## 2 Thematic Review and Analysis
### Heat Source
#### 2.4.2 Combined Heat and Power  
This section provides a detailed examination of three cogeneration types, also known as combined heat and power systems: back-pressure, extraction-condensing, and a combination of both within the same cycle (see [104; 105] for comprehensive details). Maintaining a low temperature at condensers, which heat the DH water, results in reduced pressures, increasing the steam flow through the turbine, thereby enhancing electricity output [45; 106]. * Back-Pressure  
[PERSON] et al. [107] examine the impact of various temperature schemes--70/35, 90/40, and 105/50 \({}^{\circ}\)C--on the power efficiency of a back-pressure cogeneration unit. Their findings indicate that changes in the return temperature have a minimal effect on power generation efficiency, with less than a 1% variation for a 10 \({}^{\circ}\)C change. Conversely, a 10 \({}^{\circ}\)C change in the supply temperature significantly affects efficiency: a reduction can increase efficiency by up to 2.5%, whereas an increase can reduce it by 3-4%. [PERSON] et al. [104; 108] present experimental data spanning from January to August, revealing a linear relationship between the DH return temperature and the electricity output from the steam turbine. Their findings indicate that the electrical output increases from 67.1 MW\({}_{\text{el}}\) to 67.5 MW\({}_{\text{el}}\) as the return temperature decreases from 59.8 \({}^{\circ}\)C to 55.1 \({}^{\circ}\)C.  
Figure 9 illustrates the impact of the DH return temperature on combined heat and power plant operations. Higher return temperatures require increased supply temperatures to maintain constant heat output, leading to a substantial rise in DH mass flow rate. Specifically, the flow rate increases from 745 kg/s to 1020 kg/s as the return temperature rises from 45 \({}^{\circ}\)C to 65 \({}^{\circ}\)C, necessitating a supply temperature increase from 99 \({}^{\circ}\)C to 107 \({}^{\circ}\)C [108]. [PERSON] and [PERSON] [109] evaluate the impact of return temperature on a geothermal-sourced organic Rankine cycle cogeneration unit with both parallel and serial configurations. In the serial configuration, geothermal water first heats the cogeneration unit evaporator, then the outlet medium is directed to the DH heat generation unit. The study, based on the Glewe, Germany facility, finds that decreasing the return temperature from 55 \({}^{\circ}\)C to 40 \({}^{\circ}\)C linearly boosts electricity output from 1.5 kW\({}_{\text{el}}\) to 6.8 kW\({}_{\text{el}}\), and enhances overall exergetic efficiency from 0.58 to 0.67 under a DH heat demand of 150 kW\({}_{\text{th}}\) and a supply temperature  
does not affect net power generation (constant at 3.55 kW\({}_{\rm el}\)), but further increasing it to 90 \({}^{\circ}\)C significantly reduces electricity production by 1.77 kW\({}_{\rm el}\). In another study, [PERSON] and [PERSON] [110] investigated a serial connection where the outflow from the evaporator is directly transmitted to the DH heat exchanger, without considering temperature boosts. Their control logic involves setting the evaporator outlet temperature based on DH supply and return temperatures, and subsequently adjusting the evaporation pressure for the organic Rankine cycle. Under a DH heat demand of 150 kW\({}_{\rm th}\) and a supply temperature of 75 \({}^{\circ}\)C, they observed a linear increase in electricity production from 2.2 kW\({}_{\rm el}\) to 8.7 kW\({}_{\rm el}\) and an increase in overall exergetic efficiency from 0.56 to 0.66 as the return temperature decreased from 55 \({}^{\circ}\)C to 40 \({}^{\circ}\)C. Similarly, changes in supply temperature from 60 \({}^{\circ}\)C to 85 \({}^{\circ}\)C did not affect net electricity generation, which remained at 4.68 kW\({}_{\rm el}\), while further increases to 90 \({}^{\circ}\)C resulted in a significant reduction of 2.5 kW\({}_{\rm el}\). * Extraction-Condensing [PERSON] et al. [111; 112] show power loss in extraction-condensing cogeneration units relative to DH heat load and operational temperatures.