Subsurface urban heat islands: A renewable source of energy?

Transcription

1 Subsurface urban heat islands: A renewable source of energy? Philipp Blum, Susanne A. Benz, Kathrin Menberg, Peter Bayer Institute for Applied Geosciences (AGW), Engineering Geology DonPaolo (2009) 1 KIT The Research University in the Helmholtz Association

2 Overview 1. Motivation 2. Groundwater temperatures (GWT) 3. Subsurface urban heat islands (SUHI) 4. Geothermal potential 5. Sustainable use of geothermal energy 6. Conclusion 2

3 Groundwater temperature (GWT) measurements 3

4 Subsurface urban heat islands in Germany Groundwater temperatures in m depth 4 Menberg et al. (2013) Science of the Total Environment

5 Subsurface urban heat island in Berlin Groundwater temperature-depth profile 5

6 Urban heat islands of land surface and groundwater correlate Annual mean GWT and LST of Berlin, Cologne, and Karlsruhe Spearman correlation: 0.8 ± ± ± ± LST= land surface temperatures derived from satellite data (MODIS, 1 km 1 km)

7 Urban groundwater temperatures are higher than land surface temperatures Comparison of urban groundwater temperatures (GWT) and land surface temperatures (LST) 7 Benz et al. (2015) Environmental Science & Technology

8 ΔT highest under the city center and suburbs Temperature difference ΔT = GWT LST for Berlin, Cologne, and Karlsruhe 8 Benz et al. (2015) Environmental Science & Technology

9 Subsurface urban heat islands: A renewable source of energy? 1. How much energy is in the subsurface? 2. Where is the energy from? 3. How can we use this energy? 9

10 Geothermal potential of SUHI Total heat energy in the subsurface (Q) After Balke (1977): Q Q w Q s V n C w T V (1 n) C s T Q [kj] = Total heat energy in the subsurface (aquifer) V [m 3 ] = Volume of the aquifer n [-] = Porosity ΔT [K] = Temperature reduction C w [kj m -3 K -1 ] = Volumetric heat capacity of the groundwater C s [kj m -3 K -1 ] = Volumetric heat capacity of the solids Q w [kj] = Stored heat energy in the aquifer (groundwater) Q s [kj] = Stored heat energy in the solids (sand and gravel) 10 Zhu et al. (2010) Environmental Research Letters

11 Large amount of stored energy in the subsurface Estimation of the heat energy in megacities * * by cooling the groundwater by 2 6 K. 11 Zhu et al. (2010) Environmental Research Letters

12 Subsurface UHIs are caused by humans Anthropogenic heat transport into the urban aquifer 12 Benz et al. (2015) Science of the Total Environment

13 Quantifying anthropogenic heat transport processes Heat flux model for Karlsruhe and Cologne 13 Benz et al. (2015) Science of the Total Environment

14 Mainly conductive heat transport processes Heat flux model for Karlsruhe and Cologne q heat flux; dt/dz thermal gradient; λ thermal conductivity 14 Benz et al. (2015) Science of the Total Environment

15 Heat flow mainly from buildings and elevated ground surface temperatures Heat flow of all individual heat transport processes [PJ/a] Karlsruhe Cologne 42% 16% 70% 31% 15 Benz et al. (2015) Science of the Total Environment

16 Heat flow mainly from buildings and elevated ground surface temperatures Heat flow of all individual heat transport processes [PJ/a] Estimate groundwater temperature (egwt) using land Karlsruhe surface temperature (LST), building Cologne density (BD) and basement temperature (BT): egwt = max LST LST 1 BD + BT BD 42% 16% 70% 31% Building density (BD) is a satellite-derived value. Basement temperature (BT) = 17.5 ± 2.5 C. 16 Benz et al. (2015) Science of the Total Environment

17 Estimated groundwater temperature (egwt) of Berlin, Cologne, and Karlsruhe 17 Benz et al. (2015) Environmental Science & Technology

18 Mean absolute error of estimation is 0.9 K Comparison between measured GWT and estimated egwt 18 Benz et al. (2015) Environmental Science & Technology

19 Estimation finds (most) thermal hot spots Temperature difference ΔT = GWT egwt 19 Benz et al. (2015) Environmental Science & Technology

20 High coverage of space heating Heat content, space heating demand and heat flow 32% of the annual space heating demand in Karlsruhe could be sustainably supplied by the subsurface urban heat island, in Cologne only 9%; In Karlsruhe about households could be sustainably supplied with heating energy; About tons of CO 2 could be annually saved. (reduction of 4 K) Anthropogenic heat flow in Karlsruhe Anthropogenic heat flow in Cologne Geothermal heat flow ~ 2,2 PJ ~ 1,0 PJ ~ 0,2 PJ 20 Benz et al. (2015) Science of the Total Environment

21 Increased ground source temperature [K] Increased heat extraction rates Ground source heat pump (GSHP) systems GSHP system 10% more! Increased energy potential [-] BWP (2006) Length of the borehole heat exchanger [m] 21 Rivera et al. (2016) Renewable Energy

22 Geothermal use Geothermal installations in urban areas GSHP system Energy piles GWHP system BWP (2006) Groundwater heat pumps (GWHP) system 22

23 Geothermal use Geothermal installations in urban areas Geothermal baskets GWHP system Groundwater heat pumps (GWHP) system 23

24 Geothermal use Geothermal installations in urban areas Tunnel geothermal energy GWHP system Lainzer tunnel in Vienna Groundwater heat pumps (GWHP) system 24

25 Geothermal use Geothermal installations in urban areas ATES system Tunnel geothermal energy Lainzer tunnel in Vienna GTN Aquifer thermal energy storage (ATES) system 25

26 Conclusions Urban groundwater temperatures are increased by 4 to > 6 K. Urban heat islands of land surface and groundwater correlate. Recoverable energy could cover the heating demand by years to decades. Heat flow into the urban aquifer mainly originates from buildings and elevated ground surface temperatures. 32% of the annual space heating demand in Karlsruhe could be sustainably supplied by the subsurface urban heat island (SUHI). The urban subsurface provides sustainable geothermal energy and could also be used to store energy for heating and cooling such as aquifer thermal energy storage (ATES) systems! 26