Techno-economic comparative analysis of solar thermal collectors and high-temperature heat pumps for industrial steam generation

between the economic stronghold of each technology. The evaluation is carried out through the annual energy simulations using TRNSYS and Excel spreadsheets for HTHPs, while TRNSED and OCTAVE are used for the solar thermal part. Boundary conditions for European geographical constraints have been applied to establish use cases for the analysis. The result shows that the design of a PTC system with optimal SF can reach cost parity with HTHP for most of the analyzed locations. The developed methodology serves as a valuable guide to quickly determine a preferred lower carbon heat solution, thus easing the decision-making for industries.


Introduction
"Heat is half" of the global primary energy consumption [1].The generation of heat from various fuel sources results in nearly 40 % of the global CO 2 emissions.Decarbonizing the heating supply is the "elephant in the room" and needs significant attention from policymakers to promote the right technological solution to facilitate the rapid replacement of gas, coal, and other fossil fuels.
Heat is consumed in buildings for space heating, domestic hot water, and industries to generate steam or hot water.The major focus regarding technical solutions for clean heating is often on electrification using electrical heaters or heat pumps (HPs).Residential heating demand can be decarbonized using commercial HPs, and their significance is further emphasized in Repower EU, which aims to deploy 60 million HPs by 2030, a projected 4-fold increase from current numbers [2].
It is important to note that industrial process heating demand constitutes 66 % of the EU's overall heating demand [3].In addition, the concepts of positive energy district (PED) and climate-neutral city are promising nowadays, but they have not yet included industrial heating demand within their boundaries.With the ongoing challenges in gas Abbreviations: CAPEX, Capital expenses; COP, Coefficient of performance; DNI, Direct Normal Irradiation; EIR, Electricity inflation rate; HP, Heat pump; HTHP, High-temperature heat pump; IEA, International Energy Agency; KPI, Key performance indicator; LCOH, Levelized cost of heat; LPR, Load profile; OPEX, Operation expenses; PTC, Parabolic trough collector; PED, Positive energy districts.;ST, Solar thermal; SHIP, Solar heat for the industrial process.
supply, natural gas prices have increased exponentially in the past few years, thus creating an energy-tense situation in the EU [4].This implies that a less price-volatile and reliable supply of fuels for industrial process heat should be prioritized.The process heat required in most industries is in the medium temperature range (i.e., 80 to 250 • C).Several technologies in the market can achieve this temperature with low carbon emissions, such as solar thermal (ST) collectors, high-temperature heat pumps (HTHP), and boilers utilizing green fuels such as waste biomass or biogas, or renewable electricity.
Industries typically use fossil fuel boilers to generate steam, which is used as a heat transfer fluid to carry out several processes.Retrofitting any new technology in an existing boiler system requires a detailed understanding of system boundary conditions.Economic feasibility is a crucial decisive criterion for industries to evaluate any technology.From market experience, it is realized that large multinationals can facilitate the capital expenditure (CAPEX) for an efficiency improvement process (such as the implementation of ST, HTHP) only if the payback is less than 5 years.
An indicative pre-feasibility assessment using economic key performance indicators (KPIs) can facilitate industries toward quick decisionmaking for a go/no-go decision concerning a detailed evaluation of any technology.Therefore, this paper is themed around doing a comparative techno-economic analysis for heat generation using typical boundary conditions encountered in industries.The focus is on two technologies to generate steam, i.e.(a) steam-generating HTHPs and (b) Parabolic trough collector (PTC), which is a type of concentrating ST collector.The sections provide a literature review and the current development status of using these technologies for industrial applications.

High-temperature heat pumps 1.HP use in industries
HTHPs can be a good alternative solution in industries where waste heat is available such as in drying, sterilization, papermaking, distillation, or food preparation processes [5].HPs can upgrade lowtemperature waste heat into suitable high-temperature levels.Moreover, multi-temperature HPs, which can utilize multi-temperature heat sources in different hydraulic configurations, are also an appropriate solution to increase the overall system efficiency [6].However, HTHP use is often restricted by the temperature need of an industrial plant.If the steam temperature required is within the range (e.g., low-pressure steam), an HTHP can be used as a one-to-one replacement technology for an existing boiler system.However, the redundancy of heat supply is often necessary for industries by installing a boiler parallel to the heat pump as a backup or covering peak loads.Several industrial sectors, such as food, beverage, dairy, and mining, need temperatures suitable for existing steam heat pumps.Even if the steam temperature required is higher than the HTHP supply range, it is possible to find a process that operates at lower temperatures and runs them exclusively using HPs.Another option is combining the HTHP system with mechanical vapor recompression to increase steam pressure [7].
A heat pump requires a heat source and electricity inputs for operation.Therefore, replacing existing steam boilers with HTHP increases the plant's electricity consumption.In some cases, this may require an increase in the electrical grid capacity, thus resulting in an additional cost for users.As the coefficient of performance (COP) for HPs is more than the efficiency of fossil fuel boilers, implementing HP would result in CO 2 savings in most cases.However, if the grid electricity is highly carbon intensive (such as if produced by coal), and if the existing boiler uses low carbon fuel (such as waste biomass), then a detailed CO 2 savings analysis becomes imperative to make sure that an HP solution results in positive CO 2 replacement.Hybridization of HTHPs with other backup sources (e.g., green boiler, solar thermal, thermal storage) is the way forward for industries while assuring the system's reliability.In developing countries, grid reliability is low, so hybridization becomes more critical to ensure process operation.
Process heat demand mostly occurs above 80 • C, which coincides with the starting sink temperature range of HTHPs.HTHP applications between 100 and 160 • C are mostly found in food and tobacco, chemical, and paper industries for drying, pasteurization, sterilization, evaporation, and distillation [8].The next sub-section provides an overview of different heat pump products.

Research overview and commercial suppliers
Despite promising advantages, there are still some barriers to the widespread deployment of industrial HTHPs, such as a lack of awareness and confidence in industries, integration challenges, and a lack of skilled personnel for upkeep [9].However, a significant research effort is being made to overcome these barriers.
Today, there are more than 300 good practices of industrial HPs applications in processes such as drying, washing, evaporation, distillation, and cooling [10].Currently, there are 20 projects realized for heat pumps with steam generations above 100 • C. The recently concluded HTHP symposium in the Year 2022 has attracted several researchers and industrial partners to come forward to collaborate [11].Research projects and case studies create a huge knowledge base to promote this technology's rapid deployment.IEA HPT has conducted several Annex projects dedicated to industrial HPs, such as Annex 21, 35, and 48 [12].The second phase of IEA Annex 48 on industrial HPs focuses on overcoming the existing hindrances for larger market penetration of industrial HPs.The ongoing Annex 58 (2021 to 2023) is specifically dedicated to HTHP with supply temperatures over 100 • C and aims to support understanding of the technology's potential among various stakeholders, such as manufacturers, end-users, consultants, energy planners, and policymakers [13].
A major development focus is new environmentally friendly refrigerants with low global warming potential while achieving high heat sink temperatures [14].Increasing numbers of manufacturers can offer steam heat pumps suitable for industries.These companies provide heat sink temperatures mostly between 90 • C and 200 • C, with few suppliers providing products exceeding 200 • C using steam as an energy carrier [15].The heating capacities vary from about 20 kW to 20 MW based on cycle design (single-or two-stage), refrigerant type, compressor types, internal heat exchangers, etc.The COP values range from 1.5 to 5 at temperature lifts of 130 to 40 K, respectively.IEA HPT Annex 58 project has identified at least 25 manufacturers providing steam-generating heat pumps.The technology readiness level of the recalled products ranges from 4 to 7 to a maximum temperature of up to 280 • C. The specific investment cost of the heat pumps ranges from 200 to 2 ′ 000 €/kW th .The large variation can be attributed to various technological alternatives and installation boundaries.The average lifetime of an HTHP is between 10 and 35 years and correlates with its thermal capacity, with a lifetime of up to 30 years for capacities more than 5 MW [13].

Integration approaches for HTHP
1.1.3.1.Heat pump integration with boilers.Understanding an existing industrial system is imperative for the retrofit design of any thermal solution.In an industrial setting, a boiler is typically used to generate steam, hot water, or hot oil to heat various processes.When designing such conventional systems, the knowledge of the expected peak load alone from industries is sufficient in many cases.The peak load then helps determine the size of the heat generation equipment, such as a boiler.Depending on the type of industry, the heat load can vary daily, weekly, or monthly.Planning of HTHP in an industrial system can be released in several steps, starting from measuring the existing system data, and identifying the potential for waste heat recovery [16].This can be done using pinch analysis by generating composite curves of various heating and cooling streams.Using this approach, an optimal integration point can be found for HTHP, given the techno-economic P. Saini et al. constraints.For example, HTHP can be used as a parallel operation with a boiler to generate steam to feed into the main steam header.Another possibility is to use HP only for a specific process, therefore isolating that process from the boiler [17].
1.1.3.2.Heat pump integration with various processes.The process integration of HTHP often benefits lower temperature lifts, thus better COP, than when HTHP is used for central steam generation.One such use of HTHP is for drying applications.Drying is an energy-intensive process with significant potential for waste heat recovery from outgoing moisture content in exhaust gases.Heat losses in drying process plants in Europe amount to about 11.3 EJ [18].The European goal is to develop HTHPs technically and economically feasible for heat recovery in industrial drying and dehydration processes to utilize them at higher temperatures up to 160 • C. When HP technology is deployed to 50 % of all European drying processes, it is expected to reduce 3 % to 7 % of CO 2 emissions [19].
Several EU projects have been funded to achieve this goal.Among others, the "Dryficiency" project, which intended to develop two HTHP systems expected to save energy and reduce emissions.These systems focused on drying and dehydration processes up to 160 • C. As a result, energy savings of up to 80 % were expected, and emissions reductions by 75 % compared to the existing system.As a result, two vapor compression HPs have been introduced and utilized in real production plants in industrial drying processes in three European manufacturing companies in the food and brick industries [20].
The project "FRIENDSHIP" intends to study HTHPs, heat storage systems, and solar technologies and how they can be combined to optimize the system for the intended process.The systems created from this project are expected to provide heating and cooling, ranging from 300 • C to − 40 • C [21].The goal is to understand how much emissions can be reduced using the mentioned technologies.Within the project "SuPrHeat", three HTHP systems are developed using different heat transfer fluids.These heat pumps will supply heat at temperatures up to 200 • C. Water, hydrocarbons, and CO 2 will be used in the developed heat pumps, which will be integrated into existing facilities [22].
1. 1.3.3.Heat pump integration in district heating.Besides industrial applications, HTHPs are an important bridge technology to decarbonize heat in the district heating sector [23].Large-scale HPs in district heating plants can deliver supply temperatures between 85 • C and 120 • C. In addition, renewable electricity from solar PV and wind turbines can be used to run the HPs and other system auxiliaries.The political incentives in Denmark support Danish research to develop more efficient HPs to reach 0 % CO 2 emissions by 2050.Industrial HP installations comprise 77 projects with a total capacity of around 120 MW, of which 66 cases use HPs in district heating.HPs with a combination of solar thermal collectors and large pit storage is an effective combination to achieve a nearly 100 % renewable heating system [24].

Solar thermal for industrial applications 1.2.1. Solar thermal for industries
Solar energy is often mistakenly synonymous with solar PV, which produces electricity using photovoltaic cells.However, another market segment of solar energy is known as solar thermal (ST)/solar heating solutions.ST is distinct from solar PV from technology and the final energy-form perspective.ST has a decade-long, worldwide implementation history and is the 3rd largest renewable energy source with a total installed capacity of 522 GW th .The cumulative collector area installed up to 2021 is 746 million m 2 (equivalent to 75 000 soccer fields) [25].Studies have shown that solar thermal technologies need up to 4 times less land area for the same final thermal output than solar PV [2627].In addition, there has been a significant decrease in the capital cost and LCOH for solar thermal plants in the past few years [28].
There are several types of solar thermal collectors available in the market.The selection of a collector type depends on the required temperature levels, the process medium, and the local annual irradiation characteristics.For temperatures above 150 • C, usually tracking concentrating collectors can be applied.Within the temperature range between 100 • C and 150 • C, both collector types (concentrating and non-concentrating) can be technically suitable and need project-specific evaluation.The technology selection for a specific site and application is typically based on the lowest heat generation costs during the service life.
Solar heat for the industrial process (SHIP) is a sub-sector in the market, where the collectors are often used to generate steam for the industries.The SHIP market has gained significant attention in the past few years.In 2021, a total of 78 SHIP plants were installed worldwide with a collector area of 52 ′ 000 m 2 .Up to 2021, the total installed SHIP projects approached 1000, with a cumulative collector area of 1.23 million m 2 [25].The growth rate for SHIP plants is expected to be significant, given the high heating costs that industries are facing due to the restricted gas availability.
There are many solar thermal collector types that can medium temperature range needed for many industries.Concentrating technologies are primarily used for steam generation applications.Linear fresnel collectors such as those provided by [2930] are light weight and can reach temperatures up to 300 • C. The optical losses are higher in Fresnel collectors due to the flat mirrors, which do not represent a true parabola.However, a significant advantage lies in a fixed receiver (usually glass covered) which eliminates the need for moving joints carrying working fluid.Large parabolic dish collectors with dual axis are realised in a few projects in India and Australia [31 32].Dual axis tracking leads to a point focus concentrator, and thus possibilities to reach high temperatures.Despite this, the commercial applications of such collectors is limited to only few projects.
In stationary collectors, a vacuum flat plat collector can generate low-pressure steam [33].Even though the concentration ratio for such a collector is 1 Sun (or slightly higher for non-imaging collectors), lower heat losses due to vacuum lead to higher fluid temperatures in collectors.However, due to no tracking system in such collectors, the stagnation conditions are difficult to manage compare to tracking technologies.
Out of different solar thermal technologies, PTC is widely used and holds the top spot for total capacity installed worldwide SHW.PTC collectors consist of a parabolic shape reflector with a high reflectivity surface.The sun ray's incident on the surface with an incidence angle is longitudinal, and the collector tracks the sun in a transversal plane.These rays are concentrated on a receiver, which absorbs this heat, and then heats the working fluid.There are many construction variants of PTC available commercially.The large aperture PTC collectors with glass reflector and vacuumed receivers are used in many CSP plant.The advantage lies in a high concentration ratio and lower heat losses, which helps reach the high fluid temperature required for power generation.For industrial-scale PTC collectors, several design aspects need to account for, such as modularity, low weight for roof top installation, lower cleaning cost, etc. Fig. 1 exemplifies the industrial PTC type with key components [34].The collector has no vacuum in receiver tubes, unlike in large PTC, instead the whole PTC is covered with a glass cover to avoid soiling.
Mexico has the highest number of SHIP plants installed, with a capacity weighted LCOH of 36 €/MWh, competitive with fossil fuels such as liquefied petroleum gas (LPG), fuel oil, and diesel, suggesting the potential for further market growth.In Europe, from the year 2014 to 2019, the total installed cost of SHIP projects has decreased by 40 %, reaching a value of around 550 €/m 2 aperture area.In addition, the economy of scale and policy support can further help to reduce heating costs [28].

Solar thermal integration with industrial processes
While designing the system for users, the dynamics of the solar thermal collector, heat load profile, and storage system are important [35].Fig. 2 shows a typical boiler system in industries, where red concentring circles with number marks the integration points for ST collectors.The integration points before the boiler are from 1 to 3 i.e. makeup water, condensate heating, and feed water heating.Most lowtemperature collectors are used to pre-heat the boiler feed water or make-up water temperature, as the temperature levels are appropriate for such collectors.If an economizer already exists in the boiler, the solar feed water heating often decrease the economiser's effectiveness, making feed water heating a less attractive integration point.
If integrated for steam generation (points 4 and 5), collectors can displace the fuel energy in the boiler to provide the latent heat.Therefore, integration after the boiler for steam generation allows for a higher solar fraction (SF) than integration before the boiler.There is also a possibility for process integration (number 6), where solar collector system is used to meet heat demand for a specific process with or without thermal storage.Process integration in some cases allows to have lower operating temperature in the collector resulting in high efficiency.Task 49 of IEA SHC provides detailed guidelines regarding design principles and integration recommendations for ST collectors [36].

Hybridization of solar thermal with heat pumps
Terrestrial irradiation has daily and seasonal variations.For a typical solar PV system, the grid acts as a large battery that balances the production and demand with minimal waste of electricity.However, the solar heating systems are often retrofitted with individual/stand-alone boilers to continue the operation for non-sunny hours.
As most industries have constant heating demand throughout the day (and year).ST System design with a low SF allows the solar production to always be less than the user's heat demand, thus increasing the system's utilization.Therefore, SHIP systems are typically designed with low solar fraction and backup systems.However, if an ST is designed without thermal storage, the fraction of the overall heat demand met with solar collectors be limited.
To achieve high solar fraction, steam storage is often a limiting component restricting the cost feasibility of the system.Due to its very low density, steam storage is not economical.It is usually stored as sensible heat in solid media or liquid using oil or pressurized hot water.It is observed that for a given constant load profile, the economic feasibility of the solar thermal installation decreases after a threshold solar fraction due to the need for high thermal storage capacity.As the specific heat cost of a pressurized thermal storage is higher than that of a solar thermal collector, thus large tank volumes in the system result in a relatively high cost of heating.This situation puts a financial limit on the maximum solar fraction achievable.
As industries are looking for nearly 100 % renewable heating systems, solar thermal has the opportunity to collaborate with other technological alternatives to compensate for the solar irradiation lack or fluctuation during the night and day.Fig. 3 exemplifies such a system concept to reach high renewable heating fractions using a concept involving several technologies, such as thermal storage and a heat pump driven by green electricity.The existing boiler use can be minimised if the system components are sized optimally.
Previous studies have shown that hybridizing the heat pump with solar thermal collector results in the lowest levelized cost of heating  (LCOH) compared when these technologies are used individually.Therefore, more research is needed to understand the techno-economic boundaries of solar thermal and HTHP in stand-alone and hybrid modes.This paper takes a step by looking into a comparative analysis of these two technologies.The current study is a base to investigate the combined hybrid systems in future applications.

Objectives
The central objective of this paper is a comparative analysis of both HTHP and PTC systems for steam applications using industrial boundary conditions.Previous studies have performed a general feasibility analysis for solar thermal technologies or heat pumps.However, only a few have investigated comparing these technologies on a large spatial scale with techno-economic boundaries.Moreover, there is a lack of studies comparing HTHP for steam generation with PTC-based collectors.Therefore, Platzer (2021) [37] proposed a unified cost indicator method to compare different technological scenarios for process heating in industries.The indicator includes the system's energy, economic and environmental performance, enabling decision-makers to evaluate several scenarios under the same base conditions.
The most relevant work to the current paper is by Meyers et al. (2018) [38], where the authors have developed a techno-economic comparison methodology using maximum turn-key solar investment as an indicator.This methodology can be used as a criterion to quickly compare and select between solar thermal and heat pumps based on boundary conditions.
However, the study did not consider the effect of SF on LCOH.This variation is critical to consider while comparing technologies, especially with high-temperature solar thermal, due to the lack of steam storage technologies.The LCOH of the ST system increases exponentially after a threshold SF due to the diminishing added value of heat storage.Therefore, when comparing other technologies with ST, the SF is a critical criterion to define and is not considered in previous studies.The current paper has overcome the limitations by using comprehensive variables as a comparison basis for both HTHP and ST.The paper also considers updated analysis from a techno-economic perspective capturing the recent development in PTC and HTHP while considering the effects of improved efficiency and cost reductions.

Research methodology
This study aims to assess the energy and economic performances of Industrial PTC and HTHP in the context of European climates.Fig. 4 shows the flow chart of the methodology used for analysis.First, the evaluation is carried out through annual energy simulations performed with dynamic simulation software.After this, a systematic approach is followed to provide the reader with the information needed to understand the results.Fig. 4 shows the workflow used during this study.
The analysis is carried out for 3 different load profiles with constant peak demand to capture a broad range of industrial load conditions.The geographical focus for simulations is limited to Europe.However, the results obtained are parametrized to direct normal irradiation (DNI) and can be used to assess the performance for any given location.
In Step 1, simulations for HTHP are conducted using TRNSYS for given load profiles to calculate the COP and thermal output [39].The outputs are based on a performance map obtained from an HTHP supplier (ref) for a broad range of operating conditions.
In Step 2, dynamic simulations for PTC collectors are done.The product chosen for this study is restricted to a PTC manufactured by a Swedish company named Absolicon solar collector AB [34].The product is designed for industrial applications and fits this study well.
Simulation of the PTC system is done in two sub-steps.The component performance is analyzed using TRNSED, and the system performance is simulated using the developed model in OCTAVE.Storage sizing optimization obtains each location's SF vs LCOH curve.The LCOH calculations for ST and HTHP are done using a developed model in Excel.
Finally, in Step 3, based on the results obtained, the LCOH of both technologies is compared to provide boundary conditions to identify the strong economic hold of each technology.An indicator SF limit is introduced to distinguish the economic advantage and to generalize the results.
The next section defines the critical boundary conditions and assumptions made for the simulations.

Load profiles and heating demand
The heat demand in the industries depends on the process characteristics and varies, which is difficult to capture by one study.However, the selection of load profiles to represent a significant share of industries is the focus of this paper.Three different load demand profiles are considered for the analysis.The peak heat demand is fixed at 500 kW th (steam flow of 0.8 tonnes per hour), typical of many process industries.As ST and HTHP are subjected to the same load constraints, the comparative results are not affected by the selection of peak load value.The steam demand is assumed at a constant temperature of 140 • C (saturation pressure 3.7 bar a ).The steam temperature range is commonly used in many food processing industries and fits well with temperature constraints for both medium-scale PTC and HTHP products.
The 3 chosen load profiles are explained as follows: • Continuous demand: Uniform demand throughout the year with 8760 annual operational hours, which results in annual heat demand of 4380 MWh/year.Such load profiles are prevalent in many large production factories, such as the pharmaceutical sector.• Weekday demand: Uniform demand throughout the weekdays of the year (no operation during the weekend).The annual heat demand for this case is 3132 GWh/year, corresponding to real cases in industrial load.An example of this load variation can be found in the food and beverage sector.• Daytime demand: Uniform demand only during the day (10 h per day starting 8:00 to 18:00 for whole week), resulting in an annual heat load of 1825 GWh/year.This load profile is typical for a small/ medium production facility.
A summary of the considered load profile cases for simulations is shown in Table 1.
Weekly variation for considered load profiles is shown in Fig. 5.The presented week pattern is repeated for a whole year to obtain the annual heat demand.

HTHP boundaries 4.2.1. HTHP integration
After industrial boundary conditions, the next step is to design an HTHP system and evaluate the techno-economic conditions.HTHP can be integrated at several points, for example, central steam generation for the whole plant or a specific process.This integration type will decide the inlet temperature of the fluid stream at the sink of HTHP to further convert into steam.Steam generated by the HTHP will be fed to the steam line.Therefore, the sink inlet fluid can be tapped feedwater or deaerator of the existing boiler system.The feedwater pump used for the boiler can be utilized to obtain the required flow in the HTHP.If integrated with the boiler steam header, HTHP must generate steam at slight overpressure to ensure that steam from HTHP is preferred over boiler steam.
The HTHP is designed for peak heating capacity in this study.Therefore, it is considered the sole heat source for the energy system without any backup boiler.On the source side, the available wastewater stream is considered at the inlet, which transfers heat to the HP refrigerant and exits at a lower temperature depending on the temperature glide.On the sink side, the feed water stream enters the inlet and receives heat from HP to convert to steam, which is fed to the process line.A commercial HTHP (Kobelco model SGH 165) capable of generating steam at a maximum temperature of 165 • C is used to meet the steam requirement [40].

HTHP model
The HTHP used for this study can produce steam up to maximum temperature and pressure of 165 • C and 0.8 MPa-gauge, respectively.The applied refrigerant in this HP is a mixture of R134a and R245fa.The heat pump utilizes a semi-hermetic inverter twin screw compressor.The rated COP of the modeled HTHP is 2.5, specified at source and sink temperatures of 70 • C and 165 • C, respectively.A performance map based on data from the commercial HTHP [40] is used to calculate the electricity consumption.The performance map consists of the COP of the HTHP for various temperature lifts, as shown in Fig. 6.The temperature lift represents the difference between the fluid temperature at the heat source inlet and the heat sink outlet.The heat pump has a variable speed capacity to operate at the part load conditions.The electrical consumption derived from the annual simulations is then used to calculate the LCOH.
The design temperatures for the HP model are shown in Fig. 7.The source for the HP evaporator is considered a wastewater stream with a fixed temperature of 40 • C, available throughout the year.A temperature glide of 6 K is considered on the evaporator.The resulting temperature lift of the HTHP is 100 • C, corresponding to steam temperature of 140 • C. The feedwater temperature entering the HTHP arrives at 110 • C, resulting in a 30 K temperature difference on the heat sink side.The flow rate in the source and sink are varied to obtain the designed temperature glide and thermal capacity, respectively.The heat pump is designed for peak heating demand of 500 kW; the specifications are shown in Table 2.
Other than the electricity consumption of the HTHP, there are water pumps on the source and sink side, which also consume electricity and is  important in LCOH calculations.The pumps are sized to provide the desired flow rate in the network.Pressure drop calculations are done to estimate the total head in the network using Serghide's method [41].The pump is designed for the total pressure drop of the network is 1 bar, assuming 20 % safety factor while accounting for bends and joints, etc.Based on the flow and pressure drop, a commercial pump is selected from the manufacturer catalog.The products are used to derive the pump curves (flow vs head and efficiency vs head) to simulate the working points of a given scenario and thus used for electricity consumption to calculate LCOH.

Economic inputs for HTHP
For the heat pump LCOH, it is necessary to include various costs.The analysis is done for 3 different capital expenditures (CAPEX) of 500, 1 ′ 000, and 1 ′ 500 Euro/kW th values derived from data based on implemented HTHP case studies [13].The operational costs for HTHP consider the electricity to run the heat pump compressor and fluid pumps.The O&M costs for HTHP are usually higher than those for boilers and are set to 5 % of the CAPEX value.The relevant parameters for HTHP thermo-economic modeling are shown in Table 3.The LCOH of both HTHP and PTC systems are compared for a time horizon of 15 years.The period is chosen to reflect the suitable timeline various multinational companies consider for energy-related investments.Three different electricity prices are chosen for analysis considering the range of industrial electricity tariffs in the EU.
For sensitivity analysis of LCOH, a total of 27 cases are analyzed, accounting for 3 different values of three variables (i.e., CAPEX, electricity price, and load profiles).The values of these variables are shown in Table 4.

ST simulations 4.3.1. PTC product description
The ST product considered for analysis is a PTC collector manufactured by the Swedish company Absolicon solar AB.The product T160 is a concentrating parabolic trough collector that focuses direct solar irradiance onto an absorber tube that runs along the focal line of the concentrator and contains a working fluid that gets heated when solar radiation is concentrated on it.The collector works on single-axis tracking using the astronomical watch, which tracks the solar collectors, so they always face the sun.The product can generate steam and hot water from 60 • C to 160 • C, and is therefore suitable for many industrial sectors (e.g., dairy, brewery, chemical, etc.).The collector can be categorized as a small PTC type and is certified by solar Keymark.The optical efficiency of the collector is 76.6 % based on aperture area.The key technical specifications of the collector are shown in Table 5.
The main components of a collector consist of: -Reflector, which reflects the incoming radiation onto the receiver.
-The receiver tube absorbs reflected radiation and converts it into heat; this heat is then dissipated by the agent fluid that is pumped through the receiver tube -The protective glass avoids heat losses and protects the collector from dust, snow etc.

Collector integration in the system
The overall system is divided into 2 circuits, as shown in Fig. 8.The pressurized water stays in a closed loop called the solar loop, or primary loop.Pressurized water is warmed up by circulating in the solar collectors and is later cooled down through the heat exchanger, which has its secondary side connected with a steam separator.The steam separator is a vessel that contains a mixture of steam and hot water.When the required pressure is reached in the vessel, the steam will be sent to the customer's main steam line.
The generated steam is separated from the remaining saturated liquid in the steam separator, and it has priority over the boilersgenerated steam due to small overpressure.Its title is maintained high by the small overheating.The saturated liquid separated from the steam in the steam separator is mixed with the incoming water from the deaerator and recirculated to the steam heat exchanger.The separation of steam and water (that occurs by gravity) creates inside the steam separator two regions (one of steam and one of liquid at nearly the same temperature), the volumes of which can fluctuate and absorb small power surges or reductions.
The system arrangement considers PTC collector fields with a storage system to generate steam at 140 • C with feedwater temperature at 110 • C, same as considered for HTHP.Whenever solar production exceeds the demand, heat is diverted to thermal storage.When the steam is directly used in the process, the forward temperature from solar field is 145C, to have slight overpressure in the circuit compare to boiler steam pressure.The storage system is considered a pressurized tank using

Table 2
Specifications of HP considered in this study [40].water as a storage media.When the storge is fully charged, the water Is heated to a maximum temperature of 170 • C.After discharging, the tank is cooled down, corresponding to the bottom temperature in the steam separator (approximately 150 • C).This results in an effective temperature difference of 20 • C In the thermal storage between charging and discharging.

Modelling of PTC system
A dynamic simulation of the collector performance was carried out for a statistically normal year based on climate data from Meteonorm using time step of 15 min.Simulations are based on the Solar Keymark ISO 9806 collector parameters of the Absolicon T160.
The simulation approach for PTC is based on 2 steps.In the first step, the collector is modelled without interacting with the heating load.This can be considered as if the collector operates under infinite load, and thus all the heat generated by the collector is fully utilized.The simulations are done using TRNSED, which is an add-on to TRNSYS.The collector performance parameters based on the aperture area used in the TRNSED are shown in Table 6.
The output of the component analysis results in an hourly profile of collector output with other variables.The output is used in the system model for defined industrial boundaries.System simulations are done using the OCTAVE tool, on an hourly time-step basis for a year.The tool simulates the collector interaction with the load.Several iterations are performed to obtain the collector area and storage volume for a range of solar fractions for specific loss factors.The loss factor represents the maximum quantity of heat allowed to spill from the collector at any time step and is fixed at 20 % of the collector production.The loss factor is chosen based on previous experience.Based on the simulations, a curve representing the collector area and storage volume needed for a range of solar fractions is obtained.The range of SF is restricted from 1 % to 91 %.It is possible to run simulations for SF approaching 100 %.However, this is avoided so to reduce the computational effort.
Moreover, it is uncommon to design a solar thermal system for such high SF due to the excessive tank volume required, which negates the installation's economic gains.The power consumption of the PTC system due to tracking and fluid pumps is also derived from simulation results.The collector area and tank volumes required for various solar fractions are then used to calculate the LCOH of the system, as explained in the next section.

Economic boundaries & geographical inputs
The data for PTC economic analysis includes the capital and O&M cost.The economic input values are based on data collected from the PTC manufacturer as shown in Table 7.
The simulation for PTC is done for various locations in Europe, as shown in Fig. 9.If the country's direct normal irradiance (DNI) is spatially uniform, then one city from each country is used for simulations.However, some of European countries (for e.g France, Italy and Germany), have a significant variation of DNI.Therefore, multiple cities within the same country are selected for better representation.The LCOH results of PTC for each country would be compared with the LCOH of HTHP to determine the solar fraction threshold below which ST is more economically attractive than HTHP.The annual DNI for simulated location varies from 642 kWh/m 2 (for London) upto 1848 kWh/m 2 (for Seville, Spain).The average DNI for all simulated location is 1098 kWh/m 2 .Fig. 8.A concept process and instrumentation diagram for PTC field integration in an existing boiler system for steam generation.

Table 6
Input Performance characteristics of T160 collector used for model [34].

PTC simulation results
The results from PTC simulations suggest that LCOH has a higher variation than HTHP.The reason can be attributed to a wide range of solar irradiation variations across simulated European locations.Furthermore, the LCOH also varies with solar fraction for any specific location.
The range of LCOH obtained from PTC varies is huge depening on the solar fraction.The minimum LCOH obtained is The lowest LCOH is obtained for high DNI regions (for example, cities in Spain, Portugal and Southern Italy), and at a lower solar fraction.Fig. 10 shows the variation of LCOH for PTC collectors at 3 different solar fractions (5 %, 25 %, and 50 %) and for LPR1 (8 ′ 760 h/a) using all simulated locations.A decreasing trend of LCOH with increasing DNI is due to high collector output.Furthermore, for the same DNI, the LCOH increases with an increase in solar fraction due to lower utilization of heat.
It is also important to consider that LCOH depends not only on the absolute annual DNI value but also on the temporal variation.The high temporal variation makes it difficult to achieve large SF due to the large tank volume needed, thus increasing the LCOH.This can be seen by comparing the 2 data points in Fig. 10 at 50 % SF (compared data points are marked black border).These 2 locations have nearly the same  annual DNI value of around 1500 kWh/m 2 .However, the LCOH for one location is much higher (147 €/MWh) than the other (75 €/MWh).
The range of PTC LCOH for all 3 load profiles is shown in Fig. 11.Results indicate that load profile 3 has the lowest LCOH at a high solar fraction (50 %), due to high coincidence in solar irradiation and load demand.LPR2 has the highest LCOH at any SF.This is due to the lack of heating load during the weekend in LPR2, which results in high storage volume or heat spillage from the collectors.
While comparing the LCOH of PTC or any ST product with other technologies, it is important to specify the SF at which the comparison is made.
The simulation results are used to generate SF-LCOH curve as exemplified in Fig. 12 for a location in Seville-Spain for Load profile 2 (LPR2) at PTC CAPEX of 350 €/m 2 .As seen, LCOH will have a minimum constant value up to a certain SF, till all the collector heat is utilized by the system resulting in no excess heat and no storage tank.In the curve below, the minimum LCOH obtained is 46 €/MWh from SF of 1 % to 25 %.
However, after a threshold solar fraction, the thermal production of the collector exceeds the load demand, bringing the need for thermal storage.The introduction of thermal storage adds additional cost to the system, increasing the LCOH from 45 €/MWh at 25 % SF to 67 €/MWh at 46 % SF.After this point on the curve, the LCOH increases exponentially as the thermal storage size required is very high with an increase in SF.
Increasing collector areas/tank volumes would diminish the returns for utilized heat, increasing the LCOH.This trend can be observed after SF of 46 %.
The curve shown in Fig. 12 is obtained for all the simulated locations and 3 different load profiles.It is then compared with HTHP's LCOH to obtain the corresponding SF limit.
Fig. 13 illustrates the LCOH comparison for both technologies for the Spain-Seville location.The curves shows the variation of PTC LCOH with SF for 3 different load profiles.For sake of comparison, the two horizontal lines in the graphs represent the minimum and maximum LCOH of HTHP for all simulated cases.Due to the high DNI in Spain, the minimum LCOH of PTC is always lower than HTHP for all simulated cases.It has been shown that for such a location for LPR 1, even with low CAPEX and OPEX of HTHP (bottom horizontal line), the SF limit is at 37 %.SF limit increases up to 65 % if the highest value of CAPEX and OPEX for HTHP are considered (top horizontal line).
Also, the minimum LCOH of PTC is higher in LPR2 compared to LPR1.This is due to the lack of weekend operation in LPR2, which results in high tank volume, and heat that cannot be utilized in the system, thus resulting in higher LCOH.The minimum LCOH of HTHP in LPR3 is higher than in LPR1 and LPR2.The reason for the same is a lower number of operational hours while having the same CAPEX of HTHP, which results in high LCOH.
Comparative results are shown for a low DNI location (Czech-Prague) in Fig. 14.As can be seen, the PTC LCOH for Prague in LPR.1 (a) and LPR.2(b) are even higher than the worst case of HTHP LCOH (CAP3-ELP3).This implies that for such a case and under the analysed boundaries, HTHP would be more cost-effective option for heat generation than PTC.However, there would be a change by considering LPR.3, where PTC has lower LCOH upto SF of 15 % compared to CAP3-ELP3 scenarios.The SF limit for this case is at 15.6 %.

Comparative analysis using SF limit
Based on the analysis in section 6.2, the values of SF limit for all simulated cases is shown in Table 8.The countries where no value of SF limit is defined (such as in UK-London) indicate that LCOH of ST is always higher than LCOH of HTHP in all simulated cases.These countries often have very low annual DNI, resulting in lower economic feasibility for PTC.
Using the above results, SF limit is generalized to location-specific characteristics such as annual DNI value.For each load profile, a linear correlation of annual DNI and SF limit is shown in Fig. 15   P. Saini et al. at PTC CAPEX of 350 €/m 2 .In this graph, the intersection of the DNI and HTHP curve provides SF limit below which PTC is more competitive to HTHP.
In high DNI regions (1 ′ 500 to 2 ′ 000 kWh/m 2 ), the average SF limit varies from 25 % to 55 %, indicating that ST technologies are a cheaper way of reducing the emission by at least 1/4th.In such regions, the demand up to 55 % can be met by solar thermal collectors at economical LCOH if the load profiles are favourable (LPR3).In medium DNI regions (1 ′ 001 to 1 ′ 499 kWh/m 2 ), SF limit varies from 15 % to 30 %, lower than the high DNI regions.If the boundaries favor HTHP (low CAPEX, low electricity price, and high operational hours), the DNI at any location must be higher than 1200 kWh/m 2 for PTC to compete.In low DNI   regions (500 to 999 kWh/m 2 ), the maximum SF limit obtained is 10 %.In such regions, HTHP is cheaper than PTC for low and medium CAPEX/el price value.However, if the electricity and CAPEX are high, PTC can compete with HTHP at a minimum DNI of 764 kWh/m 2 .Fig. 16 and Fig. 17 show the variation of DNI and SF limit for PTC CAPEX of 250 €/m 2 and 450 €/m 2, respectively.
Comparing the results for different load profiles, there is an increasing trend of SF limit with increasing DNI when moving from LPR.1 to LPR.3, indicating better economic results for PTC.When the consumption and production have high coincidence, for example, in LPR.3, there is a small need for storage, which would result in lower LCOH for the PTC collector, and therefore higher value of the SF limit can be seen.For all load profiles, Higher SF limit can be obtained for high DNI locations, due to high thermal output of the collectors.In LPR be obtained even for low DNI (764 kWh/m 2 ) due to a favourable match in DNI and load.The decrease in PTC CAPEX results in lower LCOH for any given solar fraction, eventually leading to high SF limit .When the CAPEX of PTC is lower, and load profiles are favorable, a solar fraction limit of 32 % is obtained, even for the lowest DNI location.This indicates that cost decrease can result in PTC as the most economic heating source.On the contrary, if the PTC CAPEX is higher (450 €/m 2 ), HTHP is always competitive irrespective of DNI for CAP1-ELP1 scenarios.Such figures can be used to do a quick cost feasibility analysis for both ST and HTHP and can be a very useful tool for designers.

Discussions
The cost aspects of any technology for low-carbon process heat assessment are extremely case-sensitive.The developed methodology serves as a valuable guide to quickly determine a preferred lower carbon heat solution just by looking at the annual DNI of any location and finding the optimal SF limit for that location.The analysis is comprehensive but restricted by the absolute values of the variables assumed.Several other aspects can change the techno-economic results in the near future.For example, the carbon tax can play a significant role in reducing the cost of heating.ST technologies consume significantly low electricity compared to HTHP technologies.If the CO 2 emission cost is accounted for, the results will favor ST technologies.Also, as the electricity grid will have more renewable penetrations, the HTHP will keep getting attractive from a cost and emission perspective.
The land usage for HTHP is much smaller compared to solar thermal collectors.This is a big advantage for HTHP, especially if the industries have limited ground or roof space for solar collector installations.HTHP, on the other hand, needs more developments for low-GWP refrigerants.
A way forward could be to use both technologies in conjunction where ST is designed up to the SF limit , and HTHP is used to meet the rest of the load.A hybrid system of optimized solar thermal collectors with a small tank volume and HTHP can produce process heat at lower LCOH compared to the technologies used individually.Technology combination is imperative to reach clean and economical industrial heat ambitions.Such a hybrid system presented in Fig. 3 could have significant potential to decrease the cost and emission and will be of focus for future studies and publications.For new energy system planning, a bivalent system where only PTC and HTHP are used together, there is a possibility to reach 100 % renewable heating fraction if the electricity for HTHP is renewable.Such system an run without any need for boiler back ups.In case of reterofitting with existing boiler system, ST can be used as a starting point to cover part heat demand.Then HTHP can be introduced in the system designed for peak heat load demand to phase off the boiler completely.
In addition, to accelerate the decarbonization aims, the EU has set an ambition to build 100 positive energy districts and smart cities (climateneutral cities) by the year 2025.However, the focus of current PEDs is mainly on residential and commercial buildings.The PED boundary unfortunately does not include industrial energy system at the moment.Nevertheless, there is a strong need to factuality the industrial decarbonisation and include industrial energy system in the PED development since it belongs to whole city energy infrastructure.If a city wants to achieve the climate-neutral goal, it must consider industrial segment.On the other hand, industry segment fits high synergies if it is considered in PED concept.For instance, the low temperature waste heat can be recovered from industries to meet space heating and DHW demand through 5th generation district heating network.In return, when PED produces extra energy, it can be exported to industries through aboveaddressed district heating via heat pumps to upgrade heat to the required temperature level.Therefore, to address the technologies for industrial heat is critical to fulfil the PED and climate-neutral city goals, such as HTHP and PTC, which can be the key technologies for a fully decarbonised urban energy system.

Conclusions
This paper compares the techno-economic aspects of HTHPs and PTC collectors for various industrial boundary conditions.The focus is on steam generation at 140 • C (3.6 bar a ), commonly used in many process heating industries.The characteristics of commercial HTHP and PTC products are used as input in the simulation model to obtain energetic results.For LCOH calculation, an excel model is used.Finally, results are P. Saini et al.

Fig. 1 .Fig. 2 .
Fig. 1.A view of glass-covered small-scale parabolic trough collector with its key components.
P.Saini et al.

Fig. 3 .
Fig. 3. Example of a hybrid system of PTC and HTHP to achieve high renewable heating fractions in industries.

Fig. 4 .
Fig. 4. Flow chart for the methodology used for this study.

Fig. 7 .
Fig. 7. Various fluid stream temperatures of the HTHP system considered for this study.

Fig. 10 .
Fig. 10. of LCOH with SF for all locations at 3 different solar fractions and load profile 1 (8760 h/a) and PTC CAPEX of 350 €/m2.

Fig. 11 .
Fig. 11.Range of PTC LCOH for 3 load profiles and 3 different solar fractions for all simulated locations at ptc CAPEX of 350 €/m 2.

Table 1
Summary of the load profiles considered for simulations.
P.Saini et al.

Table 3
Assumptions regarded in HTHP simulations.

Table 4
Different scenarios for the variables used in HTHP LCOH calculations.

Table 5
Key technical specifications of T160 PTC collector.

Table 7
Assumptions regarded in PTC T160 simulations.
P.Saini et al.

Table 9
Summary of SF limit (%) of ABSOLICON PTC (T-160) and HTHP in different European countries for LPR.1.Empty values indicate that LCOH of ST is higher than LCOH of HTHP.