Transporting industrial waste heat. The potential of using the existing sewer system.

SUMMARY The industry in the Netherlands consumes large amounts of energy for the production of heat. After the industrial processes, a substantial share of this heat is degraded to waste heat. In general, this waste heat is just being discharged to surrounding environments, like the outdoor air and surface waters. Reusing this industrial waste heat might be an interesting option in reducing the heat-related energy consumption in the Netherlands. However, installing infrastructures for the transport of this waste heat is in many cases hampered by barriers, like high investment costs and societal inconveniences. Therefore, an alternative manner of waste heat transport is desirable. Using the existing sewer system for the transport of industrial waste heat might offer a solution. From a geographical point of view, this innovative manner of waste heat transport has a significant potential. A performed analysis indicates that for about 36% of the industrial waste heat producers in the Netherlands, the sewer system provides opportunities in transporting waste heat to nearby heat consumers, like utility buildings and households. Also a technical potential is recognized for sewer-facilitated waste heat transport. The hydraulic capacity reserved for the handling of stormwater runoff is usually substantial. However, this capacity is virtually unused for most of the time. By using it for waste heat transport, an average transport-efficiency of about 57% can be achieved. Sewer-integrated heat exchangers can be used to recover waste heat from the sewer. A heat pump can additionally increase the temperature of the recovered heat. It has been estimated that on average about 55% of the effectively transported waste heat can be reused by the heat pump. Including the waste heat losses during transportation, this leads to an effective reuse of about 31% of the industrial waste heat. In the research it has been analyzed that about 30 PJ of industrial waste heat is available in the Netherlands. In combination with the geographical potential and the effective waste heat reuse percentage, this leads to a nationwide energetic saving of about 9.8 – 12.4 PJ. The corresponding yearly reduction in the emissions of CO2 is about 268 – 337 kton. Both of these reductions are in comparison to the use of conventional gas-fired boilers. It turns out that the financial potential of sewer-facilitated waste heat transport is rather limited. Only for utility buildings with a floor area larger than 5,000 m2, the investment costs are estimated to be repaid within the 15-years lifetime of the heating system. For households and smaller utility buildings, the investment costs are much larger than the savings on the running costs. The limited financial feasibility also limits the nationwide environmental potential of sewer-facilitated waste heat transport. The resulting maximum achievable energetic saving is estimated to be 6.9 PJ per year. This corresponds to a maximum reduction in CO2 emissions of about 94 kton. This maximum achievable energetic saving, leads to a reduction in the nationwide heat-related energy consumption of about 0.6%. This clearly indicates that, although sewer-facilitated waste heat transport might have a substantial potential in reducing the heat-related energy consumptions in some specific situations, on a national scale, the potential is rather limited.