Christian Budig1*, Janybek Orozaliev1, Claudia Rose1, Klaus Vajen1, Elimar Frank1#,

Ruslan Botpaev2, Alaibek Obozov2

1 Kassel University, Institute of Thermal Energy Engineering, Kassel (Germany)

2 Kyrgyz State Technical University, Department for Renewable Energies, Bishkek (Kyrgyzstan)

* Corresponding Author, solar@uni-kassel. de

Abstract

In heat and power plants with so-called open district heating nets, large quantities of cold water (e. g. 12°C) are heated up to supply temperatures (e. g. 60°C) using fossil fuels. This water, however, can be effectively preheated by uncovered collectors before heating up conventionally to the supply temperature. Due to high basic load, low inlet temperature and good climatic conditions in most parts of the Commonwealth of Independent States (CIS), extraordinary solar gains and very low solar heat costs can be achieved during the frost-free season. The objective of this investigation is to identify heat and power plants in the CIS which are appropriate for solar water preheating. For this purpose, large (or central as it is called in the CIS) operated heat and power plants were identified and evaluated. It was found that 38 out of 197 heat and power plants are in principle appropriate for this kind of solar thermal technology. In addition, this study includes an economic analysis of the technology based on previous experimental and theoretical results. Solar heat costs of less than 0.01 €/kWhth and a payback time of about 9 years are expected.

Keywords: uncovered collector, heat and power plants, district heating, CIS

1. Introduction

District heating nets for heat supply are very common in cities of the Commonwealth of Independent States (CIS) and are often combined with large heat and power plants. A representative situation can be found in Bishkek, the capital of Kyrgyzstan with similar latitude as Rome. Officially, about 350 thousands inhabitants in the center of Bishkek receive domestic hot water and energy for space heating from the central Heat and Power Plant (TEZ) of Bishkek City. The real number of consumers is estimated by the local authorities to be twice as much. The district heating net, however, shows some differences to common Central European technologies. It is constructed as an open-circuit system, where domestic hot water is taken by the consumers directly out of the net without any heat exchanger coupling (see Figure 1). Thus, the district heating net in Bishkek has to be refilled with about 3000 m3 per hour of 12°C cold water. This is carried out at the central Heat and Power Plant. Cold water is heated up to 60°C, the temperature level required during summer when no space heating is needed and ambient air temperature is usually higher that 20°C even at night.

This water can, however, be preheated using solar energy before heating up conventionally to the supply temperature. For this purpose uncovered plastic solar collectors (cf. [1]) or multicomponent # Current address: Institut fur Solartechnik SPF, Rapperswil (Switzerland)

solar thermal systems (cf. [2]) can be effectively used due to the low water inlet temperatures and high ambient temperatures.

For this application no solar heat storage is necessary because of the high basic load. Due to the low inlet temperature and excellent climatic conditions (high irradiation, high ambient temperature) very high solar gains can be achieved. For Bishkek specific solar gains are estimated to be around 1000 kWh/m2coll during the frost-free season Mai — September, which is about four times higher than the solar gains typically achieved in swimming pool heating in Central Europe.

But not all district heating nets in the CIS have similar boundary conditions so that not everywhere water preheating with uncovered collectors makes sense. Some district heating nets are not in operation in summer, but only during winter or space heating period. A number of district heating nets are constructed as a closed-circuit system with almost no need for water refilling and relatively high water return temperatures. For closed-circuit systems, other solar thermal technologies, e. g. flat plate collectors, can be used. These, however, are not yet economically competitive with still low, but rapidly increasing energy prices in the CIS and are therefore not considered in this study. Furthermore, some heat and power plants are operated in summer to produce as much electricity as possible use even cooling towers during summer. In this case, no uncovered collectors can be applied since there is no demand for low temperature heat.

The following technical characteristics of the heat and power plant are necessary in order to effectively apply the uncovered collectors or multicomponent solar thermal systems for water preheating

• high basic load (^ no storage is needed + large solar heating system)

• open-circuit or mix-circuit systems (^ low water inlet temperature)

• in operation in the frost-free period

• CHP-operation mode based on the heat load

• available area for uncovered collectors

The objective of this investigation is to estimate the potential of solar water preheating in the CIS by identifying heat and power plants where this technology can be effectively implemented. For

this purpose, an attempt was made to identify and evaluate all large heat and power plants operating in the region.