Benefits of Thermo-chemical Networks

Application cases and economic benefits of thermo-chemical networks

ABSTRACT

Thermo-chemical potential of absorption and desorption has high potential to capture and use residual heat at low temperature ranges. Due to loss-free transport and storage of the captured energy potential, long-distance transport and medium-term storage offer interesting potentials to utilize residual heat. Therefore, the aim of the EU H2020 project H-DisNet is to develop networks similar to district heating networks using thermo-chemical fluids (TCF) instead of water. The paper will give an introduction to the technology that can provide heating, cooling and drying services in one network and discuss its economics.

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First, use cases describe promising application scenarios. Requirements are derived from the use cases, first, for the novel technology and, second, the application situations, i.e. the buildings or industrial processes, in which the services are applied. This includes temperature and humidity requirements as well as further conditions of a useful application. Depending on the services requirements, features of the thermo-chemical technology, such as the used TCF, will be determined so that the thermo-chemical technology is able to satisfy the service requirements. Operation modes will be presented the show, how in specific use cases the technology would work. An outline of the operation of a network will be presented.

Second, for an economic assessment, conventional existing solutions for the services, for which thermo-chemical technology is proposed, will be described. These conventional technologies form the background for an economic comparison. The aim of the economic comparison is to show the benefits of the thermo-chemical technology for the key stakeholders involved in such a network. The aim is to provide evidence that the thermo-chemical network technology is marketable.

1. INTRODUCTION

Nowadays, an always increasing attention has been placed on reducing the energy consumption used for heating, cooling and drying with a resulting abatement in the CO2 production. As a matter of fact, a massive quantity of fossil fuel is used as primary energy source for air-conditioning and industrial operations causing a constant conversion to C02 that is swiftly rising and expediting the global climate change. It has been calculated that the energy depleted for heating and cooling of buildings (residential or in the service sector) and industrial processes accounts for 50% of the EU’s annual energy consumption [1]. This is mostly due to the fact that almost half of the EU’s buildings are old and lack in efficiency, renewable energy is narrowly used in these sectors and a huge amount of heat produced by industrial processes is dissipated into the atmosphere or into water, missing the opportunity for its recovery.

Through the development of an optimized, more efficient and less cost-consuming utilization of the energy sources, it will be possible to achieve a decrease in the energy imports, obtaining a diminution in the costs and, at the same time, an environmental benefit, represented by a reduction in the emission of greenhouse gases.

District heating is one of the possible technologies in the direction of this purpose because it remarkably concurs to a better use of the energy sources, particularly the renewable energies. Nevertheless, this technology presents several drawbacks, such as the temperature required that can preclude the utilization of some technologies that work with lower temperatures, the remarkable heat losses occurring during the transportation in pipelines and the need for integration with storage systems in order to obtain the match between the demand and the sources in time and location.

Therefore, this paper will be addressed to the description of Intelligent Hybrid Thermo-Chemical District Networks, an innovative type of district network based on the employment of thermo-chemical fluids (TCFs) instead of water as energy storage medium. Through this technology it will be feasible to obtain an energy-efficient exploitation of the resources, particularly the unemployed low-grade industrial heat and thermal renewable, leading to the achievement of a sustainable energy system. Moreover, by the usage of liquid desiccant as TCF in order to obtain a loss-free long-distance transport and a medium-term storage it will be possible to obtain significant cost reductions, making this technology absolutely interesting for citizens, workers and industry.

The paper starts in Section 2 with a description of the liquid desiccant technology in order to understand the ability of this system for heating and cooling applications. Section 3 reports the characteristics and the main advantages arising from the integration of the TCF with the district network. Section 4 detects the possible business models interested in the utilization of the Hybrid District Network. The last two section of the paper address the subject from an economic point of view, identifying the cost factors for this kind of system (Section 5) and the associated economic savings related to the different applications (Section 6).

• LIQUID DESICCANT TECHNOLOGY

The current research on Hybrid District Networks is related to the requirement of obtaining a district network which allows the connection with consumers at a greater distance, such as for the heating and cooling of residential and service buildings that are usually located far from industrial plants.

In fact, the temperature level of waste heat and renewable energy is generally too low, bringing to higher volumes that are responsible for increased energy leaks and higher costs, related to a higher expense for the pipelines. In this direction, it has to be seen the always growing interest in absorption and reversible thermo-chemical processes for district heating.

The closed district network system is a well-developed technology that employs absorption heat pumps and chillers to supply heating and cooling for residential and service buildings (!!REPETITION). However, this technology does not allow to profit from industrial waste energy or renewable energy that are located in a remote position respect to the service, besides not allowing a time shift between the source and the demand side.

For this reason, an innovative open system district heating system, based on the employment of liquid desiccant as the thermo-chemical transporter of energy, which allows to split the regeneration and absorption side and to locate them in different places, is under study.

Desiccant-based TCFs have the potential to provide simultaneous and multiple on-site functions and services, such as heating, cooling, de/re-humidification, energy storage and energy transport. Liquid desiccants exploit the hygroscopic properties of a salt (MgCl2, CaCl2, LiBr, LiCl etc.) solution for the removal of the moisture from the ambient outdoor air, until the attainment of a situation of equilibrium of its vapour pressure with that of the incoming air. For this reason, the dehumidification capacity of the desiccant can be evaluated through its equilibrium vapour pressure.

For example, an industrial process waste-heat driven air-conditioning system is shown in Fig.X in a counter-flow packed bed configuration.

FIG.

The strong TCF-solution (i.e. TCF-rich relative to water), typically a desiccant, is sprayed at the top of the absorber, ambient air (or gas) enters the absorber at the bottom and transfers its moisture to the TCF. As some heat is liberated, the TCF solution temperature rises and hence the solution vapour pressure. The heat exchange process typically takes place over a packed bed/spray tower or gravity driven wetted wall column designed with the minimum pressure drop (Jain et al., 2007) with output humidity controlled by the temperature and concentration of the TCF solution. The dehumidified air exits at the top of the absorber and can be used to meet plant specific energy demands. The warm but now diluted TCF solution leaves the bottom of the absorber and it is pumped for regeneration. The regeneration process has typically the same configuration as the absorber and it is driven by the incoming industrial process waste heat gas stream; the now diluted TCF is sprayed over this stream and water in the TCF solution evaporates, reducing the gas temperature and increasing its humidity. The now strong TCF solution is pumped back to the absorber to restart the air-conditioning process.

Industrial manufacturing plants typically have multiple demands for energy in their locality; the previously described system can exploit the low-grade process waste heat to supplement (or even replace) local demands: (1) Industrial Drying, because the ambient air (or other gases) can be dried and then cooled for utilisation elsewhere on site; (2) Heating and/or Humidification, since the ambient air is heated as it passes through the absorber, which yields a warmer and more humid gas stream that can be used locally with corresponding savings in energy demands; (3) Cooling, by utilising the dry air as the an input into an evaporative cooling system, an additional re-humidification stage can be used to produce a cooling effect and thus to supplement local air-conditioning loads; and (4) Loss-Free Energy Storage, since through the transformation of heat to TCF potential is possible to transport and store heat and TCF potential into the hybrid district network with almost total lack of energy loss. As there is significant potential for thermal energy storage thus meeting/offsetting hourly, daily and seasonal energy supply/demand.

• THERMO-CHEMICAL NETWORK TECHNOLOGY

The aim of a Hybrid Thermo-Chemical District Network is to broaden the use of district networks through the realization of a multifunctional optimized system, able to simultaneously fulfill heating, cooling and drying operations and also to be integrated with already existing thermal district networks, leading to the achievement of a more sustainable process.

Through the recovery of industrial waste heat and the exploitation of low temperature energy sources (e.g. renewables, such as solar thermal or geothermal) is possible to obtain via the regeneration process a TCF with high energy in the state of TCF-concentrate that is used as a thermo-chemical energy storage medium. This is one of the peculiar advantages of the innovative district network because the thermo-chemical energy storage in the concentrate liquid desiccant is roughly losses, offering the opportunity to enhance the storage term between hours and days, which enables to fill the mismatch in the schedule between available heat and demand, to heighten the transport distance of the heat, that can be long up to 50 km [X] with pipelines characterised by a reduced or absent insulation with a resulting reduction in costs.

This feature, together with the increased energy density of the TCF-concentrated (higher than the water, employed in the conventional district heating system) will lead to the obtaining of a very promising system from an economic point of view. Moreover, the characteristics of transport and cheapness of this cutting-edge technology enable to serve also the regions with lower heat demand.

Another advantage is that the salts used in the solution as liquid desiccants in an open district network system (MgCl₂, MgSO₄, CaCl₂, LiBr, LiCl, Ca(NO₃)₂, TEG) are in most of the cases cheap and, for the characteristics of open system, they have to be as much as possible non-toxic and environmental harmless. Particularly, the MgCl₂ (produced as by-product from sea-water processing) and the CaCl₂ (produced from industrial processes) result to be extremely cheap and hence economically viable.

The environmental benefit represented by the reduction in the primary energy consumption and in the CO₂ production is another key property of this system. Furthermore, the simpler pipeline infrastructure, which is characterised by the utilization of recyclable plastic pipes without any anti-frost protection, will allow to significantly reduce the exploitation of raw materials.

Lastly, the liquid desiccants present hygiene properties that can ensure humidity control of the process air, leading to an amelioration of the indoor comfort and forestalling the maturation of mould fungus.

• ECONOMIC EVALUATION OF HYBRID DISTRICT NETWORKS

The attainment of benefits in terms of financial, technological and environmental features are the main conditions for the spread of the thermo-chemical district network. The aim is to achieve profitability and efficiency for both suppliers and consumers, converting costs into revenues.

The implementation of this strategy could lead several benefits to different classes: (1) Citizens could profit from a monthly and yearly cost reduction for energy-effective heating and cooling calculated to be ranging from € 1500-2000 to € 300-500 [X], simultaneously achieving a better indoor comfort, ensured by the humidity control of the thermo-chemical system. Moreover, this could lead to a greater stabilization of the energy costs, because the network is mostly based on the usage of renewable energies, which cost is more predictable respect to fossil fuels, characterised by a highly volatile price. (2) Industry could also be enormously interested in the employment of district thermo-chemical networks to extent of reducing its energy costs by 4-10% with investments characterised by a payback period lower than 5 years [X] and of obtaining a sustainable process, able to decrease its energy consumption. Concurrently, this technology could lead to a more environmental harmless process with reductions in the CO₂ and air pollution, contributing to a significant improvement in the related health problems.

In order to estimate the economic potential of the technology an analysis based on the study of business cases involved on the employment of waste heat has been taken as the point of reference [x]. The main four identified sectors are: (1) Built Environment Business To Customer (B2C); for this business model, the customer base are new buildings and offices together with  the renewal of utility buildings (mostly property of the municipalities), apartments (usually possessed by housing corporations) and offices. Another possibility is the utilization of TCFs into an already existing hybrid network in order to improve its energy efficiency. Municipalities and housing corporations have a fundamental role in this business model because in most of the cases they have a previously established relationship with the formerly defined customers. To extent of achieving the success of the project is indispensable that both of the parts, public party and individuals, have an interest in saving energy and this is ensured by an equal split of the profit between the parts.  (2) Built Environment Business To Business (B2B);