, b) (with up to 20 % reduction of energy consumption), even with shape stabilized PCM (Barreneche et al. 2014, 2013a, b; Zhou et al. 2011). The inclusion of PCM in bricks (Fig. 16.10) showed similar results, reducing the thermal amplitude of the room and increasing the time delay of the peak temperature in about 3 h (Silva et al. 2012).

Fig. 16.10
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Clay bricks including PCM macrocapsules (Kuznik et al. 2008a, b)

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PCM has also been added to concrete and mortar. Cabeza et al. (2007) added 10 % microencapsulated paraffin in concrete panels and tested it in two real-size cubicles, showing up to 7 °C of indoor temperature oscillation decrease; Entrop et al. (2011) added microencapsulated PCM in concrete floors with a reduction of the floor temperature of 7–16 %; Alawadhi and Alqallaf (2011) added PCM to a concrete roof with cone frustum holes (Fig. 16.11); and, more recently, Desai et al. (2014) added PCM to cementitious composites to be used as thin panels for façades; and Joulin et al. (2014) in cement-mortars.

Fig. 16.11
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Schematic representation of a concrete roof with frustum holes filled with PCM (Diaconu and Cruceru 2010)

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PCM may also be added in shutters, window blinds and windows (which become the known translucent PCM walls). Soares et al. (2011) proposed a southward PCM shutter system to take advantage of the solar energy for winter nighttime heating, while Weinlaeder et al. (2011) used an interior sun protection system with vertical slats filled with PCM in office rooms. PCM was used together with a transparent insulation material (TIM) by Manz et al. (1997) to develop a selective optical transmittance solar radiation system. On the other hand, hollow glass bricks filled with PCM (Fig. 16.12) were studied for thermal management of an outdoor passive solar test room by Bontemps et al. (2011).

Fig. 16.12
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Hollow glass bricks filled with PCM (Biswas et al. 2014)

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3.3.2 Environmental Impact

Life cycle assessment (LCA) is a good tool for the environmental evaluation of buildings and has been used in residential and non-residential buildings and in civil engineering constructions, as well as for construction product selection and for construction systems and processes evaluation (Cabeza et al. 2014). PCM inclusion in buildings was assessed with LCA by De Gracia et al. (2010) and Castell et al. (2013), who showed that the addition of PCM (paraffin and salt hydrate) in the building envelope, although decreasing the energy consumption during operation, did not significantly reduce the global impact throughout the lifetime of the building. Aranda-Usón et al. (2013) concluded that PCM in buildings can reduce the environmental impact, but that the reduction is strongly influenced by the climate conditions and the PCM introduced.

4 Αctive Systems

4.1 Introduction

The use of TES in building active systems is an attractive and versatile solution for several applications, such as the implementation of renewable energy sources in the HVAC of the building for space heating and/or cooling, the improvement in the performance of the current installations, or the possible application of peak load shifting strategies. In this section, the use of TES in active systems will be discussed, highlighting the performance of free cooling systems and building integrated active systems, such as thermal activated building system (TABS), and heat pumps with TES.

4.2 Free-Cooling Systems

In a free-cooling system, the TES medium is charged when outdoor temperature is lower in comparison to indoor, and this stored cold is discharged when required by the cooling demand. The advantage of free cooling using TES in comparison to night ventilated cooling is that the cold stored can be discharged whenever it is needed by circulating ambient or room air through the storage (Waqas and Ud Din 2013). Since there is a small thermal gap between the charging source (low outdoor temperature at night) and the desired indoor temperature (thermal comfort), the use of PCM as the storage medium is recommended because of its high energy density within the phase change. As previously stated, the free cooling sequence consists of two modes of operation: the charging process, when the PCM is solidified, and the discharging process, when cooling is supplied to the inner environment from the storage (Fig. 16.13).

Fig. 16.13
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Free-cooling working principle. a Charging process (working of PCM storage during nighttime), and b discharging process (working of PCM storage during daytime) (Hed and Bellander 2006)

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This operating principle allows the storage to be placed and designed in various locations according to the application or requirements. Several authors have investigated the implementation of the storage in external heat exchangers (Turnpenny et al. 2001; Zalba et al. 2004), or packed bed systems integrated into the ventilation system (Arkar and Medved 2007; Arkar et al. 2007), while others integrated the storage in a building component, such as ceilings (Yanbing et al. 2003).

These free cooling systems consume around 9 % less electrical energy than conventional air conditioning units of the same power; however, the initial investment is 10 % higher, with a payback period of around 3–4 years (Arkar and Medved 2007). The main obstacle that this technology faces is the difficulty of ensuring the full solidification of the PCM in certain climates or summer periods, in which the night temperatures do not drop below the phase change range long enough. This problem can be overcome by enhancing the thermal conductivity of the PCM and using appropriate control strategies.

4.3 Building Integrated Active Systems

The active systems based on TES have been implemented using external storage systems or integrating the storage in the building elements. This last option avoids the occupation of space for the implementation of these technologies and makes use of large heat transfer surfaces between the storage and the application. The integration of the TES in the building can be done using the core of the building (core, floor, walls) in external solar façades , in suspended ceilings, ventilation systems, PV systems and water tanks, as shown in Fig. 16.14.

Fig. 16.14
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Integration of the TES in buildings (Navarro et al. 2014b)

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Regarding the use of the core of the building for the integration of the TES system, several diverse systems have been developed and tested. Navarro et al. (2014a) tested an innovative active slab with PCM in its hollows, in which air is the heat transfer fluid. On the one hand, the system stores solar energy during sunny hours in winter and provides a heating supply when required by the demand; on the other, during the summer it solidifies the PCM located inside its hollows, using the low night temperatures and supplies cooling in peak load hours (Fig. 16.15).

Fig. 16.15
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Operating principle of the active slab with PCM: a heating, b cooling (Navarro et al. 2014a)

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4.3.1 Integration of the TES Into the Core of the Building

Other storage systems that are integrated into the core of the building are the TABS , which makes use of a building element as a TES system, such as ceilings or envelopes, usually adding embedded pipes through water flows (Fig. 16.16). They are usually charged with the low temperatures at night (free cooling source) and designed to discharge passively the cooling store in the building structure (Saelens et al. 2011). The large area of these systems allows a high heat transfer rate between the space and the structure, and even the low thermal difference between them, which makes the system appropriate for managing internal heat gains (Lehmann et al. 2007).

Fig. 16.16
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TABS principle (Pavlov and Olesen 2011)

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The typical daily cycle of the application of TABS is shown in Fig. 16.17, which is composed of the following steps:

Fig. 16.17
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Typical temperature development of room air, concrete slab surface and water during 24-h period (Koschenz and Dorer 1999)

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  • Initial temperature of the room air and surface (1)

  • Heat sources are switched on, during the first interval of the heat sources releases its convection portion to the room air (1–2)

  • Radiative portion is absorbed by the storage capacity of the surfaces (2–3)

  • The heat sources are switched off at (3), and a similar pattern is found in the fast and slow heat discharge of the convective and radiative portion, respectively

  • The system is charged by the water pipes for the following daily cycle.

The activation of the building mass using pipes embedded in the main concrete structure significantly reduces the cooling peak load, which drives to a reduction in the cooling capacity of the HVAC systems up to 50 % (Rijksen et al. 2010). These systems based on sensible heat storage (typically the use of concrete) have been used commercially because of low investment costs and because it provides favorable comfort conditions (Lehmann et al. 2011). Moreover, in order to increase the TES capacity of the system, the addition of microencapsulated PCM to a concrete deck (Pomianowski et al. 2012) has been explored. Furthermore, TABS with two different shape-stabilized PCMs have been investigated. The two layers of PCM have different melting temperatures, so one is used for heating purposes and the other for cooling (Jin and Zhang 2011). Despite the benefits that the use of latent heat storage can provide to the performance of TABS, its inclusion in commercial systems has been limited because of the cost.

4.3.2 Integration of the TES in External Façades

The implementation of storage in external façades, such as double-skin façade, has been discussed to reduce the risk of overheating and increase the system efficiency in both winter and summer (Fallahi et al. 2010). Moreover, De Gracia et al. (2012) tested the thermal performance of a ventilated façade with macro-encapsulated PCM in its air chamber. During the heating season, the façade acts as a solar collector during the solar absorption period (Fig. 16.18).

Fig. 16.18
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Operational mode of the system during winter (De Gracia et al. 2012)

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Once the PCM is melted and the solar energy is needed for heating, the heat discharge period begins. This discharge period is performed until no more thermal energy is needed or can be provided by the system. The authors registered a reduction of 20 % in the electrical energy consumption of the installed HVAC systems because of the use of this solar-ventilated façade .

Furthermore, the same system was tested for cooling purposes, using low temperatures at night to charge the PCM and/or provide free cooling . The authors developed a numerical model and evaluated the performance of the system under different weather conditions (De Gracia et al. 2015). It was shown that the use of this latent heat-activated system is suitable for temperate climates according to the Köppen Geiger climate classification (Kottek et al. 2006), while the potential of the system to provide benefits in arid and equatorial climates is very limited.

4.3.3 Integration of the TES in Suspended Ceilings and Ventilation Systems

A significant number of old buildings need serious, energetic retrofitting in order to accomplish the standards defined by the European Directives (Directive 2010). Hence, the integration of TES components in the suspended ceiling, such as actively charged radiant panels filled with water (Roulet et al. 1999) or PCM (Koschenz and Lehmann 2004), is a good solution as a cooling or heating system.

Moreover, the TES units can be located in the ventilation ducts behind the suspended ceilings as in the work proposed by Yanbing et al. (2003), in which PCM is solidified during the nighttime and discharged during the peak load hours as in Navarro et al. (2014a). The system was tested, and it was proved that its use improves the thermal comfort level of the indoor environment. Furthermore, Monodraught Ltd developed and commercialized a cooling and ventilation system called “Cool-Phase” (Cool Phase 2014), which consists of an air-handling unit and a storage device based on macro-encapsulated panels of PCM. This equipment, shown in Fig. 16.19, can provide space heating by a heat recovery cycle and cooling, making use of low night temperatures.

Fig. 16.19
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TES integrated in the ventilation system. Cool-Phase system (Cool Phase 2014)

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4.3.4 Integration of the TES in the PV System

The storage implemented in the PV system provides two main benefits. On the one hand, it can profit from the heat behind the PV panels for heating purposes, and on the other, the PV panels increase their performance when cooled. The conventional aluminium-fined PV panel with natural ventilation can reduce the temperature rise of the PV; however, the use of a PV/PCM system can greatly reduce the temperature rise of the PV. The PCM should have a flash point considerably higher than the maximum operating temperature of the PV system, and it should be non-flammable and non-explosive (Abhat 1983). The use of PCM was shown to provide significant PV temperature control and thus increased electrical conversion efficiency (Huang 2006).

4.3.5 Integration of the TES in Water Tanks

Water tanks are a widely used TES device for solar systems or domestic hot water facilities, but because of the high volume they occupy, their integration into the building has only recently been addressed. This integration was done by architecturally including the storage in the living areas (Fig. 16.20), or by using ground-integrated tanks. The use of underground storage is justified if seasonal TES strategies are considered. There are four main types of underground systems for seasonal TES (The European Project EINSTEIN 2014) (Fig. 16.21): tank thermal energy storage (TTES); pit thermal energy storage (PTES); borehole thermal energy storage (BTES); and aquifer thermal energy storage (ATES).

Fig. 16.20
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Solar water tank integrated in a living room (Solaresbauen-Grundlagen 2014)

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Fig. 16.21
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Types of underground seasonal thermal energy storage (The European EINSTEIN Project 2014)

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4.4 Use of TES in Heat Pumps

The use of heat pumps with TES systems are presented as a promising technology to shift electrical loads from high-peak to off-peak periods, thus serving as a powerful tool in demand-side management (DSM) (Arteconi et al. 2013). The benefits of this technology are focused on the energy cost savings by using thermal energy stored during low-cost electricity tariffs. The incorporation of these systems in the buildings has been investigated both for space heating and cooling. The TES is implemented in the heat pumps, not only for load shifting, but for achieving important energy savings on the defrosting system of the outdoor unit, or for heat recovery applications as well (Moreno et al. 2014a, b).

PCM thermal energy storage systems were coupled to conventional heat pumps for space heating in Agyenim and Hewitt (2010), which concluded that due to the low thermal conductivity of the PCM, the average storage tank size needed to cover the heating demand of a semi-detached house was 1116 l, which ruled out the use of PCM for this application, unless the thermal conductivity is enhanced. On the other hand, if the PCM storage tank is implemented in a modern low-temperature heating system, such as that studied by Leonhardt and Müller (Leonhardt 2009), the switching between the on and off of the heat pump is reduced, which represents cost savings. Moreover, the heat pumps with PCM were also studied in combination with other energy sources, such as solar (Kaygusuz 1999; Niu et al. 2013) or geothermal energy (Benli and Durmus 2009).

As previously written, a PCM module is implemented in air source heat pumps to solve the frosting problem in the outdoor coil, which limits the air passage area and dramatically decreases the performance of the unit. Currently, the most widely used defrosting method consists of a reverse-cycle defrost, which means that one defrosting is required; the outdoor unit acts as a condenser; and the indoor unit as an evaporator, which is costly and produces discomfort in the inner environment. The PCM can be integrated in the cycle to absorb waste heat from the compressor and release this heat during the defrosting period (IEA 1990) (Fig. 16.22). This system increases the COP of the system, reduces the energy consumption for defrosting (Dong et al. 2011), and increases the thermal comfort of the indoor environment as well (Minglu et al. 2010).

Fig. 16.22
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Schematic diagram of operation for experimental ASHP unit (left) and physical view of PCM heat exchanger (right) evaluated by Jiankai et al. (Dong et al. 2011). 1 Compressor; 2 four-way reverse valve; 3 PCM heat exchanger; 4 indoor coil; 5 capillary tube; 6 outdoor coil; and 7 gas-liquid separator; F1–F5 solenoid modulating valves

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The use of PCM in heat pumps for space cooling is based on the cold storage from the evaporator, and the later use of this cooling during peak-load hours. Ice has been used as the storage media, implemented in heat storage tanks (Fang et al. 2009) or spherical capsules packed beds (Fang et al. 2010). In addition, Moreno et al. (2014a, b) implemented two PCM tanks in a heat pump cycle, one connected to the evaporator (cold TES tank) and the other to the condenser (hot TES tank). This allows the system to store cold and heat at once in order to cover the demand and apply peak load shifting strategies during hot or cold periods.

The use of waste heat from the compression cycle of the heat pump during the cooling season is based on the concept of storing the heat released by the condenser in a PCM tank and using it for domestic hot water (Gu et al. 2004). The ability to totally or partially fulfill the demand of domestic hot water might contribute in an important reduction of the CO2 emissions in this field (Zhang et al. 2011).

5 Conclusions

This chapter provides the state of the art of the studied active and passive TES technologies integrated in the building sector, including sensible, latent, and thermochemical storage systems. Passive techniques are based on increasing the thermal inertia of envelopes and building elements, which provides thermal stability and hence reduces the energy consumption for space heating and cooling. The correct use of these passive systems requires temperate weather conditions, in which the storage can be passively discharged to the outer environment during nighttime in the cooling season, and enough solar radiation to be charged during the winter. Moreover, the inclusion of TES in active systems has been investigated for the implementation of renewable energies for space heating and/or cooling, for improving the performance of the current installations, and for using peak load-shifting strategies.

TES shows great potential for reducing the energy demand of buildings and/or improving the energy efficiency of their energy systems. Several groups and associations (i.e., IEA, RHC Platform, EASE) have identified this potential. However, some of challenges that have to be faced are reduction of the cost, increase of the compactness of the systems, increase of the energy density of the materials and systems, increase of the thermal conductivity of the materials, and development of new materials.