How to design a flexible air-conditioning for logistics centers?

The constant development of the consumer market and e-commerce makes it more and more companies feel the need to increase their storage space, which relatively contributes to the development of the business. Easy-to-store goods are able to make do with infra-red light or dark radiant heaters and directly heated hot air systems. Decentralised solutions are useful for large spaces that are ideally suited to demanding goods such as food, medication, electronics components and convenience products.

Fig. 1: The market for logistics buildings continues to grow. For investors, third-party utilisation is playing an increasingly important role. As a result, hall heating systems, too, need to respond flexibly to the requirements of its users.

Experts in the sector have become more cautious with underfloor heating as a large space heating system. The authors of the Factbook "Zukunft Hallenheizung" (Perspectives of Large Space Heating), published by the Figawa – Bundesvereinigung der Firmen im Gas- und Wasserfach e.V. – Technisch wissenschaftliche Vereinigung, Köln, describe underfloor heating as being too slow in response for the needs of dynamic warehouse logistics. Also, "the uniformly distributed arrangement of the heat transfer can restrict the ways in which the space can be used. In addition: "Under-floor heating limits the locations where machines and rack systems can be anchored, a fact that can become problematic later on if halls are repurposed or transport vehicles exert high point loads on the floor.

Because of the current trends in hall logistics and the legislative framework (EnEV and EEWärmeG), Hoval considers the best medium- to longterm solution for investors, hall owners and hall operators to be a combination of central heat generation and decentralised recirculation heating. Together with a technical facility equipment planner, Hoval developed a modular turnkey solution that consists of heat generation units, recirculation heaters and zone control modules.

Fig. 2: Function model of the Hoval "large space heating" solution with a heat generator and a decentralised recirculation heater

Due to its modular design, this scalable system can be adapted to almost any hall size. Its structure not only simplifies planning and the tendering process but also shortens project implementation times since virtually all product steps are standardised and the entire solution comes from a single source.

The hall solution consists of the following basic modules:

• Hoval UltraGas twin boiler unit
• Control module
• TopVent recirculation unit with Air-Injector for the uniform distribution of heat throughout the entire building space. Stratification (temperature layering in a room) is only 0.15 K/metre, a value that is unattainable by most other large space heating systems.

Fig. 3: Decentralised recirculation units can be optimally integrated in the hall infrastructure, without becoming an obstacle for racks, craneways and supply lines.

One advantage of a modular solution with a central heat generation unit and decentralised recirculation units is that customers can choose between several different energy sources. Alternatives to natural gas, which is by far the most common heat source in large spaces, are heat pumps, pellets, wood chips or solar energy, or the integration of these energy sources with conventional systems. In addition, the Smart Grid function enables the coupling of line-bound energy sources (natural gas, electricity, district heat) with regenerative energy sources (pellets, wood chips, solar heat) by means of dual fuel heat generation. As a result, operators can take better advantage of fluctuating energy supplies (wind, photovoltaic power) and can create switch-off potential for negotiating more favourable rates for line-bound energy sources. Due to the expansion of renewable energies and the fluctuating nature of this supply, Hoval sees dual fuel heat generation as growing in significance in the future.

A wholesaler for food and convenience products would like to expand its logistics network. The challenge was to systematise the heat, ventilation and control function modules to the extent where planning, tendering and project implementation could be realised in a very short time. In the case at hand, the logistics distribution centre (145 m long, 125 m wide, 12 m high) is designed for short turnaround times. The fabric heat loss is 910 kW or 4.2 W/m3. The requirement was to divide the hall into four room temperature control zones and to operate and control these separately on the basis of the storage quantity and turnaround time. Experience has shown that energy savings of 10 to 20 percent can be achieved through the needs-based operation of control zones as a function of turnaround time and storage quantity.

The heat generator is a twin boiler plant (UltraGas UG 900D) with a nominal heat output of 2 x 450 kW. The two gas condensing boilers with a standard efficiency of 109% are operated in such a way that both boilers run on a modulating basis. In condensing boilers, boiler efficiency under partial load is higher than under full load; consequently, overall efficiency increases by around 2 % when boilers are operated in parallel on a modulating basis. A further increase in efficiency is achieved by means of the cascaded return feed to the boiler, in which the two return lines – from the recirculation heaters (low) and the drinking water heating (high) – are sequentially fed to the exhaust heat exchanger. This adds another 9 percentage points to the boiler efficiency.
Apart from the "UltraGas" standard module, alternative heat generation methods are the oil boiler (UltraOil), heat pump (Belaria or Thermalia), bio-mass boiler (BioLyt) and solar energy.  

The core module of the "large space heating" solution is the decentralised TopVent DHV-9/C recirculation unit, developed for use in halls with mounting heights of 4 to 25 m. This duct-free system greatly simplifies installation without affecting the hall infrastructure (cranes, rack and conveyor systems, supply lines, etc.). The absence of air ducts also reduces the measures needed to assure compliance with Hygiene Directive VDI 6022, and power requirements for the air supply are decreased as well. 

In the project described here, 29 TopVent units were installed to efficiently ventilate the space. The air flow rate per unit is 8,700 m³/h and the maximum heat output is 137 kW. Due to the integrated, automatically adjustable vortex air distributor, or "Air-Injector", the range of the air stream can be precisely adjusted to the mounting height and rack infrastructure.

Fig. 4: The Air-Injector unit adjusts the air stream to the spacial configuration in its vicinity with a special fuzzy logic algorithm in the controller.

Compared to designs with conventional recirculation heaters, the Hoval solution requires fewer units and smaller air volumes for the same hall size because of its more efficient air distribution. This lowers investment and installation costs. 

Functionally, the patented Air-Injector consists of a deflector and a jet nozzle that is responsible for the formation of an air supply stream with the maximum possible range of coverage. Added to this is a so-called vortex device that rotates the air current, thus increasing the scatter angle (induction) of the air current. The shape of the current is similar to that of an inverted mushroom with a thin stalk and a large cap. This geometry achieves optimal current flow conditions, offering high comfort in occupied areas with a maximum mounting height of 25 m. With this type of air supply, temperature stratification is reduced to approx. 0.15 Kelvin per metre of mounting height.

Conventional air distribution systems operate with a stratification of up to 1.0 K/m. The Air-Injector, on the other hand, creates a more uniform temperature distribution in the hall while avoiding the formation of energy-consuming warm air pockets under the roof, a phenomenon typical for high spaces.

Storage and transfer halls must be highly flexible and varied in use. This also applies to leased halls, the utilisation of which is shaped even more strongly on logistics strategies and economic influences. Hoval has therefore developed a zone-based control system for its decentralised large space heating system. This approach enables needs-oriented, time and temperature-based heating, ventilation and cooling of different hall areas and thus helps lower energy costs. 
Independent of the zone controller, each device and its Air-Injector can be individually and autonomously actuated via the controller using fuzzy logic control algorithms. The control function that performs the air stream range calculation takes into account the temperature difference between the supply air and room air in the occupied areas, varying the supply air flow rate and the air discharge angle accordingly. This avoids draughts. If it should become necessary to expand the hall, the modular control concept can be readily extended.

With its modular and scalable system building block design, the Hoval "large space heating" solution offers maximum flexibility for all types of halls and hall usages. The case study describes the development of a solution that can be adapted to the customer's current and future logistics needs. As a modular system, it is easy to plan, fast to install and quick to commission.

Fig. 5. With the Hoval solution, the planner is able to present a turnkey heating, ventilation and control concept for all large space heating needs. This saves time and costs.

It also improves profitability for investors. In addition: Through the formation of control zones, the large space heating system can effectively respond to different hall utilisation rates and types and to different kinds of goods being moved.
With the option of a dual fuel heat supply drawing on conventional and regenerative energy sources, the  Hoval solution can flexibly respond to future energy supply concepts with Smart Grid functions and to the stipulations of renewable energy laws.

Dimensions (L/W/H)                                  145/125/12m

Room temperature                                    Approx. 15°C

Outside temperature                                               -12°C

Heating times                                              24 hours/day

                                                   ~ 240 heating days/year

Fabric heat loss                                                    910 kW

Specific fabric heat loss                    Approx. 4.2 W/m³

Total energy costs                               71,915.00 €/year

(basic price, 2010:

                                                                   Heat 50 €/MWh

                                                         Electricity 84 €/MWh)

Specific energy costs per year:

Storage area                                                      3.97 €/m²

Hall volume                                                       0.33 €/m³

Heat

Annual heat consumption as per VDI 2068

                                           Approximately 1,176 MWh/a

Annual heating costs                                   58,800.00 €

Electricity

Annual electricity costs for 240 days Approx. 156 MWh

Annual electricity costs                                13,115.00 €

Investment costs (end customer price in Germany)

TopVent DHV-9/C unit                                 87,000.00 €

Mechanical installation                                  7,200.00 €

Hydraulic installation                                   25,500.00 €

Electrical installation                                    17,500.00 €

UltraGas boiler unit                                      61,000.00 €

Total investment costs                              198,200.00 €

Specific investment costs

Storage area                                                   10.93 €/m²

Hall volume                                                       0.91 €/m³

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