Industrial Process Heat Applications Research Articles

Fabrication, Modeling, and Testing of a Prototype Thermal Energy Storage Containment

Abstract Increasing penetration of variable renewable energy resources requires the deployment of energy storage at a range of durations. Long-duration energy storage (LDES) technologies will fulfill the need to firm variable renewable energy resource output year round; lithium-ion batteries are uneconomical at these durations. Thermal energy storage (TES) is one promising technology for LDES applications because of its siting flexibility and ease of scaling. Particle-based TES systems use low-cost solid particles that have higher temperature limits than the molten salts used in traditional concentrated solar power systems. A key component in particle-based TES systems is the containment silo for the high-temperature (>1100 ∘C) particles. This study combined experimental testing and computational modeling methods to design and characterize the performance of a particle containment silo for LDES applications. A laboratory-scale silo prototype was built and validated the congruent transient finite element analysis (FEA) model. The performance of a commercial-scale silo was then characterized using the validated model. The commercial-scale model predicted a storage efficiency above 95% after 5 days of storage with a design storage temperature of 1200 ∘C. Insulation material and concrete temperature limits were considered as well. The validation of the methodology means the FEA model can simulate a range of scenarios for future applications. This work supports the development of a promising LDES technology with implications for grid-scale electrical energy storage, but also for thermal energy storage for industrial process heating applications.

Impact of Temperature and Optical Error on the Combined Optical and Thermal Efficiency of Solar Tower Systems for Industrial Process Heat

Concentrating solar thermal (CST) power towers can provide high flux concentrations at commercial scale. As a result, CST towers exhibit potential for high-temperature solar industrial process heat (SIPH) applications. However, at higher operating temperatures, thermal radiation losses can be significant. This study explores the trade-off between thermal and optical losses for SIPH applications using a collection of three case studies at operating temperatures that range from 900-1,550 °C. We assume blackbody radiation to represent the thermal losses at the receiver and we use ray tracing to estimate the optical losses. The results show the impact of process temperature on the maximum attainable system efficiency, as well as the higher flux concentration requirements as the temperature increases.

Comparative techno-economic and environmental analysis of a relocatable solar power tower for low to medium temperature industrial process heat supply

The primary objective of this study is to promote the adoption of solar power tower (SPT) technology in low to medium-temperature industrial process heat (IPH) applications and compare it with other heat generation alternatives. Through comprehensive techno-economic and environmental analyses, a proposed relocatable SPT-based IPH plant (PR-SPT-IPH), with a capacity of 1 MWth, is thoroughly evaluated against traditional concentrating solar-thermal power (CSP) technologies like parabolic trough collectors (PTC) and linear Fresnel reflectors (LFR), as well as natural gas (NG) and photovoltaic (PV)-based IPH systems. The techno-economic assessment of the PR-SPT-IPH is performed using an in-house developed MATLAB code, while the system advisor model (SAM) is used to simulate equivalent PTC-, LFR-, and PV-based IPH plants. The primary techno-economic assessment metric is the levelized cost of heat (LCOH), while the environmental analysis quantifies the avoided greenhouse gas (GHG) emissions and other pollutants resulting from NG combustion. The results show that the PR-SPT-IPH plant achieves the lowest LCOH of 2.42 cents/kWhth, generating annual thermal energy of 5.62 GWhth with a capacity factor of 48.27 %. With certain assumptions, this cost can be further reduced to 1.5 cents/kWhth, meeting the U.S. Department of Energy targets. The PR-SPT-IPH plant could annually avoid emitting 1112 kg of CO2, 54 kg of particulate matter, 1201.76 kg of NOx, and 5.08 kg of SO2. It could also save annual external costs between $43,100.28 and $153,194.67, while the annually saved fuel cost could reach $151,632. The study indicates that the PR-SPT-IPH plant surpasses PTC-, LFR-, NG-, and PV-based IPH plants both technically and financially. With reduced capital costs and appropriate incentives, the PR-SPT-IPH plant emerges as an economically and environmentally viable choice for low to medium-temperature IPH applications.

Impacts, Barriers, and Future Prospective of Metal Hydride‐Based Thermochemical Energy Storage System for High‐Temperature Applications: A Comprehensive Review

This article summarizes various thermochemical energy storage (TCES) systems based on their thermochemical properties, operating temperature, and storage density. Metal hydride (MH)‐based TCES is a potential energy storage system due to its higher energy storage density (>100 kWh m−3), higher operating temperature (>500 °C), relatively better reaction kinetics, and minimal or no energy losses (theoretically). MHs operate at various temperature ranges, and based on temperature, they are classified as low‐temperature MH and high‐temperature MH. This article highlights potential metal alloys operating above 300 °C, with an energy storage density of more than 100 kWh m−3, suitable for concentrated solar thermal power generation and industrial process heating applications. Magnesium (Mg) alloy‐based hydrides have shown good cyclic stability (up to 1500 cycles) at a temperature above 400 °C. The lower cost of material, higher energy storage capacity, better reversibility, and higher thermal stability of Mg‐based alloys have been explored in several small‐scale experimental investigations. The thermal energy storage (TES) efficiency of MH‐based TCES systems is reported ≈ 90%, which is significantly higher than sensible and latent TES systems. Challenges associated with MH‐based TES systems, such as heat transfer enhancement, cost reduction, and chemical and thermal stability, have been discussed in detail.

FORECASTING THERMAL ENERGY DEMANDS FOR VARIOUS PROCESS INDUSTRIES USING MACHINE LEARNING TECHNIQUES

Despite the fact that India has vast solar energy potential, the process industries are dependent on conventional fossil fuels for their thermal energy needs. Solar thermal energy is a viable option for industrial process heating applications to mitigate the utilization of conventional fossil fuels. The utilization of solar energy in the process industries to meet their energy demands helps to reduce the carbon footprint, and eventually will help India balance its energy needs by reducing the import of crude oil. The exploitation of solar energy in the process industries requires the assessment of future thermal energy demands. In this study, since the data points were nonlinear in nature, support vector regression (SVR) and long short-term memory (LSTM) algorithms were applied to forecast the thermal energy demands of various process industries. The process industries considered in this study were textile, food processing, leather and footwear, chemical and pharmaceutical, dairy, iron and steel, and automotive. Data from 1998 to 2014 were used for training and data from 2015 to 2017 were used for testing. The mean absolute percentage error (MAPE) was used as a performance measure metric to measure the performance of the SVR and LSTM algorithms. The average MAPEs obtained for given industries by the SVR with the genetic algorithm (SVRGA), SVR with grid search (SVRGS), and LSTM algorithm were 7.56%, 8.34%, and 11.10% respectively. The SVRGA outperformed the SVRGS and LSTM algorithm for the given training and testing data.

Artificial neural Network-Based LCOH estimation for concentrated solar power plants for industrial process heating applications

A study was condacted to assess the feasibility of a 5MWt solar-only parabolic trough concentrated solar power (CSP) for industrial process heat (IPH) in various locations across Morocco. Furthermore, the initial plant design has been optimized by considering the number of full-load storage hours and the size of the solar field. Additionally, this study employs a Multi-Layer Perceptron (MLP) model, featuring five input parameters(storage hours, solar multiples,Direct Normal Irradiation, latitude, and longitude), two hidden layers, and one output parameter(levelized cost of heat (LCOH)). The model has been trained using Levenberg-Marquardt (LM) training algorithms. According to the study, the Errachidia site was found to be the most efficient location, generating 21983.56 MWhth at a LCOH of 3.31 ¢/kWth for the initial design. The optimal plant design was determined to have a solar multiple of 1.5 across all investigated sites. The statistical analysis revealed that the MLP model proposed in the study, which had 43 neurons in the hidden layer, demonstrates the lowest errors, including the root mean square error (0.0071) and the coefficient of variance (0.1852), and achieved the highest coefficient of determination value (0.99999) during the training process. Furthermore, the disparities in LCOH values between simulated data and the proposed MLP model across six different Moroccan cities, with errors ranging from 0.01% to −4.07%, confirm the reliability of the MLP model for predicting LCOH.

Numerical and experimental studies of thermal performance enhancement for parabolic trough solar collector using none-circulated CuO/synthetic oil nanofluid

PurposeOne of the existing and commonly used solar energy harvesting devices is the parabolic trough solar collector (PTSC). Because of their ability to operate in low and medium temperatures, parabolic trough concentrators are widely used in power generation plants and industrial process heating applications. Therefore, the investigation of how different operating conditions affect these devices’ overall efficiency has received a great deal of attention in the recent decade. This study aims to enhance the thermal performance of the PTSC and reduce the system cost.Design/methodology/approachIn the novel configuration, a noncirculated nanofluid absorbs solar radiation through a glass wall. The base fluid was synthetic oil (5W30), and the nanoparticles used were copper oxide. The heat captured is immediately absorbed by the water circulating inside the copper tube immersed in the nanofluid. ANSYS FLUENT 15.0 was used for carrying out computational fluid dynamics simulations for two models of single and triple copper tubes. The experimental results obtained from a test rig constructed for this purpose were compared with the numerical outcomes of the single copper tube model.FindingsThe findings of the simulation demonstrated that performance was superior for the single copper tube model over the triple copper tube model. The numerical findings of the single copper tube model were compared with the experimental results. The numerical and experimental results differed from 3.17% to 5.6%. Investigations were carried out to study the effects of varying the volumetric flow rate of (20, 40, 60 and 80 L/h) and water inlet temperatures of (300, 315 and 330 K) on the effectiveness and performance of the newly developed model. Additionally, two nanofluid volume fractions of 0.05% and 0.075% were used for investigating their effect on the performance of the novel configuration. According to the findings, the highest thermal efficiency of 55.31% was recorded at 0.075% concentration and 80 L/h volume flow rate.Originality/valueIn this study, a novel direct absorption solar collector configuration using a noncirculated nanofluid was designed to enhance the thermal efficiency of PTSC. This new approach makes it possible to boost the thermal performance of the PTSC and lower the system’s cost.

Modeling and performance analysis of solar parabolic trough collectors for hybrid process heat application in Kenya’s tea industry using system advisor model

Among the Concentrated Solar Collector (CSC) technologies, Parabolic Trough Collector (PTC) is the most mature and commercialized CSC technology today. Currently, solar PTC technology is mainly used for electricity generation despite its huge potential for heating, especially in industrial process heat (IPH) applications. Though the technology is well-developed and successfully used in many developed countries, there is barely any development in Kenya. This paper studies the techno-economic feasibility of a solar PTC-assisted tea drying process in one tea factory that currently relies on biomass for process heat, in the tea producing area of Kericho, Kenya. The plant integrating parabolic troughs is modelled and a yearly simulation performed using System Advisor Model (SAM) software. The weather data are derived from ground measurements at Kericho meteorological weather station. SAM is used to model the impact of the principal design parameters, i.e., solar multiple (SM), thermal energy storage (TES) and hybridization percentages, on solar–biomass plant configurations, and to reveal the optimum case. The studied impacts are linked to the annual energy production and the optimal size which minimizes the levelized cost of heat (LCOH). Analysis of monthly variations of energy production by the solar PTC reveals that even when the solar system is designed to its maximum capacity (SM of 3 and TES of 24 h), some months will still require hybridisation with biomass to fully meet the energy demand. TES must also be incorporated in the solar PTC design to maximise on energy production. The hybrid solar–biomass plant with TES provides optimal performance when SM is 1.8 and TES is 24 h. This results in LCOH of 1.85 US cents/kWh, which is 25% cheaper than using biomass only as is the current practice. Furthermore, integration of solar PTC has a positive impact on carbon footprint and considerably reduces annual greenhouse gas (GHG) emissions by 9817 tons of CO2-eq, and annual fuel wood consumption by 16,462 m3 (equivalent to 23.51 acres of mature grown trees).

Investigation of a Compound Parabolic Collector with a Flat Glazing

The compound parabolic concentrator is a promising technology for efficient solar irradiation exploitation at low- and medium-temperature levels. This collector type can be used in a series of applications, such as solar cooling, desalination, and industrial process heat applications. This work presents a novel compound parabolic concentrator that presents satisfying efficiency and low cost due to the use of flat glazing and not an evacuated tube receiver. More specifically, the goal of the present investigation is based on the energy and exergy analysis of a compound parabolic collector with flat glazing, which has a concentration ratio of 2.81. The collector is examined thermally and exegetically, aiming to calculate the efficiency of different operating inlet temperatures. Moreover, the solar unit is studied by a developed computational fluid dynamics model in the SolidWorks Flow Simulation tool. Emphasis is given to the calculation of the convection losses of the receiver tube with the internal air inside the collector. The heat convection coefficient is calculated, and the distribution of the thermal losses, convection, and radiation is presented. Furthermore, the temperature levels of the absorber, the cover glass, and the top thermal loss coefficient are found. The thermal efficiency of the solar unit was 77.4% for inlet temperature at 10 °C and 32.6% for inlet temperature at 110 °C. It was calculated that the maximum exergetic performance of the solar unit is 10.19% for operation at 90 °C, while the thermal efficiency for this case is 41.57%. Additionally, the temperature distributions for different cases are included in the present work.

Analytical characterization of improvised design of parabolic trough solar collector

The analytical characterization of a mini-parabolic trough collector (mini-PTC) designed for industrial process heating applications that require hot water in the range of 75 to 90 °C is presented in this work. The analysis is carried out for Pune, Maharashtra, India from October to May. The average slope FRUl and average intercept FR(τα) are evaluated. The paper also presents the performance analysis of mini-PTC. The maximum optical efficiency (ɳ) is found out to be 61.9 % while the average value of a slope is 0.8282 W/m2-K. The analytical results indicate that mini-PTC offers a prospect for Industrial Process heat applications requiring temperatures up to 90 °C.

Parametric study of evacuated tube collector solar air heater with inserted baffles on thermal network for low-temperature applications

The great challenge in the evacuated tube solar air heater for the industrial application is choosing the optimum thermal networks to get target temperature for the desired air flow rate. In this work, theoretical analysis is done to investigate the influence of the significant design parameters such as evacuated tube length, diameter and number of tubes in a collector on the thermal performance and to design a thermal network model for achieving the desired temperature at specified flow rate of air for industrial process heat applications. Further, establishing an empirical correlation to simplify the thermal network design process. Based on the results of design parametric study, optimal parameters identified for a collector system are 20 numbers of evacuated tubes having dimensions of 1.8 m length, 58 and 47 mm outer and inner diameters. It is noticed that a considerable drop in temperature rise attained in each collector while increasing the number of collectors in series network. The enviro-economic analysis reveals that the overall useful energy gain per annum can be achieved around 8252 kWh which will reduce 17.16 tonne of carbon dioxide emission through the utilization of a thermal network comprising six modules working at 500 kg/h mass flow rate of air. The correlation relates the attainable effective thermal with the non-dimensional operating parameters such as mass flow number, temperature ratio, and number of collectors. The correlation can be used to design a solar evacuated tube thermal network and the proposed solar evacuated tube thermal network can be effectively employed for fulfilling the industrial process heating applications require hot air between 80 and 120 °C.

Design aspects of a fixed focus type Scheffler concentrator and its receiver for its utilization in thermal processing units

Fixed focus type Scheffler concentrators introduced by Wolfgang Scheffler are popularly used for different low to medium industrial process heat applications. These Scheffler concentrators are available in the standard sizes of 8 m2 and 16 m2. The present paper provides a detailed stepwise procedure for the design of a standing type 16 m2 Scheffler concentrator and its fixed focus receiver, intended to be utilized in a solar thermal processing unit for essential oil distillation at our institute. The dome type receivers of boiler steel grade material, SS-304, are selected for the present application. The diameter of the receiver is estimated based on the analysis of the image formed at the receiver of the Scheffler concentrator. The crossbar equations, seasonal parabola calculations and aperture area calculations for the Scheffler concentrator have also been presented in the paper. The geometrical concentration ratio for the Scheffler reflector dish and receiver system are also evaluated for any given day of the year. The design procedure presented in the paper can be used for designing any suitable size Scheffler concentrator and its receiver as per the energy requirement of the application.

Performance Evaluation of Solar Air Collector in Sensible Heat Storage by Forced Convection with Cherry Powder

In practice different kinds of solar air heaters are available for domestic heating and industrial air heating purposes, space heating, process heating and agricultural applications. Advantage of this system is to use of renewable energy source that is free in cost and an eco-friendly, economical and simple device. There are so many researcher who focuses to improve the efficiency and to maintain the continuous heat supply of the double pass solar air heater using sensible storage material. The heat transfer and friction characteristics due to the sensible storage material of solar air heaters have been reviewed in this research. A double pass solar air heater was fabricated and integrated with thermal storage system and powdered cherry pits are used as a thermal storage medium. The performance of this heater was studied for different configurations. The solar heater integrated with thermal storage delivered comparatively high temperature. The efficiency of the air heater integrated with thermal storage was also higher than the air heater without thermal storage system. The study concluded that the presence of the thermal storage medium at the absorber plate is the best configuration.

Thermal performance of an evacuated tube solar collector with inserted baffles for air heating applications

The thermal performance of an evacuated tube solar air collector with inserted baffles (ETSACIB) was investigated experimentally and analytically investigated under the indoor solar thermal simulator. The effects of the operating parameters such as flow rate, inlet air temperature, baffle length and solar insolation on the thermal behaviour of ETSACIB system were analyzed. Experiments were performed different flow rates of air (100–1000 kg/h). The maximum temperature rise of 42.8 °C was achieved with the lowest thermal efficiency of 35.31% at the flow rate of 100 kg/h. The experimental results show that a heat transfer enhancement in terms the thermal efficiency is obtained with inserted baffle arrangements. An increase in the inlet temperature of air causes a decrease in temperature rise as well as the thermal efficiency of the collector. The results indicated that the increase in baffle length has a positive impact on temperature rise and thermal efficiency but induces higher pumping power. It is concluded that the developed evacuated tube solar air collector has sufficient potential for industrial process heating applications.

Experimental performance of a parabolic trough collector system for an industrial process heat application

Manufacturing is responsible for 60% of the fuel consumption in Cyprus and the industrial sector is the second biggest fuel consumer, mainly for steam production. Thus, the use of parabolic trough collector (PTC) systems for the production of steam or hot water can be a promising solution for the industrial sector. This study presents the first industrial PTC system in Cyprus, installed at the biggest soft drinks factory. The system consists of 288 m2 of PTC, a steam generator and concrete thermal energy storage (CTES) in order to keep the system dispatchable. To achieve that, two operation strategies are developed which are controlled automatically by the main processor of the system. The first strategy is enabled when there is a steam demand and the second when the energy can be stored directly to the CTES. Both strategies are tested, and it is shown that under Strategy 1 the PTC system can produce 940 litters of steam per day, and under Strategy 2 it can store 107.3 kWhth. In two months period tests, it is proved that it can supply the required amount of steam to the factory even when solar radiation is low, with the support from the CTES.

Integration of a parabolic-trough solar field with solid-solid latent storage in an industrial process with different temperature levels

The aim of this work is to analyse the integration of solar industrial process heat with novel latent heat storage systems by means of a representative case study that has a demand profile with different temperature levels. To that end, an innovative thermal storage system is proposed to support the contribution of the solar field to the industrial heat demand. The storage system considered is based on the latent heat of the solid–solid transition of pentaglycerine and it seems particularly interesting for industrial applications with space limitations and low temperature differences. Additionally, it may suppose further advantages in terms of corrosion, degradation and low cost of raw material. Thermal tests have confirmed the suitability of pentaglycerine as storage material for such applications. In this case study, a solar field with parabolic-trough collectors coupled to a pasteurization process has been considered. To analyse the expected behaviour of the proposed system, a simulation model has been developed in TRNSYS. This model defines, for the first time in the literature, a complete set of operation modes and control strategies specific to concentrating solar collectors for industrial process heat applications with latent storage, able to deal with the required variable temperature levels. Annual simulations have been performed using locations and meteorological data of Graz (Austria) and Plataforma Solar de Almería (Spain). The simulation results show that the percentage of annual heat demand covered with solar energy could be increased from 20% to 27% in Graz or from 40% to 52% in the PSA by using 3 h of latent heat storage.

Modeling, simulation and outdoor experimental performance analysis of a solar-assisted process heating system for industrial process heat

Industrial process heating applications in the temperature range of 50°–250 °C consumes about 35% of the global fossil fuels, wherein solar thermal is envisaged as an alternative option. Outlet temperatures of both flat-plate collector (FPC) and photovoltaic thermal collector (PVT) are not high enough to serve industrial processes. In this article, a solar assisted process heating (SAPH) system, wherein an FPC integrated in series with a PVT, has been designed to produce low to medium temperature process heat. TRNSYS simulation models have been developed for the isolated FPC and PVT as well as integrated SAPH (PVT-FPC) systems and simulated models have been validated through outdoor experimentation. Results show that SAPH system generates thermal energy as high as 1420 W with a thermal efficiency of 75% and an exergy efficiency of 12.72%. Performance mapping ascertains that SAPH system performs better at lower mass flow rates and under higher radiations. This well-engineered integration offers unique and sustainable solution to meet industrial process heat demand at a minimum fossil fuel usage. Such SAPH system with the compatible numbers of FPCs and PVTs can be implemented to accomplish low to medium temperature process heat requirements.

Compound parabolic solar collector – Performance evaluation as per standard test method and actual field conditions for industrial process heat application in Indian context

Industrial process heating has been identified as a potential field for solar thermal applications. In this paper, a comparative study was carried out on the performance of an individual solar collector under laboratory conditions and performance of the collector array under field condition. Compound Parabolic Concentrator (CPC) has been used as a solar collector for the application of boiler feed water preheating. The characterization of CPC has been done in liquid phase with water as a working fluid as per the standard, IS 16648 (Part-5):2017. The optical efficiency of the CPC is found to be 64.8% under standard test conditions. In the field, total 200 number of CPCs, each having 3 m2 area, were installed. The performance monitoring and evaluation of the actual installation was carried out over a period of 10 months. The system was designed to supply average 448,500 kWh- Thermal/year, saving the equivalent amount of fossil fuel. The comparative study of the performance of CPC collector under standard conditions and actual operating conditions in the field shows significant variation. The average variation in the performance was about 9% with a maximum variation of 14.7%. The major reasons behind the lower performance output than expected, attribute to the heat losses through the collector manifold and piping (3.9%), soiling of reflector and absorber of CPC (1.72%), uncertainties of instruments (1.49%) and other unaccounted losses like dislocation of the evacuated tubes from the original position, loss of vacuum through evacuated tubes, shadow effect, heat utilization pattern etc. (1.89%), which should be considered while designing the solar field to meet the required output.

Optical and thermal performance analysis of a micro parabolic trough collector for building integration

Medium temperature thermal energy (100–400 °C) is widely used in industrial processes and in large buildings. The presented work investigated the feasibility of a new solar collector which was designed to harvest solar energy from factory rooftops for industrial process heat applications. The proposed solar collector was comprised of parallel micro-parabolic troughs, vacuum tube receivers, and an internal tracking mechanical contained in a glazed box which can be easily mounted on large buildings. The system does not require external rotational tracking and achieves a concentration ratio of ~4.2 in a ~150 mm height, so it can be easily integrated with buildings. The optical performance of the presented collector is analysed and modelled theoretically and numerically, considering the effect of shading, inclination, and orientation. Furthermore, a transient thermodynamic model is developed to calculate its thermal efficiency and stagnation temperature, along with the effect of vacuum pressure, beam radiation, and emissivity of selective coatings. The theoretical analysis, verified by TRNSYS simulations and an outdoor experiment, revealed that the annual optical efficiency of the system was about 66.7% and the thermal efficiency was about 59.3% at 200 °C, if the collector was inclined to local latitude angle. These results reveal that the proposed design is competitive with evacuated flat plates (i.e., the TVP collector which has a thermal efficiency of ~36% at a normalized temperature difference of 0.2). Further, if the collector were to be installed on the vertical façade of a building, the theoretical model estimated that the optical and thermal efficiencies would be 44.1% and 37.5%, respectively. An economic analysis indicated that a levelized cost of heat energy of 0.51 $/kWh can be obtained. Overall, since the proposed collector has a simple structure and a low-profile, this study indicates it is promising for medium temperature solar thermal heat production for industrial processes and/or for multi-effect absorption chillers.

Design and Analysis of Parabolic Trough Solar Water Heating System

Renewable energy technology is one of the prospective sources which can meet the energy demand and can contribute to achieve sustainable development goals. Concentrated collectors are widely used in solar thermal power generation and water heating system also. It is very popular due to its high thermal efficiency, simple construction requirements and low manufacturing cost. This paper is concerned with an experimental study of parabolic trough collector for water heating technology. It focuses on the performance of concentrating solar collector by changing the reflector materials (aluminum sheet, aluminum foil and mirror film). In Bangladesh, it is possible to use low cost solar concentrating technologies for domestic as well as industrial process heat applications. The line focusing parabolic trough collectors have been designed, developed and evaluated its performance by collecting solar radiation, inlet and outlet water temperature, flow rate, efficiency etc.