Solar heating plants with flat plate collectors and parabolic trough collectors

Abstract

Denmark is the frontrunner country worldwide on the total installed capacity and numbers of large solar district heating plants. By the end of 2017, more than 1.3 million m2 solar collector fields have been constructed in Denmark. Denmark is also the first and only country with a mature-commercial market of large solar heating plants in the world. Most collectors in the existing solar district heating plants in Denmark are flat plate collectors. The thermal performance of flat plate collectors decreases with the increase of operation temperature in the plant. Parabolic trough collectors have attracted lots of attention in the temperature level 70-150 ℃ in recent years, due to the good efficiency at high temperatures. A hybrid solar district heating plant with flat plate collectors and parabolic trough collectors in series was investigated in this study. Firstly, as large-scale solar district heating plants have been witnessed sustainable growth in magnitude and number, requiring significant initial investment in Denmark, the importance of the “bankability” of the solar radiation data and collector field yield used to plan, design and operate solar heating plants increases as well. Solar radiation analysis was carried out in this study. Total solar radiation on a tilted surface is one of the most important variables used to simulate the thermal performance of flat plate collectors, while direct normal irradiance is the main input for parabolic trough collectors. Calculated direct normal irradiance from global radiation based on empirical models for Danish solar radiation conditions was introduced and the method was validated by measurements. Calculated total radiation on a tilted surface based on the hybrid empirical models only from available global radiation was proposed and has good agreements against measurements. A numerical model based on TRNSYS-GenOpt was set up and validated. Both measured and simulated thermal performances of a hybrid solar heating plant with flat plate collectors and parabolic trough collectors in series were investigated. Comparisons on the thermal performance of both collector types was carried out. Potential of the hybrid solar heating plant based on weather data from the design reference year was also presented for Denmark. Thermal-economic optimization of the hybrid solar heating plant based on Levelised Cost of Heat was carried out. Optimal solar collector areas in hybrid solar heating plants was determined based on the current investment costs. Furthermore, analysis on different future scenarios, such as different parabolic trough collector price levels and heat demand, was carried out to determine optimal configurations for hybrid solar heating plants in the near future. It is found that hybrid solar heating plants are feasible in Denmark. This study shows the potential thermal performance of the investigated hybrid solar heating plant with flat plate collectors and parabolic trough collectors in series in Denmark. The new concept might also be used for other regions. This PhD thesis consists of 2 parts: Part I is the summary of main results achieved; Part II is the peer-review scientific papers.