Advanced Energy Transmission Fluids for District Heating and Cooling
Under Annex III of the International Energy Agency's Implementing Agreement on District Heating and Cooling, the Experts Group on Advanced Transmission Fluids continued its program of research, development, and testing that was originally initiated in 1987. Member country representatives from Canada, Denmark, Finland, Germany, The Netherlands, Sweden and the United States developed a research program that addressed major concerns relating to the use of friction reduction additives (FRAs), and looked further into the problems associated with advanced district cooling technologies. Based on a review of proposals submitted by several research groups, the member country representatives selected four projects to be carried out under this annex, and activities were initiated in early 1991.
The performance of heat exchangers in systems utilizing friction-reducing additives was identified as an area of concern, since earlier testing had shown that FRAs caused a degradation in heat transfer performance proportional to improvements in friction reduction. Bruun & Sorensen was therefore asked to undertake a project to test heat exchanger performance in a system using these additives, and to develop alternatives to overcome any losses in heat exchanger performance.
A second concern of the Expert Group centered on the potential environmental and health effects associated with surfactant friction-reducing additives, the leading candidate chemical under consideration for use in district heating systems. The University of Dortmund performed a comprehensive literature review and assessment of the environmental effects of surfactants, and performed tests to determine their adsorption properties in various soil types as a means of assessing the potential impacts of district heating system leakage and spills on the local environment.
In Annex II, the Experts Group performed a preliminary evaluation of the potential for increased corrosion of district heating and cooling system metals due to their exposure to friction-reducing additives. Under Annex III, the Ohio State University expanded this evaluation to test several candidate additives in a variety of water compositions and in combination with corrosion inhibitors. Testing was also carried out at two temperature ranges common to district heating system operation.
Also under Annex II, research and testing of ice slurry mixtures as a means of improving the energy transport capability of district cooling systems was carried out. In an effort to move this exciting technology out of the research stage toward practical consideration as a viable concept for building cooling, Annex III supported Energy, Mines and Resources Canada in the development of a manual which provides design guidelines and alternatives for the use of ice slurries in district cooling systems.
Each of these four research projects has now been completed, and the results of the efforts are presented in the project report. In addition, a fourth annex addressing advanced transmission fluids has been approved, and several new research and testing projects will be initiated in the technology area in the near future.
Summary of the project reports
This report describes the results of research and testing undertaken by the International Energy Agency (IEA). Under Annex III of the IEA's program, the Experts Group on Advanced Transmission Fluids for District Heating and Cooling Applications undertook four research projects that addressed: performance of plate heat exchangers operating with friction reduction additives; environmental effects of surfactant friction reduction additives; use of ice slurries in district cooling systems; and corrosion effects of friction reduction additives.
Performance of Plate Heat Exchangers operating with Friction Reducing Additives
Bruun & Sorensen Energiteknik AS of Risskov, Denmark undertook an investigation of the effects of friction-reducing additives on the performance of plate heat exchangers. An experimental plate heat exchanger (.7MW) was installed in the district heating centre in Lind, Denmark, in parallel with the existing main (10 MW) heat exchanger at the centre. Over a ten month period, tests were conducted using the friction reduction additive "tenside" at concentrations of 100, 215 and 400 parts per million (ppm). Supply and return temperatures and flow rates were measured, as was the pressure loss across the primary (additive) side of the experimental heat exchanger. The heat transfer coefficient (a) and heat transmission coefficient (k) of the heat exchanger were then calculated.
The results of testing showed that the calculated heat transfer coefficient for the plate heat exchanger operating with tenside can approximate that of pure water, if flow velocity across the exchanger plates is increased. The higher the concentration of tenside, the greater the velocity must be increased, however. This same characteristic is also apparent for the exchanger heat transmission coefficient, with the K-value being improved at the same overall flow rate by reducing the total gap area of the heat exchanger plates.
In order to compensate for the reduction in heat transfer associated with the use of friction-reducing additives, higher flow velocities through the exchanger must be employed while maintaining the required effective exchanger surface area.
Three design alternatives can be considered:
- Utilize longer plates. Elongated plates will compensate for the reduced heat transfer coefficient without significantly changing other exchanger parameters. By allowing the exchanger to remain as a single pass system, this alternative minimizes total pressure losses across the heat exchanger and maintains all piping connections on the same side of the plate exchanger for ease of installation and servicing. In a new plant installation where sufficient space can be made available for the longer plates, this alternative would appear to be the most practical.
- Increase the number of plates. If an existing plant is being modified to utilize friction-reducing additives, it may be possible to add additional plates to the heat exchanger to compensate for the reducted heat transfer coefficient. However, preliminary calculations indicate that the number of plates required to maintain return temperatures in the primary district heat system may be substantial (up to 100% more).
- Convert to a two-pass system. In an existing plant where space and cost constraints may be apparent, converting the plate heat exchanger into a two-pass configuration may be possible. The associated velocity and turbulence increases will improve heat transfer coefficient and lower primary side return temperature. The negative aspects of this alternative include a significant increase in system pressure loss, and problems associated with more complex piping connections.
Environmental Effects of Surfactant Friction Reduction Additives
The use of friction-reducing additives in district heating and cooling systems must take into consideration their impact on the environment, and the health and well being of all living organisms that may come into contact with the additives during their use and disposal. The University of Dortmund therefore undertook an investigation of one popular class of friction reducing additives, cationic surfactants, to assess their hazard potential when used in district heating and cooling applications. This effort included a comprehensive review of the available literature pertaining to cationic surfactants, combined with testing of the adsorption characteristics of surfactant solutions in selected soil types.
The structure of the cationic surfactants under consideration for use in district heating systems is very similar to that of other cationic surfactants in wide use today. As such, their toxicological and ecological properties are similar:
> 1 mg/l
> 1 mg/l
Two important characteristics of cationic surfactants serve to reduce the exposure risk associated with their accidental release into the environment:
- they form neutral salts when in contact with anionic surfactants that are present in all waste waters and surface water.
- they adsorb on all solid surfaces.
To assess the potential for damage due to leakage in a district heating or cooling system employing cationic surfactants, adsorption testing was conducted using several common soil types. The results of these tests indicated that surfactants are irreversibly adsorbed, with the rate of adsorption by the amount of clay present in the soil.
The influence of Friction Reduction Additives on corrosion rates of District Heating System Materials
The use of friction-reducing additives in district heating and cooling systems provides an opportunity to reduce system costs and improve operating performance. However, the potential exists for these additives to have deleterious effects on metal piping and other distribution system components. The IEA therefore conducted a preliminary study, in 1990, of the corrosion effect of cationic surfactants in solutions of tap water, deionized water, and tap water containing corrosion inhibitor. Subsequently, a more detailed study of the corrosion effects of friction reduction additives was determined to be required, and was under taken by the Ohio State University under this IEA annex.
Pitting corrosion rate susceptibility and corrosion rate tests were carried out on AISI type 304 stainless steel, SAE 1112 (API Grade B) carbon steel, copper 90/10 copper/nickel, and 60/40 copper/zinc samples. Corrosion environments included tap water, deionized water, deoxygenated water, deionized/deoxygenated water, and a 50% tap water- 50% deionized water mixture. Tests were carried out at temperatures between 60 and 110 °C, and also at 25 °C to approach district cooling system temperatures, with pH values varied between 7 and 10. The effects of several friction additives on pitting susceptibility and corrosion rates were assessed. Additional tests were conducted using corrosion inhibitors in combination with friction reduction additives.
The results of testing showed that, with the exception of pitting susceptibility in tap water, the addition of Habon-G or Ethoquad T/13-50 (plus NaSal) friction reduction additives either improves or has no negative effect on pitting susceptibility or corrosion rates of the iron-based or copper-based metals typically found in district heating and cooling systems at temperatures of 60 and 110 °C, and in some cases gave improved results. However, adverse corrosion effects were found when friction reduction additives containing chlorides were tested.