Usability of sediment heat energy

The usability of sediment heat energy was studied in Papers I–IV. The key elements of usability are availability and renewability. Papers III–IV investigate whether sediment heat is annually renewable energy and how thoroughly the recovery of heat balance in the sediment layer takes place during the summer. A further aspect of sediment heat´s feasibility as a heat source is its energy-saving ability. This hinges on correct planning and sizing of the heat-collection network.

These issues are clarified in Papers I and II.

Results of paper I:

The novelty of Paper I “Renewable, carbon-free heat production from urban and rural water areas” was that it was the first scientific article presenting the sediment heat energy system in Suvilahti and its new approach to renewable energy production. It also introduced the innovative “flower” pipe (later Refla). Paper I´s objective was to describe the sediment heat system and provide an analysis of the heat energy consumption and energy-saving ability.

The sediment heat energy system´s capability to extract heat from the sediment was verified by the optical short-term temperature measurements (DTS) which clearly indicated a drop in the sediment temperature from the normal temperature of 8 °C during the period of heating. The heat extraction rate from the sediment heat-collection pipes was evaluated to be 40–50 W/m (Aittomäki 2001). The sediment heat based low-energy system worked properly.

The annual heating-related energy consumption (including the hot water) of one household (floor area 234.5 m²) connected to the low-energy network in Suvilahti was 9 000 kWh. The energy consumption per square meter was 38 kWh/m². The average annual energy consumption of a new, low-energy, single family house (140 m², 4 people, house location in temperature zone I or II) in Finland is 15 000 kWh for heating and hot water (Motiva 2019). This equates to an average energy consumption per square meter of 107 kWh/m², more than double the figure for the Suvilahti sediment energy house. Therefore, the sediment energy system provides evident energy-saving capabilities.

Paper I compared the sediment heat system with other ground source heat systems. Its comparison is summarized here in Table 2 (modified from Table 1 in Paper I). Sediment heat is mainly generated by solar energy. A sediment heat

system is not sensitive to damage due to the position of its pipes in the sediment layer. The heat extraction rate of sediment heat system is at a very competent level compared with other ground source heat systems. Sediment heat is suitable for urban areas.

Results of paper II:

The novelty of Paper II “Correlation between temperatures of air, heat carrier liquid and seabed sediment in renewable low-energy network” was its evaluation of the adequacy of pipeline sizing by means of temperature correlations. Studying the delay of temperature changes in the sediment layer was also novel. This work underpinned Paper II´s objective of studying the adequacy of network sizing with the help of possible correlations between ambient air, heat carrier liquid and sediment temperatures, based on the measured data during 2014.

The high correlation between the heat carrier liquid temperature and sediment temperature was observed in Paper II. In particular, there was strong correlation between the liquid temperature and the next month´s sediment temperature, as well as between the liquid and sediment temperature of the same month. This correlation was seen to indicate that the low-energy system was working correctly.

In winter, the sediment was getting cooler due to the usage for heating. In summer, the sediment was warming due to the cooling of the houses and the warmer ambient air temperatures.

Table 2. Comparison of different ground source heat systems (GSHS).

System Water course heat Ground source heat Bedrock heat Sediment heat

Main heat source Solar energy Solar energy Geothermal energy Solar energy

Annual renewal Yes Yes No Yes

Number of heat collector

units > 20 Yes Yes Typically not Yes

Pipeline`s sensitivity to

damage Yes Not very No No

Main direction of pipe(s) Horizontal Horizontal Vertical Horizontal

Vertical depth of pipes In the bottom of a water

body 1.2–2 m 100–300 m 3–4 m inside the sediment

layer (from the bottom of a water body) (Lieskoski 2014) Approximate heat extraction

rate W/m 15–28 W/m (Banks

2012) 100 W/m (Banks 2012) 20–92 W/m (Stober et

al. 2013) 40–50 W/m (Aittomäki 2001)

Figure 6. The original temperature data of DTS measurements from seabed sediment in year 2014.

Except in August, the sediment temperature curve (Fig. 6) was noticed to rise slightly up to the end of the pipe (300 m distance from the shore), even in winter.

This might indicate that this network is over-sized for its energy demand. The recovery of sediment heat was observed using temperature curves for one year.

The inlet temperature of the heat carrier liquid is higher than the sediment temperature during June and July due to the cooling of houses (Fig. 7). Conversely, house heating reduced the heat carrier liquid temperature compared with the sediment temperature. The cooling of houses is observed as a peak in sediment temperatures in September. Although the ambient air temperature is falling after July, the sediment continues to warm up until September.

Figure 7. Temperature data from January to December 2014.

The high and significant correlation between the ambient air temperature and temperature of sediment one or two months later was also observed. The sediment temperature was indicating the previous weather conditions (Fig.7.) This delay reveals the heat loading and it has to be taken into account when utilizing sediment heat.

Results of Paper III:

The novelty of Paper III “Seabed sediment as an annually renewable heat source”

was its study of sediment heat renewability. It is the first time that long-term measurements have been made and analyzed. Paper III´s objective was to verify annual renewability of sediment heat or the possible cooling of the sediment. The effects of long-term usage of heat for a low-energy network were studied.

The follow-up sediment temperature measurements (Fig. 8 and 9) showed that sediment had been fully reloaded every year. The highest values of sediment temperatures were measured in every autumn. This indicated the annual accumulation of heat.

Figure 8. Seabed sediment temperatures against the distance from shore from March 2014 to August 2014 in Liito-oravankatu. Temperatures increased after the winter months.

Within the first 200 m distance from the shore, the slope of the temperature curve was bigger in March and April due to the energy intake. However, from May to August, the sediment was loaded by heat. It could be observed in the temperature curves, which became more horizontal.

Figure 9. Seabed sediment temperatures measured versus distance from shore from March 2015 to August 2015 in Liito-oravankatu. Heat loading observed as increased temperatures in the sediment layer.

Paper III studied the influence of energy usage on sediment temperatures in the long term. The comparison was made in heating years of 2008–2009, 2013–2014 and 2014–2015. Temperatures were compared between the month with the highest sediment temperature value (in autumn) and the month with the coldest sediment temperature (in winter). The temperature differences during the three studied years were 9.7 °C, 11.1 °C and 11.2 °C respectively (Fig. 10). The use of the energy did not cause a permanent decrease in the temperature rate of the sediment during the several years period of the Vaasa Housing Fair area.

Figure 10. The between-month difference in sediment temperatures for the months with the highest and the lowest temperature values in the periods 2008–2009, 2013–2014 and 2014–2015. The polynomials of second degree are drawn as trend lines.

Results of Paper IV:

The novelty of Paper IV “Seabed sediment – a natural seasonal heat storage feasibility study” was its study of natural seasonal heat storage. Its objective was to estimate the annual amount of thermal energy charged into the sediment by the Sun. The estimation was compared with the amount of energy that was exploited (see 4.3). In addition, the annual loading of sediment heat was studied with the help of long-term temperature measurements.

A three-year measurement period indicated such regular sediment temperature differences between the warmest and the coldest months of the year that it was apparent there was distinct natural loading of heat energy (Fig. 11).

Figure 11. Sediment temperature differences between the warmest and the coldest months during three annual loading periods in 2014, 2015 and 2016 in Ketunkatu as a function of length and distance of the cable from the shore.

Papers I–IV provide the answer to RQ1: sediment heat energy was found to be annually renewable and the recovery of heat in the sediment layer during summer was observed to be complete. The energy-saving ability of the sediment heat based low-energy network was also verified. Correct planning and sizing of the heat- collection network were observed to be important elements for the usability of sediment heat energy.