Abstract
This paper addresses the challenges the policymakers face concerning the EU decarbonization and total electrification roadmaps towards the Paris Agreement set forth to solve the global warming problem within the framework of a 100% renewable heating and cooling target. A new holistic model was developed based on the Rational Exergy Management Model (REMM). This model optimally solves the energy and exergy conflicts between the benefits of using widely available, low-temperature, low-exergy waste and renewable energy sources, like solar energy, and the inability of existing heating equipment, which requires higher exergy to cope with such low temperatures. In recognition of the challenges of retrofitting existing buildings in the EU stock, most of which are more than fifty years old, this study has developed a multi-pronged solution set. The first prong is the development of heating and cooling equipment with heat pipes that may be customized for supply temperatures as low as 35 °C in heating and as high as 17 °C in cooling, by which equipment oversizing is kept minimal, compared to standard equipment like conventional radiators or fan coils. It is shown that circulating pump capacity requirements are also minimized, leading to an overall reduction of CO2 emissions responsibility in terms of both direct, avoidable, and embodied terms. In this respect, a new heat pipe radiator prototype is presented, performance analyses are given, and the results are compared with a standard radiator. Comparative results show that such a new heat pipe radiator may be less than half of the weight of the conventional radiator, which needs to be oversized three times more to operate at 35 °C below the rated capacity. The application of heat pipes in renewable energy systems with the highest energy efficiency and exergy rationality establishes the second prong of the paper. A next-generation solar photo-voltaic-thermal (PVT) panel design is aimed to maximize the solar exergy utilization and minimize the exergy destruction taking place between the heating equipment. This solar panel design has an optimum power to heat ratio at low temperatures, perfectly fitting the heat pipe radiator demand. This design eliminates the onboard circulation pump, includes a phase-changing material (PCM) layer and thermoelectric generator (TEG) units for additional power generation, all sandwiched in a single panel. As a third prong, the paper introduces an optimum district sizing algorithm for minimum CO2 emissions responsibility for low-temperature heating systems by minimizing the exergy destructions. A solar prosumer house example is given addressing the three prongs with a heat pipe radiator system, next-generation solar PVT panels on the roof, and heat piped on-site thermal energy storage (TES). Results showed that total CO2 emissions responsibility is reduced by 96.8%. The results are discussed, aiming at recommendations, especially directed to policymakers, to satisfy the Paris Agreement.
Highlights
These sample results conclude that the sustainable solution for decarbonization with low and ultra-low district energy supply temperatures lies on the demand side by low-exergy heating and cooling equipment without requiring temperature peaking of any kind, i.e., electrical or non-electrical
If the transition of the standard heating equipment is possible only by oversizing them like adding more radiator sections or adding more radiator units, mostly in series in the latter case, which applies for many moderately old buildings, pressure heads do increase. This increase is coupled with higher pumping power at the start-ups in an on-off type of controls facing larger volumes of heat transfer fluid accelerated in the entire system due to oversizing
This paper has shown the importance of retrofitting the building stock for ultralow heating and ultra-high cooling towards satisfying the Paris Agreement goals
Summary
In many EU countries, half of the residential stock comprises buildings, which were built before 1970, when the first thermal efficiency regulations were not in place yet [7]. FFiigguurree22..EExxeerrggyyTTrriaianngglelewwitihthththrreeeessidideessrreeppreresesenntitninggεdεedmem,,εεsusupp,, aanndd tthhee uunniitt tthheerrmmaall eexxeerrggyy ssuuppppllyy,, EEXXHH. For a narrow margin of sensitivity required about the maximum ψR at low supply temperatures as desired by the EU roadmaps, uncertainties in the supply temperature must be minimal This is a critical issue for control systems of 5DE districts with renewables in terms of exergy because Tsup is low, and ∆T‘eq must be kept minimum, making the condition hard to satisfy. ΨR, which is 0.40 is obtained at Tsup = 30 ◦C At this supply temperature, the insulation must be too heavy that the Q‘ will be about 50% of the design heating load, which is not practical. TThhisis, ,aattththeessaammeetitmimee, ,mmeeaannssaa∆ΔCCOO22eemmisisssioionnssrreessppoonnssibibiliiltiyty: : ΔCO | = 0.27 ×∆(0C.7O52|−1 =0.400.267) ×= (00.0.7953−kg0.C40O62)/k=W0-.h093 kg CO2/kW-h slisltgietgneanaimtiHmetHecetcaeortaorresrrestyrtehytteaoheaomnemnmtmihtusiheusltethltpithppeioperlotimrleteimeneranrtal0itla0e.i2ale.x27lxoe7oerfsgrftpsgayptoa.ynownA.dwAdesnersnfreogflegoreelerncetnthcetrterheiracreiattcpyittopioyponwopen.wee.oTrTefohrghfeeegexrenereexefnereofgreorrayrgertaey,idto,tihenodthseneitessreitosexsryxteosieertyllgirdelgayl.dyna.dBndoBeopesoottshtptrithutorliunicolgtci.nignto.iniontientferafornaomndmd With such an emissions responsibility in mind at the steam generation step, the second major exergy destruction and emissions responsibility take place at step (4), which concerns the steam heating system in the building. District energy systems are on the rise [15] but without referring to the 2nd Law
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