This study analyses the possibility of constructing combined district heating and cooling systems with a common return pipe, i.e. three-pipe systems (3P) instead of conventional four-pipe (4P) systems. In a 3P system the return temperatures vary widely during the season, from being close to the heating system return temperature at winter time to being close to the cooling system return temperature at summer time, depending on the combination between heating demand and cooling demand. In the chosen example in this paper (50% cooling load case) the temperatures are about 48?C (winter) and 25?C (summer), respectively. This lower return temperatures can be used for more effective operation of the biomass cooling plant by using stack gas condensers more effectively. On the other hand, this means also that more energy has to be supplied to the heating circuit and at the same time, a higher cooling capacity is necessary to cool down the return flow to the cooling supply temperature. In practice it means that in the 3P system energy is transferred from the heating circuit to the cooling circuit and either must be rejected by the cooling tower of the cooling circuit, or partially recovered by means of of a heat pump. The condenser of the heat pump is connected for preheating the water of the heating circuit.
The general presumptions for the 3P systems investigated in the study are that the 3P system uses the same pipe trench as the 4P system and that both the heating and cooling systems, 4P as well as 3P, are layed at the same occasion over the same consumer area. However, it was assumed that in average only each 10th customer was connected to the district-cooling network. Furthermore, it was assumed that the network out of the heating/cooling plant is divided into two main stems. In the basic analysis, only the cost of the cooling equipment and pipes were included in the economic analysis, whereas the biomass heating plant was taken to be the same in all models. As to the operating cost, the price of electricity was chosen to 350 SEK/MWh and that of biomass fuel to 80 SEK/MWh. This cost difference reflects a typical situation in Swedish energy market. The investment costs were converted to annual costs by means of annuity rate of 8% (20 years, 5% interest).
In the study, three different models are compared for 4P and 3P systems, respectively. Model 1 is the base case for completely separated heating and cooling systems in the 4P case; in 3P heat exchangers are used for recovering part of the energies, keeping the heating return temperature at 30?C whenever the common return temperature TR is below this value, and keeping the cooling temperature at 30?C, whenever TR is above that temperature.
In Model 2, a heat pump is interconnecting the supply pipes of the cooling and heating circuits, recovering energy by means of its evaporator cooling, the cooling supply pipe and preheating the heating medium at the entrance of the biomass plant. The maximum preheating temperature (summer time) is 80?C (two-stage heat pump). By that way a large amount of the heat rejected in Model 1 can be recovered and the total energy of such a system is quite close to that of the totally separated 4P Model 1 system. However, an important part of the heating energy is electrical (compressor) work, and hence normally this is an expensive way of producing heat.
For avoiding this disadvantage of Model 2, in Model 3 part of the cooling was achieved by a pre-cooling tower (3P system only), bringing down a large amount of the common return temperature at summer time to the cooling supply temperature and leaving only as much energy as necessary for the delivering the summer load to the heat pump. This system uses totally more energy than Model 2, but much less electricity and can therefore under some circumstances be more economic than Model 2.
In all 3P cases, extra or larger equipment (heat exchangers, cooling towers, heat pumps) are used compared to the 4P systems. But the investment costs for this equipment can be compensated by lower costs for omitting one return pipe. So generally speaking, if cooling investment and pipe investment costs (heating plant costs not included) are compared, it can be seen that 3P systems generally have lower investment costs compared to 4P systems.
The opposite holds for the operating costs (fuel and electricity). Those of the 3P systems are far above (Model1 and 2) or slightly above (Model 3) that of 4P systems. That also means that 3P systems are more energy intensive than 4P and therefore should in general not be applied for the reason of energy economy.
In some occasions, where electricity prices are low, the comparison gives results indicating that the total annual costs of the 4P systems and 3P systems are quite similar at low cooling loads (25%), i.e. cases where the energy consumption of 3P and 4P systems does not differ too much. For these cases with low electricity prices, some favourable conditions for 3P systems could emerge. However, the differences are quite small and such systems need more detailed analysis of every application.
Borlänge, 2001. , p. 76