Standard hydraulic circuits for small-scale heat pump plant Dr. Thomas Afjei University of Applied Sciences Basel Institute of Energy Fichtenhagstrasse 4 CH-4132 Muttenz E-mail: t.afjei@fhbb.ch Summary In the first phase of the project entitled 'standard hydraulic circuits for small-scale heat pump plant' (STASCH), hydraulic circuits for small-scale heat pump plant were evaluated from both practicability and error tolerance [1] standpoints. The project was carried out on contract to the Swiss Federal Office of Energy. The project benefited from experience gained in field trials (FAWA), publications on previous standard circuits (RA- VEL, SWKI, FWS) and quality assurance projects. The investigations concerned circuit variants for a range of heat sources and hot water heating systems for new building (supply temperature 35-45 C) and renovation (supply temperature 60 C). In the second phase of the project, circuits that proved viable in the field and offer sizeable implementation potential will be simulated using MATLAB/SIMULINK and the CARNOT Blockset. They are then compared and evaluated, and their number finally minimized. Assessment criteria are: comfort, overall annual efficiency, investment required, annual costs and fault tolerance. Seven standard systems are presented (see Fig. 1): Standard system 1: without, heating only Standard system 2: without, with heating and hot water Standard system 3: with series, heating only Standard system 4: with series, with heating and hot water Standard system 5: with parallel, heating only Standard system 6: with parallel, with heating and hot water Standard system 7: with combination and solar collectors, with heating and hot water Objectives The objectives are: definition of a simple and efficient standard circuit for each of the applications listed: - air, ground and solar heat sources - heating and hot water via heat pump, separate (direct, electric) or solar hot water - new building (T supply <45 C) and renovation (T supply <60 C) reduction of the number of standard circuits to an absolute minimum preparation of design guidelines for an output range of 5 to 25 kw. The final object is to produce a guideline of standard circuits with dimensioning instructions.
Two steps towards a viable standard system The Swiss Heat Pump Test Center (WPZ) in Winterthur-Töss has established clarity with regard to practicable heat pump designs. The task of STASCH will be to establish similar clarity on the plant side. This mainly involves provision of the following three planning tools: 1. Standard applications Classification of possible applications 2. Standard systems Hydraulic circuit + control system + layout 3. Selection matrix Which standard system is best suited to what standard application? This results in a logical procedure in two steps as follows: Step 1 Step 2 Choice of standard system using the selection matrix Layout of the chosen standard system (each standard system having its own separate design data sheet) It is recommended that the planner/plumber should adopt these two steps in designing each new heat pump installation. They will be further detailed in the following based on current experience. Substantial changes and additions must however be expected in the ultimate version. Step 1: choice of standard system using the selection matrix Standard applications The starting point is always a particular application (= standard application). For this, an optimum hydraulic circuit (= standard system) must be found and not vice versa. In certain renovation markets, it is particularly important to take this circumstance into account. The first step is to classify the applications and establish a small number (as few as possible) of standard cases. To establish an adequately detailed classification, present knowledge suggests that it is sufficient to answer three questions, each of which has two or three 'multiple choice' answers: 1. What is the hot water system? hot water not via heat pump hot water via heat pump (solar backup for hot water possible) hot water via heat pump with solar backup for heating and hot water 2. Is the flow rate essentially constant, AND/OR is hydraulic separation necessary, i.e. does the system have: a single controlled heating branch, percentage of thermostatic regulators < 25% a single controlled heating branch, percentage of thermostatic regulators 25% several controlled heating branches (irrespective of number of thermostatic regulators) 3. How high is the inertia of the radiator system? wet-laid floor heating system, with (a few) or without additional radiators dry-laid floor heating system, OR wet-laid floor heating system with a significant proportion of radiators, OR radiators only Depending on the answers to these three questions, one of 3 x 3 x 2 = 18 standard systems will result. Standard systems From discussions held in workshops on the question, it was evident that the most frequent systems are those illustrated in Fig. 1.
Fig. 1: Standard systems Note: in place of circuit diagram 7, a simpler version that includes a heat exchanger is available (see Fig. 2) Legend: WP=Heat Pump; WW=Sanitary Hot Water; WA=Heat Emission System; SP=Buffer Storage Selection matrix The selection matrix based on Fig. 1 is shown in Table 1. Additional comments: for parallel, the ' ' symbol is shown, since a series (having only a single pump) would be more suitable for standard system 7, no such comparison is possible, since this is the only system that permits the use of solar energy for heating and hot water (here, it may well be better to deploy solar energy for hot water alone)
Standard applications Without heating without hot water with hot water Standard systems Heating in series Heating in parallel (in the return) without hot water with hot water without hot water with hot water Heating with integrated hot water 1 2 3 4 5 6 7 N 1 K T 2 N 3 * O V T 4 * E N 5 ** T 6 ** K N T 7 8 W V T 10 N 9 E N 11 ** T 12 ** N 13 K T 14 S N 15 V T 16 N 17 E T 18 What is the hot water system? O hot water not via heat pump W hot water via heat pump (solar backup for hot water possible) S hot water via heat pump with solar backup for heating and hot water Is the flow rate essentially constant, AND/OR is hydraulic separation necessary, i.e. does the system have: K a single controlled heating branch, percentage of thermostatic regulators < 25% (effectively constant flow rate) V a single controlled heating branch, percentage of thermostatic regulators 25% (effectively variable flow rate) E several controlled heating branches (irrespective of number of thermostatic regulators) How high is the inertia of the radiator system? N wet-laid floor heating system, with (a few) or without any additional radiators T dry-laid floor heating system, OR wet laid floor heating system with a significant proportion of radiators, OR radiators only Recommendations (based on current experience): system recommended system recommended under certain conditions system not recommended system banned inappropriate combination * version with spill valve ** version with mixing valve for several heating branches Table 1: selection matrix
Step 2: layout of the chosen standard system Based on the given matrix of standard systems, the most suitable standard system was chosen in step 1. This provides only the basic hydraulic circuit, to which possible variations, the control system and details of the layout must be added. In their final form, the standard systems may be represented by seven data sheets containing the following sections: schematic diagram of the hydraulic circuit with possible variants (as in Fig. 1) basic data (differing as between new building and renovation) selection of variants selection of control system layout Selection of variants It is certainly true that the fewer the number of variants, the simpler and safer will be their implementation in practice. However, a certain minimum number of variants is unavoidable (see dashed line in Fig. 1): in the flow (standard systems 3 and 4) spill valve (standard systems 3 and 4) three or four connections to parallel (standard systems 5 and 6) auxiliary electric heating (standard systems 3, 4, 5 and 6) several heating branches (standard systems 5, 6 and 7) solar backup for hot water only (standard systems 2, 4 and 6) Further variants not shown in Fig. 1 may be appropriate as follows: external heat exchanger for hot water (standard systems 2, 4 and 6) interfaces for heat pumps with additional heaters, condenser and subcooler Selection of control system The choice of control system is mainly determined by the standard system selected: Standard systems 1+3 Heating: outdoor temperature controlled return temperature control (with or without defrost control) Standard systems 2+4 Heating: outdoor temperature controlled return temperature control (with or without defrost control) Hot water: alternative system with 100% heat pump heating (with or without independent solar control) Standard system 5 With a single heating branch: Heating: outdoor temperature controlled buffer temperature control (with or without defrost control) With several heating branches (hydraulically separated): Buffer charging: outdoor temperature controlled stepwise charging of buffer (with or without defrost control) Heating: Outdoor temperature controlled supply temperature control Standard system 6 With a single heating branch: Heating: outdoor temperature controlled buffer temperature control (with or without defrost control) Hot water: alternative system with 100% heat pump heating (with or without independent solar control) With several heating branches (hydraulically separated):
Standard system 7 Buffer charging: outdoor temperature controlled stepwise charging of buffer (with or without defrost control) Hot water: alternative system with 100% heat pump heating (with or without independent solar control) Storage charging with heat pump in winter: Stepwise charging of the central section of the (three-way valve is through-switched, lower pump switched on) Storage charging with heat pump in summer: auxiliary heating of the upper section of the (three-way valve is switched to branch, upper pump switched on) Solar charging: preheating of the lower section of the (three-way valve through-switched), heating of the central and upper section of the (three-way valve switched to branch) Heating: outdoor temperature controlled supply temperature control (one or more heating branches possible) The circuit in Fig. 2 is much simpler and will be included as a variant in the study. Fig. 2: Inclusion of combination in all systems studied by FAWA. Legend: SW-WP=Brine/Water Heat Pump; BWW=Sanitary Hot Water; SP=Storage; WA=Heat Emission System It is also necessary to consider where the sensors and thermostatic regulators should be located, which values have been preset and how much hysteresis is present in the heating controller. Temperature control of individual rooms using thermostatic regulators is always possible for standard solutions 3-7. With the standard systems, it is always possible to include the room temperature as a control parameter on condition that a suitable reference room and measurement point can be found (albeit hardly feasible in an apartment house).
Including the room temperature as a control parameter can serve several purposes: temporary lowering (i.e. parallel displacement) of the heating characteristic as long as the fault persists successive optimization of the heating characteristic (so-called 'optimizer') display of the room temperature as a secondary control parameter in optimizing the heating characteristic Experience gained over the last 15 years with secondary room temperature control has tended to be negative since: the advantage of thermostatic temperature control of individual rooms is partly lost room temperature control is not a substitute for individual control! the combination of room temperature control with thermostatic regulators carries with it a certain risk indeed combining room temperature control and thermostatic regulators in the reference room is fatal! it is difficult to find a viable reference room having a suitable measurement point, and it is quite possible that errors will occur in setting this up. Also, room utilization can change. Optimizers have largely disappeared from the market as a result of negative experience had with them. Recommendations for simulation in the second phase All standard systems (1 to 7) shown in Fig. 1 are to be simulated. Heat pumps fitted with additional heaters, condensers and supercoolers will not be simulated. However, assuming that sufficient project capacity is available, or should these features prove essential in the course of the project, they will be included as black boxes with suitable hydraulic interfaces. The simulations will be based on a detached house with 4 occupants (2 adults and 2 children). It will also be interesting to study changes in patterns of use, e.g. where 4 adults replace 2 adults and 2 children. The comfort criteria (measure of well-being) are calculated based on the statistical and empirical comfort relationship determined by O. Fanger shown in Fig. 3, which is also contained in SIA180:1999. Dissatisfied People [%] Room Temperature [ C] Fig. 3: Percentage of persons dissatisfied (PPD) as a function of room temperature in an office in summer [1]. Note: according to O. Fanger, 5% dissatisfied is the optimum.
Literature [1] Afjei, Th., Schonhardt, U., Eicher, H.P., Erb, M., Gabathuler, H.R., Mayer, H., Zweifel, G., Achermann, M., Renaud, P.: Standard hydraulic circuits for small heat pump installations, interim report SFOE project 36228, Swiss Federal Office of Energy 2001.