## 2 Thematic Review and Analysis
### System-Level Optimization
#### 2.5.1 Operation Temperature  
The supply temperature in DH systems is determined by the DH operator, based on factors such as weather compensation curves or fixed supply temperature schemes. The return temperature reflects the cooling capacity of consumer units in response to the specified supply temperature, as well as control and cascade considerations within the system components. Consequently, the supply/return temperature configuration varies significantly across DH systems, as each system design process must account for specific operational temperature parameters [1; 54; 123].  
[PERSON] [29] highlights the challenge of determining the optimal supply temperature, which requires site-specific analysis considering the type of heat production units and the anticipated return temperature from other DH system components. CIBSE [69] recommends optimizing temperature to account for lifespan costs and environmental impact, based on annual analysis. [PERSON] [54] generally advises using a weather compensation curve to minimize DH heat loss, especially at low loads, though exceptions may apply if pump consumption and return temperatures are deemed excessive. This underscores the need for system-level temperature optimization and accurate commissioning of weather compensation curves.  
[PERSON] et al. [11] underscore the necessity of system-level temperature optimization by addressing the trade-off between lowering supply temperatures, which reduces DH heat loss, and potential increases in pipe diameters that could lead to greater heat loss. High supply temperatures are expected to yield low return temperatures, assuming only heat emitter performance. Therefore, optimizing supply temperature requires considering DH heat loss, the efficiency of heat production units, and the operation of end-user components such as substations and indoor units.  
[PERSON] and [PERSON] [1] indicate the importance of site-specific optimization with examples. For instance, one example involves selecting a high supply temperature to achieve a low return temperature that facilitates flue-gas condensation at the heat source. Conversely, in another scenario involving a combined heat and power plant, a low supply temperature is chosen to enhance the power-to-heat ratio in the cogeneration cycle or to improve the coefficient of performance in a large heat pump plant.  
[PERSON] et al. [26] illustrate the optimal supply temperature levels for different heat production units within the same district. For an extraction-type cogeneration unit, the ideal supply temperature is identified as 66 \({}^{\circ}\)C. Conversely, for a central large-scale heat pump, a supply temperature of 63 \({}^{\circ}\)C is recommended. Considering both types of heat production units, the final return temperature is observed to range between 22 \({}^{\circ}\)C and 27 \({}^{\circ}\)C, with the higher return temperature associated with increased pump electricity consumption compared to the cogeneration unit.  
In a case study of the G\(\acute{\text{a}}\)vle DH system, [PERSON] [45] examined the impact of return temperature on various system components, including the Johannes cogeneration plant. This plant's heat production involves three stages: flue-gas condensation, back-pressure from the low-pressure turbine, and extraction from the mid of the high-pressure turbine. The study observed significant reductions in pump consumption and DH network heat loss, alongside enhanced recovery from the flue-gas condensation unit, as depicted in Figure 14.