Exchanging and Cascading Heat and Cold 6391 Exergy

The First Law of Thermodynamics states that energy is never lost, whereas the Second Law describes that processes develop towards a state of increasing entropy. Hence, entropy embodies the non-useful waste energy evolving during processes. Exergy is the useful part, the part that can perform work. It is a measure of energy quality.

Processes can be energy-efficient, but in terms of exergy this efficiency can be totally different if the initial exergy level is predominantly converted to entropy. For instance, a boiler may have an energetic efficiency of 95%, but considering the gas flame of 1500°C, a lot more can be done with it than just heating up houses to 20°C. The exergetic efficiency is approximately 15%. If the gas flame heat were used in the

Fig. 6.16 Suitability of the soil in the province of Groningen for vertical heat exchangers (IF Technology, 2005), and the energy yield that it can produce (1 PJ = 278 GWh)

metal industry, the exergetic efficiency would approximate 100%. Therefore, energy of a high-quality level should be used for high-grade functions before it transforms into a lower-quality state, which can still be useful to low-grade functions. The Low-Ex Approach

The current energy system in a region (Fig. 6.17) is characterised by an influx of primary energy - fossil fuel - into every function present in the area. A power plant, which is also fuelled by fossil energy, generates electricity and every function produces waste and waste heat. The latter is emitted into the air and water.

The low-exergy (low-ex) approach strives for limiting exergy losses between and during process steps. This would imply feeding an exergy quality close to the required level, losing little exergy during the process step and finding a secondary function that can make use of the output level. High temperatures would only be

Fig. 6.17 Graphical example of energy provision in the current system (Source: (jij) even Vastleggen?)

Fig. 6.17 Graphical example of energy provision in the current system (Source: (jij) even Vastleggen?)

used in heavy industrial processes, of which waste heat could be used in lower-grade functions as manufacturing processes, horticulture, and subsequently for residential heating (the cascade of Fig. 6.18). Residences and agriculture can eventually again 'feed' the power plant with biomass and waste.

Thus, four instead of just one function would be served by the same amount of primary energy. In our current system each step requires high-quality input of exergy.

On the regional scale the low-ex principle has implications for planning. Low-caloric heat cannot be transported over long distances: heat losses would be too big. Therefore, spatial functions should be concentrated and mixed: horticulture near industry, residences near horticulture. Before this can be established, the energy potentials of the region should be investigated.

Figure 6.19 presents an inventory of heat produced in the environs Delfzijl. The amount of heat is defined by size of the respective circle and the temperature is defined by the colour. In this figure, heat demands are also depicted at the darkest colour of red, 90°C, which is the traditional temperature for conventional heating systems. As can be seen, industrial waste heat is at a lower level. Nowadays,

Fig. 6.18 A more sustainable, low-exergy system that uses heat cascading (right)

however, we are able to design buildings that require low-temperature heating systems, enabling reuse of industrial waste heat. Tuning Heat and Cold Supply and Demand

A better tuning of excess and shortage of heat and cold can save a lot of energy. In most urban settings in temperate climates there is simultaneous demand for heat and cold in a significant part of the year: dwellings need heat from October to April, whereas modern offices already require cooling from March to December. By the selection of specific functions, balance can be established. This commences with the analysis of daily patterns: dwellings of families with working parents and kids at school need a comfortable indoor climate in the morning and evening, whereas this applies to offices, schools and retail during the day. A well-deliberated mix of functions with utilities that enable exchange and storage of energy would to the extreme only require a resuming demand for heat or cold (not both), which can be solved with a function that only needs this heat or cold (swimming pool, skating rink etc.).

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